SLIM-SFA-1013 rev 0
SAW Filter / Amplifier for 1013 MHz
Released 12-11-11. Original release. Updated
12-13-11. Note regarding IMD Updated
12-17-11. Add section - Test, Evaluation,
and Characterization Updated 2-18-12. Add
sections h. and i. Updated
5-2-12. Add link for downloading schematic
as Express file. Updated
5-16-15. Change capacitor C5. Parts list now
Rev A.
SLIM-SFA-1013, SAW Filter / Amplifier,
size-A
All needed documentation can be found within this web page. a.SKSLIM-SFA-1013
rev 0
Schematic. b. PWB-SFA rev 0,
PWB artwork, in
ExpressPCB software. Use for ordering from Express. c.Component
Layout. Use for locating parts on board. d.PLSLIM-SFA-1013 rev
A,
Parts List. e.Construction f.Test Results g.
Test, Evaluation, and Characterization h. Spurious Responses i.
Conclusions
The SLIM-SFA-1013 is a combination of three cascaded
SAW
Filters and an amplifier. The purpose of this module is to replace the
Coaxial Cavity Filter used in the MSA. There are two expressly separate
circuit sections on a single pwb. It is the intention to cut the pwb
before assembly. Although both sections are intended to operate
together, it is important to separate the sections so that full
perimeter fencing can be installed on each section. This is very
important to minimize
section cross-talk and maximize ultimate filter response rejection.
The basis of this design was to band pass the
center frequency of 1013.3 MHz, which is the nominal First I.F. of the
MSA. A more important design feature is to maximally attenuate
frequencies near 1034.7 MHz, which is the image frequency of the MSA
input. Test results indicate attenuation at this image frequency is
about -91 dBc. This is not a good as the Cavity Filter (better than
-100 dBc) but it is very good compared to the minimum attenuation
required for Spectrum Analyzer operation, which is -70 dBc.
The total gain for the design is approximately 0 dB.
The combined loss of the attenuators and the filters is compensated by
the amplifier's gain of approximately 18 dB. The maximum input power of
any of the SAW filters is +10 dBm. Therefore, the maximum input to the
Filter/Amplifier section is +13 dBm. The maximum
input to the
dual filter section is +10 dBm. When the dual cascaded
SAW Filter section follows the Filter/Amplifier, the maximum input to
the Filter/Amplifier section must be kept below -2 dBm.
When substituting this module for the MSA's Cavity
Filter, connect Mixer 1 output to the Filter/Amplifier
section,
followed by the Dual Filter section, followed by the input to Mixer 2.
Although this web page is specific to the
SLIM-SFA-1013, other filter frequencies could be used for other
projects. The vendor specifies several SAW Filters with the identical
footprint. They can be found at www.golledge.com.
12-13-11 Note:
There is one area of concern that I have overlooked and should mention.
IMD,
Inter modulation Distortion. Because the Bandwidth of this scheme
is
about 18 MHz it is possible that two incoming signals separated by 10.7
MHz will enter Mixer 2. The product of the two signals, 10.7 MHz will
not be displayed as a spur. It will manifest itself as a noise floor
increase. The noise floor might even "modulate" at a rate of the same
modulation of either of the input signals. This interference will be
most pronounced when using a very wide bandwidth Resolution Filter (the
two signals don't have to be exactly 10.7 MHz apart). More information
is presented later on this page. This interference is highly
unlikely when using the Coaxial Cavity Filter, since its bandwidth is
much narrower, about 2 MHz.
a.
SKSLIM-SFA-1013 Schematic. Click
to download Express file.
The ERA-33SM (U1) can be replaced by a
variety of MMIC amplifiers. Also, the 10 volt supply voltage can
be reduced to as little as 5 volts, along with R1 and R2 value
reductions. This would decrease the amount of heat generated by R1 and
R2. This would allow faster temperature stabilization of the SAW/Amp
section.
b. PWB-SFA, PWB Artwork. Click
to download Express file. Use for ordering from ExpressPCB.
c.
Component Layout, Top
(Component)
View
This layout can be used for locating or
placement of components.
Connectors J1-J4 and P1 are mounted on the bottom side (ground plane)
of
the PWB. Size is 1.2 inch x 1.2 inch.
d.
PLSLIM-SFA-1013, Parts List Master, future changes
will be made on this web page only.
PLSLIM-SFA-1013
Rev A, 5/16/2015
Des Value
Mfg. Part
Number Digikey
Number Cost
C1 .01 uF
C0805C103K5RACTU
399-1158-1-ND
0.04
C2 .1 uF
C0805C104K3RACTU
399-1168-1-ND
0.05
C3 .1 uF
C0805C104K3RACTU
399-1168-1-ND
0.05
C4 .1 uF
C0805C104K3RACTU
399-1168-1-ND
0.05
C5 10uf/35v ceramic
GMK316F106ZL-T 587-1352-1-ND
0.35 Rev A, deleted
C5 10uf/16v AVX,
TPSB106K016R0500 478-5230-1-ND
0.65 Rev A, better esr cap
C6 100 pF
C0805C101K5GACTU
399-1121-1-ND 0.05
C7 .01 uF
C0805C103K5RACTU
399-1158-1-ND
0.04
C8 100 pF
C0805C101K5GACTU
399-1121-1-ND 0.05
C9 100 pF
C0805C101K5GACTU
399-1121-1-ND
0.05
FB1 Ferite Bead
BLM21PG221SN1D 490-1054-1-ND
0.06
J1 SMA Female Molex,
538-73391-0070 WM5544-ND
3.78
J2 SMA Female Molex,
538-73391-0070 WM5544-ND
3.78
J3 SMA Female Molex,
538-73391-0070 WM5544-ND
3.78
J4 SMA Female Molex,
538-73391-0070 WM5544-ND
3.78
L1 68 nH
LQW18AN68NJ00D
490-1178-1-ND
0.39
FL1 1013/15 SAW
Filter, PN MA08712, Model TA0477A
FL2 1013/15 SAW
Filter, PN MA08712, Model TA0477A
FL3 1013/15 SAW
Filter, PN MA08712, Model TA0477A
P1 Conn, 2 pin
S1012-36-ND (row of 36 pins)
S1012-36-ND 0.06
R1 75 1/4W
RHM75.0F
RHM75.0FCT-ND
0.08
R2 75 1/4W
RHM75.0F
RHM75.0FCT-ND
0.08
R3 294
MCR10EZHF2940
RHM294CRCT-ND 0.04
R4 294
MCR10EZHF2940
RHM294CRCT-ND
0.04
R5 17.8
MCR10EZHF17R8
RHM17.8CRCT-ND 0.04
R8 0
MCR10EZHJ000
RHM0.0ACT-ND
0.04
R9 294
MCR10EZHF2940
RHM294CRCT-ND 0.04
R10 294
MCR10EZHF2940
RHM294CRCT-ND 0.04
R11 17.8
MCR10EZHF17R8
RHM17.8CRCT-ND 0.04
R12 294
MCR10EZHF2940
RHM294CRCT-ND
0.04
R13 294
MCR10EZHF2940
RHM294CRCT-ND
0.04
R14 17.8
MCR10EZHF17R8
RHM17.8CRCT-ND 0.04
U1 ERA-33SM Minicircuits
ERA-33SM
1.72
PWB Circuit Board
PWB-SFA
3.28
Note: The cost of the SAW filters is not included.
e. Construction
Cut the PWB-SFA into two sections with a hacksaw blade that has a
cut less than .075 inch.
Sand the sides of each section to the perimeter ground planes. This
will allow easy installation of the perimeter fencing.
See the Construction Tips
web page for good advice on the following procedures.
Install top components using your favorite soldering method. I
still use the old fashioned low wattage soldering iron and a hot plate
from a coffee brewer.
Clean the board.
Clean it again, because I know you could do better!!
Modify the SMA connectors for a flush fit to the pwb.
Mount P1 and J1-J4 from the bottom side of the boards.
Clean the boards again.
Test. This is a "did I short anything out" type test. The test results
will not be anywhere close to "good".
Install the three internal fences.
Clean the boards again.
Install the perimeter fencing.
Clean the boards again.
Test before installing the top covers. The results will be better than
the first test, but still not "great".
Install the top covers.
Test. Results should be very good.
Raw PWB.
Components installed.
Showing one of the internal fences.
Sections Integrated
f. Test Results
(Click on images for larger display)
SAW/Amplifier Section, Data taken by Neal Martini.
As a comparison, the following plot is one I (Scotty) took
of a newly completed SAW/Amp Section.
Marker 5 is not at -130 dBm. It cannot be
measured since this MSA range is limited to 1036 MHz. This can be seen
as the "fall-off" just above Marker 4.
Marker 4 is the attenuation near the image frequency of the MSA (1034.7
MHz). Neal got a rejection ratio of -31.4
dBc and I got -32.3 dBc. Very close for two different sets of SAW
filters. This is a good indication that the SLIM is
reproducible.
Dual Section, data taken by Neal Martini.
The input section of the module is fully shielded.
It shows excellent attenuation characteristics and minimal crosstalk.
Well within the desired design goal.
As a comparison, the following plot is one I (Scotty) took of a newly
completed Dual SAW Section.
Marker 1 is not really at -130 dBm, but it is below
-120 dBm. Marker 4, the attenuation near the image frequency of the MSA
(1034.7 MHz) is the important data point. Neal got a rejection ratio of
-62.2 dBc and I got -65.5 dBc. Very close for two different sets of SAW
filters.
Final Results of Cascaded Sections, data taken by Neal
Martini.
Both sections cascaded together, the
SAW/Amp section first.
As a comparison, the following plot is one I (Scotty) took
of the cascaded sections.
Markers 1, 2, and 6 are not really at
-130 dBm, but they are below -120 dBm.
Marker 4, the attenuation near the image frequency of the MSA (1034.7
MHz) is the important data point. Neal got a rejection ratio of -89.5
dBc, but his shielding was not complete. I got -99.1 dBc. Actually,
Marker 6 (1034.875 MHz) is closer to the image frequency
and should be used. Neal got -97.7 dBc and using -120 dBm for my Marker
6, I got -107 dBc. Anything better than -90 dBc is excellent
g. Test,
Evaluation, and Characterization
The purpose of this section is to offer
my personal notes while building the SLIM-SFA-1013. These are the steps
and notes as I progressed.
Dual Cascaded SAW Filter Section (no
attenuators)
1. Acquired raw PWB and SAW Filters.
2. Cut sections of pwb and trim edges.
3. Install SMA connectors.
4. I swept the dual SAW section to determine amount of J3 to J4
crosstalk on this pwb. It is
difficult (if not impossible) to predict what a pwb's crosstalk will
be. A "perfect" pwb would have no input to output RF energy flow. In
reality, the input to output crosstalk needs to be better than the
expected input to output isolation of the SAW filters.
This plot simply proves that this pwb does have some
crosstalk. The white trace is used as a reference. It is a transmission
response of two SMA connectors in open air, about 1 inch apart. The
yellow trace is the actual J3 to J4 crosstalk response. The
Verification MSA has an upper limit of 1035 MHz, as shown by the abrupt
loss near the end of the sweep. The input signal level to J3 is -12.2
dBm at 1035 MHz, and indicating about -99 dBm at J4. Therefore,
crosstalk is about -86.8 dBc.
At this point in testing we don't know how much of the crosstalk
is due to air transmission (J3 to J4 acting as antennas), surface
effect (RF flowing across the surface of the pwb), or substrate
conduction (RF flowing inside the FR 4 material). In any case, we want
to minimize it so that the resulting crosstalk does not materially
affect the SAW Filter responses. We predict that the two cascaded
filters should have at least -62 dB of rejection ratio at the frequency
of interest (1034.7 MHz). It is essential that pwb crosstalk be
minimized at least 10 dB below this number. That is, pwb crosstalk must
be better than -72 dBc at all frequencies near our SAW Filter. This
pwb's crosstalk of -86.8 dBc does meet the goal, but there
are several thing we are going to do to this board which may make the
crosstalk worse.
5. I placed a small brass box over the J3 area. The purpose was to
eliminate all air transmission from J3 to J4. I did not plot the
response, but the total crosstalk dropped below the MSA noise floor
(the -130 dBm line). This would conclude that all of the crosstalk
measured in step 4. was attributed to air transmission. This is good
news. It means that the design using plated-thru holes to minimize substrate
conduction was successful. Air transmission crosstalk is
far easier to minimize than surface effect or substrate conduction. The
same brass box over the J4 area resulted in the same response (as
predicted).
6. I installed three temporary 100 pfd capacitors in place of R8, R11,
and R14. A used a fourth 100 pfd to bridge over the uninstalled FL 3,
This is to provide a low impedance path from J3 to J4 so that I can
test the true substrate conduction crosstalk under FL2. I added the
total perimeter fence using .2 inch height coffee can material (tin
plated steel). This picture looks inverted, but connected to the MSA
(under the blue cloth) the signal direction is from left to right
(Tracking Generator Output to J3 and J4 to MSA Input).
The following plot shows the crosstalk with only the capacitors and
perimeter fence in place. The top white trace is J3 to J4 crosstalk.
The yellow trace is the same, but with a temporary lid on the module.
This crosstalk looks extremely high, but it is due
to the 4 coupling capacitors that lowers the impedance from J3 to J4.
We need to shield the circuit internally to minimize the air
transmission effect before obtaining viable data for the substrate
conduction crosstalk under FL2.
7. I added an internal fence over the ground strip that separates the
input
and output of FL2.
The white trace is a reference, as used in the
previous plot in step 6. The yellow traces are my attempt at holding a
temporary top cover over the J3 section. This shows the crosstalk
passing through
the FR4 material from the input pad of FL2 to its output pad. With an
input at -12.3 dBm and output at -100 dBm, this crosstalk is about -88
dBc. Although this crosstalk seems high, it is actually pretty darn
good. Consider this: these two input/output pads are separated by only
.050 inch. If the pads were not separated with a ground strip and
ground vias, the internal FR4 crosstalk would be horrible. As it is, we
only need crosstalk to be about 10 dB better than the expected SAW
filter
response at 1034.7 MHz, which is predicted (by specification) to be -31
dBc. We beat this
by 57 dB. Woo-Hoo!
Now let's see how a single SAW performs with this
really good pwb.
8. Install FL2. (Holy Flyspeck Batman, this thing is small!).
If you construct this board according to the
Construction procedure, the SAW will be installed before the SMA
connectors and the perimeter fencing. This allows solder paste and
reflow technique. This would be the easiest way to install this SAW.
However, I could not do this since I wanted to characterize the pwb
before installing the SAW. Here is how I installed the SAW using a
standard fine tip soldering iron. I flipped the SAW on its back and
applied a tiny solder bead on the input lead. I then positioned the SAW
in its correct position on the pwb. I then applied the soldering iron
tip on the pwb trace as close to the input lead as possible. The heat
transfer caused the solder bead on the SAW to flow onto the pwb's
trace. I then soldered the output of the SAW in the same manner,
but adding solder this time. I then soldered the 4 ground corners using
the same method. I cleaned with 99% isopropyl. ( I let dry and then I
cleaned it again).
9. I added the internal fence over FL2.
I cut the fence to size (roughly) using coffee can
lid and metal shears. I used a Dremel Tool with a carbide cut-off wheel
to trim
for final shape and added the notch for the SAW. I soldered the
internal
fence sides to the perimeter fence first and then to the ground strip
than extends under the SAW. While still hot, I added solder to the
fence directly above the SAW and moved the soldering iron tip down to
the top of the SAW and let the solder flow. The SAW accepted the solder
quite nicely. The dual section, completed. Notice that there are three
seperate lids.
10. Sweep for results. There is a temporary lid tacked over the J3 area
only.
Results are extremely close to both Neal's data and
the Vendor's specs. Insertion loss is (-12.27 dB/-15.9 dB) = -3.63 dB.
Attenuation at the image frequency (1034.5 MHz) is (-15.9
dB/-47.86 dB) = -31.96 dB.
The following wide band sweep shows the out-of-band
attenuation at lower frequencies. With my set-up I cannot sweep above
1035 MHz.
11. I removed the temporary 100 pfd at FL3 and installed the SAW at
FL3. I fashioned a second internal fence and installed it over FL3 in
the same manner as
done for FL2.
Insertion loss is (-12.27 dB/-18.21 dB)
= -5.94 dB. Attenuation at the image frequency (1034.5 MHz) is (-18.21
dB/-88.54 dB) = -70.33 dB. I do not show a wide band sweep
below 988 MHz because it would be a waste of web page space. The
isolation is well below the noise floor of the MSA.
You might notice that I showed the insertion loss of the
single FL2 SAW to be -3.63 dB. Here, I show the combined insertion loss
of FL2
and FL3 to be -5.94 dB. This would indicate that the
insertion loss of the FL3 SAW is (-5.94/-3.63) = -2.31 dB. However,
after installing FL3 I noticed that the Tracking Generator SMA
connector was loose. This might account for the descripancy. I am
inclined to believe that each SAW has an insertion loss of -5.94 / 2 =
-2.97 dB.
This shows the 3 dB bandwidth (2 cascaded SAW's) to
be 16.4 MHz. It also shows that the in-band response has over 1 dB of
"slope". Not really a concern and we can't do anything about it, anyway.
I added both internal 3 dB attenuators and added the
lids
permanently. This is the sweep of the finalized section:
The insertion loss is now (-12.27
dB/-24.18 dB)
= -11.91 dB.
I added a 2.8 dB pad on each port (center trace). Then added
another 2.8
dB pad on each port (bottom trace).
The top trace is a carry-over from the previous
sweep. Adding the external attenuators causes distortion.
Weird!
12. I did a quick test of phase change versus temperature
change. I
will do more, but preliminary testing indicates that the phase change
is inversely proportional to temperature change. That is, as
temperature increases, the phase decreases. It is about 1 degree of
phase for each 1 degree F. Center frequency insertion loss does not
change at all over a
20 degree temperature change. I will do further testing to see if the
image attenuation changes vs. temperature.
This concludes the characterization of the Dual Cascaded SAW
section.
SAW/Amplifier Section (no attenuators)
1. Install SMA connectors
2. Install .25 inch perimeter fence
3. Install C8 and C9 as a perminant installation
4. Temporarily install two 100 pfd capacitors. One across the pads that
R5 will occupy and one across the pads that U1 will occupy. This is to
create a low impedance path from J1 to J2 so that I can measure the
isolation between the two pads that FL1 will occupy.
5. Temporarily install the FL1 internal fence. It has no cut-out for
the SAW. The SAW is not installed yet.
7. Tack a temporary lid over the J1 area.
8. Measure the transmission from J1 to J2 to characterize
the crosstalk passing through
the FR4 material from the input pad of FL1 to its output pad.
I would expect it to be the same as the crosstalk number taken during
the Dual Filter Board testing, which was -88 dBc.
The input power is -12.3 dBm and the output is -100
8 dBm. Therefore the FL1 gap crosstalk leakage is -88.5 dBc. We needed
at least
-41 dBc and we beat that by 47.5 dB. By golly, we have
another winner.
9. Install FL1 and its internal fence. (no amplifier or
internal pad)
10. Tack a temporary lid over the J1 and J2 areas. Sweep
for characterization.
Insertion loss is (-12.27 dB/-15.74 dB)
= -3.47 dB. Image point (R) is 32.3
dBc. Quite similar to the results of FL2 taken earlier. However, the
out-of-band response is about 10 dB poorer than FL2. I'm not going to
spend much time trying to determine why the difference. Since the
out-of band levels are at least 10 dB below the image response it won't
make a difference.
Here is a wide band sweep of the response.
(no amplifier or internal pad)
Here is a narrow sweep to show the 3 dB Bandwidth.
(still no amplifier or internal pad)
I added the internal 3 dB attenuator and the MMIC
amplifier.
Permanently installed lids. The SAW/Amp section is now complete.
The
following plots are sweeps with external attenuators. The
top trace is with two external 3 dB attenuators, one on each port (J1
and J2).
It is somewhat distorted due to it
overloading the MSA input. The maximum input without distortion is -11
dBm. The center trace is with 6 dB attenuation on
each
port. The MSA is within its linear dynamic range. The bottom trace is
with 9 dB attenuation on each port.
According to the top trace, the insertion loss
(gain) is (-12.27 dB/-5.14 dB)
= +7.13 dB. This is taken with 6 dB of external attenuation, so the
total insertion gain of the SAW/Amp section is +13.13 dB.
Integrating the Two SAW Sections (no
amplifier or attenuators)
The previous paragraphs show data of the two
sections during construction and after completion. During construction,
I tested the uncompleted sections cascaded together. The purpose was to
get total insertion loss and total bandwidth data for 3 series SAW
filters with no amplification or internal attenuators. These are the
results.
1. I connected the MSA's Tracking Generator to J3 of Dual SAW. Its J4
is connected to J1 of
SAW/Amp section. Its J2 is connected to the MSA input. The Dual SAW
section has no internal attenuator resistors. The SAW/Amp section does
not have its amplifier or internal 3 dB attenuator resistors installed.
A coupling capacitor is bridged over each of the missing components.
This simulates a test of 3 series SAW filters.
The heading of this graph is confusing. It really
should read "Data of 3 series SAW Filters".
The total insertion loss is (-12.3 dBm - 20.94 dBm)
= -8.64 dB. The image point (R) is displayed as being below -120 dBm,
which is the lowest level this MSA can measure effectively. There is no
calibration below -110 dBm. Even if the image point was at -110 dBm,
the image rejection would be -89.06 dBc. Obviously it is better than
this, but at this point in test, I don't know precisely what it is. I
will determine this later.
This plot is impressive. The design stipulates 3 dB
attenuators between SAWs to minimize interaction. However, I'm
beginning to think the SAWs can tolerate feeding each other without
padding. I will know more once the attenuators are in place.
2. Plot the 3 dB points of the 3 SAWs in cascade. (no
MMIC amplifier or internal pads)
Yuck. We may need those pads, after all. The peaking
may be caused by the interaction the SAWs are having with each other.
It is also possible that the MSA is causing interference, since I am
sweeping through the 1013.3 MHz of its First I.F. Anyway, the
relevant information is the 3 dB bandwidth, which is about 14.9 MHz.
Integrating the Two
completed Sections (full shielding, internal pads, and amplifier)
The following plots are the responses to
the
SLIM-SFA-1013 in its final configuration. Tracking generator to J1, J2
to J3, and J3 to MSA input. The top trace is with no
external attenuators on the ports (J1 and J4). The center plot is with
a 3 dB pad on each port. The bottom trace is with 6 dB of
attenuation on each port.
The Tracking Generator power to J1 is -12.3 dBm.
Therefore the total gain of the two cascaded sections is +1.1 dB. Yes,
those internal attenuators are needed to help isolate the SAWs from
each other. I did try adding a 3 dB attenuator in front of
FL2 to smooth
things, but it made no difference.
SAW Input Impedance Test
According to the data sheet, the SAW is close
to 50
ohms but not exactly. I ran a Reflection measurement (S11) of J3
(SAW/Amp section)
to see what the impedance really is.
This is what I got:
As suspected, the port is not 50 ohms at 1013 MHz, it is closer
to 75 ohms.
Here is an S22 Reflection measurement of the SAW/Amp
Section (J2). Basically, it is an output impedance measurement of the
ERA-33 amplifier.
One would expect to see a flat 50 ohm response.
However, this takes place only if the ERA-33 amplifier's input
impedance remains constant. Here, the SAW filter is absorbing and
reflecting
the amplifier's reverse coupling (reverse isolation) energy, and that
changes the
source impedance of the amplifier's input. Here is the Smith chart for
the above data.
The following plot is a comparison of the SLIM-SFA-1013
to the Coaxial Cavity Filter. The Cavity Filter is tuned to
1014 MHz, which is the First IF of the Original MSA (from which the
filter was removed). I made no attempt to re-tune the Cavity just for
the purpose of this comparison. It has an insertion loss of -8 dB.
The SLIM-SFA-1013 has an insertion loss of
+1.2 dB.
The following plot is a narrower sweep, showing the
S21 phase of both filters. The SAW traces are yellow and the Cavity
traces are white. Since the sweep is only 5 MHz wide, the S21 Magnitude
of
the SAW is a straight line at the top of the graph.
The phase delay through the Coaxial Cavity Filter is
a total of 360 degrees, 90 degrees for each cavity. The phase delay for
the SAW Filters seem to be less, but this is deceiving, as this is plot
is for a (relatively) narrow frequency sweep.
The following plot shows the S21 for the SAW Filters
for its entire 3 dB bandwidth.
Here, you can see that the phase delay is 3 complete rotations (1080
degrees) within its 3 dB bandwidth. This is 360 degrees for each SAW
Device.
h. Spurious Responses
For some MSA builders, the SAW
Filter
approach is easier to implement than constructing the Coaxial
Cavity Filter. However, this approach does have its
limitations. Replacing the Coaxial Cavity Filter with the wider band
SAW Filters may
result in more spurious responses while
in the Spectrum Analyzer Mode. These spurious responses are somewhat
predictable, and easily differentiated from real input signals. But
still, they can be a nuisance, especially for novice spectrum analyzer
users. The following paragraphs will explain how the "predictable"
spurious responses are created. I should point out that spurious
responses are created in an MSA with either the Coaxial Cavity Filter
or the SAW Filters. I will address only those responses that will be
higher using the SAW Filters, as opposed to the Cavity Filter.
I
define a spurious response as "the Magnitude indication of an input
signal that does not exist". As an example, when the Spectrum Analyzer
has a measurable Magnitude response when it is commanded to 100 MHz,
yet there is no input signal at 100 MHz. The measurement is called a
"Spurious Response". The MSA's commanded frequency (100 MHz) is called
the "Spur Frequency", even though it does not exist.
The actual signal that creates the spurious response can enter the MSA
via its
Input, be self-generated internally by the MSA, or it can be a
combination of both. These signals are called "Spur Generators".
In almost all cases, a spurious response is created
by the
product of two or more frequencies within a mixer. In the MSA, there
are two mixers, Mixer 1 and Mixer 2. Ideally, there
would be
only two RF
frequencies that enter Mixer 2 and produce the proper IF of 10.7 MHz.
These two RF
frequencies are IF1 and PLO2. IF1 is the product of Mixer 1 and enters
Mixer 2 at its R port. PLO2 is the 2nd local oscillator and is fixed at
1024 MHz, entering Mixer 2 at its L port.
Ideally, the band pass (Cavity or SAW)
filter between Mixer 1 and Mixer 2 would allow only an IF1
frequency of 1013.3 MHz to enter Mixer 2. The product of Mixer 2 would
then be PLO2 -
IF1 = 10.7 MHz. However, nothing is "ideal" and we must deal with
"real" components.
Mixers are not "ideal"
and they will produce output frequencies with an action called harmonic
mixing.
"Mixer-Generated Inter modulation Distortion", "Intermods", and
"Single-Tone IMD" are some of the terms used for this mixing
action. I will generalize this form of mixing action as "IMD" or
Inter Modulation Distortion. IMD is not very efficient, but none
the less, it is how spurious responses are
created in the MSA. IMD occurs in both Mixer 1 and Mixer 2.
For either mixer, this is the mathematical formula:
±(M*LO) ±(N*RF)
= Mixer Output
: where M and N are harmonic multipliers (0 through infinity),
and LO and RF are the two frequencies to be mixed.
For Mixer 2, the formula can be re-written as:
±(M*PLO2) ±(N*IF1)
= Final IF
The frequencies of PLO2 and Final IF are fixed (1024 MHz and 10.7 MHz).
Therefore, the formula can be re-arranged to find what IF1 frequencies
will create
a spurious response:
IF1 = (±Final IF
±M*PLO2)/N, or
IF1 = (±10.7
±M*1024)/N
By substituting M and N with multipliers,we can
calculate all of the (single) IF1 frequencies at which Mixer 2
can create a spurious
response at 10.7 MHz. Usually, responses that are created by
harmonics greater than about 9 are insignificant. Therefore, I will
show only those IF1 frequencies using harmonics of 1 through 10. Also,
I will show only those IF1 frequencies that can pass through the SAW
Filters with less than 90 dB attenuation (990 MHz - 1035 MHz).
The following table
shows the IF1 frequencies that will potentially produce a spurious
response in Mixer 2. Harmonic MultiplesIF1 Frequency
using M*PLO2 - N*IF1 = IF2
for M=1 and N=1,
IF1 =
1013.3 MHz (The "normal" or Fundamental
IF1)
for M=2 and N=2,
IF1 =
1018.65 MHz
for M=3 and N=3, IF1 =
1020.43333 MHz
for M=4 and N=4, IF1 =
1021.325 MHz
for M=5 and N=5, IF1 =
1021.86 MHz
for M=6 and N=6, IF1 =
1022.21667 MHz
for M=7 and N=7, IF1 =
1022.47143 MHz
for M=8 and N=8, IF1 =
1022.6625 MHz
for M=9 and N=9, IF1 =
1022.81111 MHz
for M=10 and N=10, IF1 =
1022.93 MHz
using N*IF1 - M*PLO2 = IF2
for M=10 and N=10, IF1 =
1025.07 MHz
for M=9 and N=9,
IF1 =
1025.18889 MHz
for M=8 and N=8, IF1 =
1025.3375 MHz
for M=7 and N=7, IF1 =
1025.52857 MHz
for M=6 and N=6, IF1 =
1025.78333 MHz
for M=5 and N=5, IF1 =
1026.14 MHz
for M=4 and N=4, IF1 =
1026.675 MHz
for M=3 and N=3, IF1 =
1027.56667 MHz
for M=2 and N=2, IF1 =
1029.35 MHz
for M=1 and N=1, IF1 =
1034.7 MHz (The Image Frequency)
There are three mechanisms for producing these IF1
Frequencies: Case 1. Directly, by PLO1. The
MSA can be commanded to a frequency at
which PLO
1 will generate the IF1 frequency. The frequency of PLO1
will always be 1013.3 MHz above the command frequency.
Example: Command MSA to 5.35 MHz. (PLO1 = 5.35 + 1013.3 = 1018.65
MHz). The power level of PLO1 at the input to the IF1 Filter depends on
Mixer 1, L to R port
isolation, but will be in the range of
-20 dBm to -35 dBm. In my Original MSA, it is approximately
-38 dBm. Case 2. As a mixing
product of Mixer 1. When an input
frequency
combines with PLO 1 (in Mixer 1)
to create
one of the IF1 Frequencies. This is called Single-Tone
Inter Modulation Distortion. Case 3. Two or more input
frequencies to the MSA will combine in
Mixer 1
to produce the IF1
Frequencies. This is a form of Two-Tone Inter Modulation Distortion.
Case 1. Direct Generation
The following table shows the MSA Frequency
Commands that will directly create the IF1 frequencies. This test is
performed with the Original MSA,
using
the SAW Filters. The external input to the MSA is terminated into
50 Ω (no signal input). IF1 (PLO 1) is
approximately -38 dBm on the input of the SAW Filters. The
power level of IF1 at the input to Mixer 2 is dependent on the
frequency response of the SAW Filters. The Indicated
Magnitude response level is a relative measurement. That is, if a
"real" 5.35 MHz signal at -96 dBm were input to the MSA, the Magnitude
measurement would be "-96 dBm". Here, the MSA is commanded to measure a
5.35 MHz input signal, but is actually measuring the internal PLO1
frequency power and indicating a spurious response measurement of -96
dBm. IF1 FrequencyIndicated Magnitude Response Level
IF1 = 1013.3 MHz,
MSA Commanded to 0.0 MHz
-31 dBm
IF1 = 1018.65 MHz,
MSA Commanded to 5.35 MHz
-96 dBm
IF1 = 1020.43333 MHz, MSA Commanded to
7.13333333 MHz <-120 dBm
IF1 = 1021.325 MHz, MSA
Commanded to 8.025 MHz <-120
dBm
IF1 = 1021.86 MHz,
MSA Commanded to 8.56 MHz
<-120 dBm
IF1 = 1022.21667 MHz, MSA Commanded to
8.91666667 MHz -110 dBm
IF1 = 1022.47143 MHz, MSA Commanded to
9.17142857 MHz <-120 dBm
IF1 = 1022.6625 MHz, MSA Commanded
to
9.3625 MHz
<-120 dBm
IF1 = 1022.81111 MHz, MSA Commanded to
9.51111111 MHz <-120 dBm
IF1 = 1022.93 MHz,
MSA Commanded to 9.63 MHz
<-120 dBm
IF1 = 1025.07 MHz,
MSA Commanded to 11.77 MHz
<-120 dBm
IF1 = 1025.18889 MHz, MSA Commanded to
11.8888889 MHz <-120 dBm
IF1 = 1025.3375 MHz, MSA Commanded to
12.0375
MHz
<-120 dBm
IF1 = 1025.52857 MHz, MSA Commanded to
12.2285714 MHz <-120 dBm
IF1 = 1025.78333 MHz, MSA Commanded to
12.4833333 MHz <-120 dBm
IF1 = 1026.14 MHz,
MSA
Commanded
to 12.84 MHz
<-120 dBm
IF1 = 1026.675 MHz, MSA
Commanded
to
13.375 MHz
<-120 dBm
IF1 = 1027.56667 MHz, MSA Commanded to
14.2666667 MHz <-120 dBm
IF1 = 1029.35 MHz,
MSA
Commanded
to 16.05 MHz
<-120 dBm
IF1 = 1034.7 MHz,
MSA
Commanded to 21.4 MHz
<-120 dBm
A measurement of <-120 dBm
does not necessarily mean that there is no spurious
response. -120 dBm is the noise floor of this MSA. (The
responses shown will not be
accurate for every MSA using the SAW Filter topology. However, the
testing does indicate that the spurious responses will likely exist.)
This test was repeated with the MSA using the Coaxial Cavity
Filter. None of the IF1 frequencies produced a measurable response.
This is due to the much narrower bandwidth of the Cavity Filter.
Case 2. Single-Tone IMD
The following table shows the MSA's
Magnitude
response levels when an input signal is combined with PLO 1 to create
the IF1 Frequency. The
external input to the MSA is a fixed frequency at five different power
levels: 0
dBm, -10 dBm, -20 dBm, -30 dBm, and -40 dBm. The MSA is commanded to
the Input Frequency plus a Frequency Offset. The
IF1 Frequency is produced by the mixing
action of the input signal and PLO 1. Mixer 1 output power level is
approximately -7 dB below the input power level. The
power level of IF1 at the input to Mixer 2 is dependent on the
frequency response of the SAW Filters. For testing, I used an input
frequency of 30 MHz (at the 5 power levels). However, any input
frequency could be used as long as the same Frequency Offset were added
to the MSA Command Frequency. For example, in the first step, the MSA
is commanded to 30 MHz. In the second step, the MSA is commanded to
35.35 MHz, etc.
The other 13 IF1
Frequencies were tested, but none created a measurable Magnitude level.
The
responses shown will not be
accurate for every MSA using the SAW Filter topology. However, the
testing does indicate that the spurious responses will likely exist.
This test was repeated with the MSA using the Coaxial Cavity
Filter. None of the IF1 frequencies produced a measurable response.
This is due to the much narrower bandwidth of the Cavity Filter.
Case 3. Two-Tone IMD
Multiple input signals to the MSA will combine
with PLO1 in
Mixer 1 to create an IF1 that can produce a spurious response in Mixer
2. There are literally millions of different frequencies that can
produce this effect with both the SAW Filter and Cavity Filter. I will
concentrate on only one circumstance that
will affect the MSA with a SAW Filter much more than an MSA with a
Cavity
Filter. This is when the difference between two input frequencies, F1
and F2, is within the 90 dB bandwidth of the MSA's Final Resolution
Filter. Also, the product of each input frequency and the frequency of
PLO 1 must be within the 90 dB bandwidth of the First IF Filter. This
may seem like a rare event, but it is not. Input signals with wide band
modulation occur quite frequently. For example, two Digital TV channels
are shown in the following Spectral Scan.
The MSA is tuned to 113.25 MHz, the guard band between the two
TV channels. Marker 1 is indicating RF power at 107.91 MHz.
Marker 2 is indicating RF power at 118.614 MHz. The two signals meet
the first criteria that their difference is within the 90 dB bandwidth
of the Resolution Filter (118.614-107.91 = 10.704 MHz).
Since the MSA is tuned to 113.25 MHz, PLO 1 is at (1013.3 MHz +
113.25 =) 1125.55 MHz.
The product of F1 and PLO1 is 1125.55 - 107.91 = 1017.64 MHz.
The product of F2 and PLO1 is 1125.55 - 118.614 = 1006.936
MHz.
The 90 dB bandwidth of the SAW Filter is from 990
MHz to 1035 MHz. Therefore, both the F1 and F2 products of Mixer 1 meet
the second criteria. Since both signals will pass through the SAW
Filter, they will mix together in Mixer 2. (1017.64 MHz - 1006.936
MHz = 10.704 MHz). In the above scan, there is no indication that a
spurious
problem exists in this situation. Here is why:
The power level of each signal entering Mixer 2 is about -98 dBm, well
below a power level required to create an IMD Magnitude response. If
the input signal levels were high enough, we would see that the signal
level
at 113.25 MHz would be greater than the noise floor of the MSA (-120
dBm). We will perform that test in the next scan.
This form of Two-Tone IMD is not likely in an MSA
using the Coaxial Cavity Filter. This is because the second criteria is
difficult to meet with the narrower bandwidth of the Cavity Filter.
In the following scan, two input signals have
amplitude great enough to create severe Two-Tone IMD.
The odd behavior of the IMD is the combination of Two-Tone IMD and
other IF1 spurious combinations. We are dealing with four fundamental
signals, which create a large combination of distortion products.
The following scan is a continuation of the previous scan. In each
successive sweep, the power of both input signals are attenuated 10 dB.
The
power level of the IMD drops dB for dB as the input powers are
attenuated.
The bottom trace is when both input powers are -43.9 dB and -43.6 dB.
It can be safely assumed that if both input signals are less than
-50 dBm, the IMD will be too low to be measured. The following scan
shows those results.
The only way to decrease
this type of Two-Tone IMD is to change the Final IF from 10.7 MHz to a
frequency greater than about 20 MHz. A Final IF of 21.4 MHz is an
option, and will decrease this IMD. However, to totally eliminate this
potential IMD, the Final IF must be greater than about 28 MHz. 28 MHz
is
the -50 dBc attenuation points on the SAW Filter skirts.
Options:
1. Increasing the First IF to 1017.3 MHz will reduce the amplitude of
the spurious responses, but not totally eliminate them. This is a
simple change to the Hardware Configuration Manager to "tell" PLO 2 to
operate at 1028 MHz instead of 1024 MHz. Changing the First
IF to 1021.3 MHz is a better option, which will be detailed in the
next section.
2. Changing the Final IF to a frequency greater than 20 MHz would
have
a better affect of reducing all spurious responses. However, narrow
Resolution Filters at higher frequencies are difficult to build or
acquire. Also, a Final IF greater than 30 MHz will require an MSA
hardware change. Both Mixer 2 and the SLIM-IFA-33 have cut-off
frequencies of 30 MHz. Also, the MSA/VNA has not been tested with a
Final IF greater than 21.4 MHz.
3. Changing the MSA topology from Dual Conversion to Triple Conversion
would eliminate the spurious responses described in the previous
paragraphs. However, new spurious frequencies may be introduced,
depending on what frequency schemes are used. This would be a big
change to both MSA Hardware and software. The software would have to
include both topologies of an MSA (Dual and Triple Conversion). It
would be more prudent to establish a Hardware/Software scheme totally
separate from the present MSA.
Exploring Option 1
The IMD levels created in Cases 1 and 2
can be somewhat
reduced by
operating the
MSA's First IF at the higher frequency of 1017.3 MHz (or 1021.3 MHz)
rather than 1013.3
MHz. This is because the IF1 frequencies are higher and are
positioned on the upper slope of the SAW Filter. This is a
simple change to the Hardware Configuration Manager, to tell the
software that PLO2 is to operate at 1028 MHz (or 1030 or 1032 MHz)
instead of
1024 MHz.
For a study of Option 1, spurious response data was taken with the
First IF at four different frequencies. The sweep labeled "PLO2 at
1024" shows the
SAW Filter Band-Pass Response with the First IF at 1013.3 MHz, the
MSA's normal frequency configuration. Markers are placed on the
response trace to indicate how effective the IF1 spurious frequencies
are attenuated through the SAW Filter. The actual frequency at each
marker is the Center Frequency (1013.3 MHz) plus the marker frequency.
PLO2 at 1024
Marker 1 is 1013.3 + 5.35 = 1018.65
MHz. This is the frequency that creates the 2*M - 2*N spurious
response. As we can see, this MSA frequency configuration is not very
effective in reducing the spurious frequencies below Marker 3.
The MSA frequency configuration is
changed to take advantage of the upper skirt (slope) of the SAW Filter.
Here, the markers still indicate where the IF1 spurious
frequencies will reside.
PLO2 at 1028
The Marker 1 frequency at 1017.3 + 5.35 = 1022.65 is
the 2*M - 2*N spurious response. It is attenuated by
5.4 dB. However, all higher spurious response frequencies are much more
attenuated. Changing the MSA to this frequency configuration does not
result in fewer spur frequencies, but it does attenuate their final
spurious responses.
The PLO2 is operating at 1030 MHz, causing the First
IF to be at 1019.3 MHz. PLL 2 must use a Phase Detector Frequency of 2
MHz instead of the normal 4 MHz.
PLO2 at 1030
The Marker 1 frequency at 1019.3 + 5.35 =
1024.65 is
the 2*M
- 2*N spurious response. Marker 5 indicates where the Image Frequency
will fall.
Finally, the MSA frequency
configuration is changed to take
maximum advantage of the upper skirt (slope) of the SAW Filter. Here,
the
markers still indicate where the IF1 spurious frequencies
will reside.
PLO2 at 1032
The Marker 1 frequency at 1021.3 + 5.35 = 1026.65 is
the 2*M
- 2*N spurious response. It is now greatly attenuated. And, all
spurious response frequencies higher than that are even more attenuated.
The following table shows the Case 2 spurious
responses with the four different MSA frequency configurations. The
responses shown are not absolute, but can be used as a comparison to
each other. It should be noted that the input signal creating the IMD
response has a power level of 0 dBm. This is a much higher signal power
level that would normally be input to the MSA. Therefore, this data is
a worse case situation. As a rule of thumb, when the input is lowered
by 10 dB, the spurious response will decrease about 20 dB.
Numeric Spur Freq Offset
1013.3/1024
1017.3/1028 1019.3/1030 1021.3/1032
1M-1N
0.0 MHz
0 (ref)
0
(ref) 0
(ref) 0
(ref)
2M-2N
5.35 MHz
-43 dBc
-51 dBc
-75 dBc
-101 dBc
3M-3N
7.13333333 MHz
-53 dBc
-93 dBc
4M-4N
8.025
MHz
-75 dBc
5M-5N
8.56 MHz
-93
dBc
6M-6N
8.91666667
MHz -102 dBc
7M-7N and greater spurious responses are below the measurable noise
floor of the MSA.
A response that has its measurement blank is below the
measurable noise floor of the MSA.
The following is a comparison of the Case 1 spurious
response with the different PLO2 frequencies:
Numeric Spur
1013.3/1024
1017.3/1028 1019.3/1030 1021.3/1032
2M-2N
-96 dBm
-107 dBm
-118 dBm <-120 dBm
Others are too low to be measurable.
It would seem that the best option is to change the
MSA's frequency configuration to use 1021.3 MHz as its First IF
frequency. However, there is a concern. 1021.3 MHz is on the upper
skirt of the SAW Filters. This can be seen as Marker 1 on the following
"Temperature Shift" graph. SAW Filter
characteristics will change with temperature.
It is likely that the SAW frequency response will shift enough over
temperature to cause the absolute Magnitude measurements to change. The
two traces in the following graph are the Magnitude responses of the
SAW Filters.
The white is taken during a cold start. The yellow is after a warm-up
of 10 Minutes. Temperature Shift
The markers shown on the graph are for cold. They
are not displayed for warm. However, they are listed below:
Marker Cold
Warm
Change vs Temperature
L -30.21 dBm
-30.18 dBm .03 dB
1 -34.67
dBm -35.26 dBm
.59 dB
The change at the center frequency of the filter (Marker L, 1013 MHz)
is minor, but the change at the operating point (Marker 1 at 1021.3
MHz) is significant. The main contributor is the SAW Filter (FL1) in
the SAW/Amp section, which is getting extremely warm from the ERA-33
circuitry. This heating effect could be minimized with one or both of
the following modifications:
1. The ERA-33 is normally running at 38 ma of current. As
designed, this section consumes about .4 watts.
This allows full compliance to the Minicircuits specifications of gain
and compression. However, we don't need +13 dBm at compression. We
could get by with a much lower compression point, about +2 dBm. By
lowering the current to 15 ma (from its nominal 38 ma) the compression
point would be about +6 dBm. The total power consumption would be
reduced to .15 watts. The gain will be reduced a little, but we don't
need much gain with this SAW Filter replacement for the Cavity Filter.
This is easily accomplished by changing the values of R1 and R2 to 200
ohms. They can be 1/10 watt, 0805 size.
2. The power consumption of the ERA-33 at 15 ma is about .06 watts. The
rest of the power (.09 watts) is consumed by the bias resistors, R1 and
R2. These could be positioned outside of the module where their heating
would not affect the SAW filter. At 15 ma, R1+R2 should be about 400
ohms. A 390 ohm axial lead resistor could be placed in the 10v feed
line, external to the module. The internal R1 and R2 would be replaced
with 4.7 ohm resistors.
3. The ERA-33 could be replaced with a lower compression, lower current
MMIC device. The MAR-6 would do this. (R1 and R2 would need
re-calculating.)
I performed an experiment where I lowered the input
voltage to the
SAW/Amp section from 10 volts to 6.8 volts. This lowered the MMIC
current from 38 ma to 17.5 ma, a total power dissipation of 118 within
the module. R1 and R2 were left in place, dissipating 23 mw each. The
MMIC dissipates 72 mw. This decrease in power consumption (from 400 mw
to 118 mw) made a significant difference in the temperature rise of the
module. The total gain only decreased 1.7 dB. The gain vs temperature
change at 1021.3 MHz (Marker 1 in the previous sweep) decreased only .2
dB (was .59 dB).
i.
Conclusions
When substituting the SLIM-SFA-1013 in place of the
Coaxial Cavity
Filter, the best MSA results will be obtained if these suggestions are
followed:
1. Use 1021.3 MHz instead of 1013.3 MHz as the 1st IF to
minimize CASE I spurious responses (< –101 dBc)
2. Use 1021.3 MHz instead of 1013.3 MHz as the 1st IF to minimize
CASE II spurious responses (< –120 dBc)
3. To avoid Two Tone IMD problems (CASE III), reduce MSA
input levels for strong signals using external attenuation.
4. Lower the MMIC current in the SAW/Amp section to reduce temperature
effects.
5. Allow the MSA a minimum of 10 minute "warm up"
stabilization before any calibrations or important data measurements.
