Updateded
7-03-10. Added section on Thermometer used for
testing. Updateded
7-22-10. Add more temperature data. Updateded
8-09-10. Add "Comment on Compensation" at end of page.
The MSA can be used in ambient
temperatures
that vary from below
freezing (<+32 F.) to desert conditions (>+100
F.). Ambient temperatures in normal home usage would typically
range from 65 F. to 85 F. The MSA's frequency accuracy and both
Magnitude and
Phase Measurements are affected by temperature changes to components
within the MSA. If the
temperature of each sensitive component could be regulated to a fixed
temperature, the
accuracy of the MSA would rival the most expensive commercial
Analyzers. This is not practical for the MSA. The MSA's
components that
are the most sensitive to temperature variations are: Master Oscillator: It has
a positive frequency vs. temperature coefficient of approximately
.15 ppm (parts per million) per degree F. This means if the temperature
of the Master Oscillator changes 1 degree F, its frequency will change
.000015 percent. For a 64 MHz oscillator, this is 9.6
Hz. All frequencies generated within the MSA are dependent on the
stability of the Master Oscillator, since it is the only
frequency reference in the MSA. Final Xtal Filter (Resolution
Filter): Temperature variations of this filter will change
Insertion Loss (Magnitude variation) and Center Frequency
(Phase variation). Crystal filters (narrow BW) are usually much more
susceptable to temperature variations than lumped element filters (wide
BW).
I characterized the Final Crystal Filter
(SLIM-MCF-FL096) used as the primary path (Path 1) in the Verification
MSA. The Magnitude change vs. Temperature change is
negligible. It is less than .01 dB / 1
deg F. The resonant center frequency will decrease at
a rate of -4.51 Hz / 1 deg
F, which is also a phase vs. temperature coefficient of -.777 deg / 1
deg F (a negative temperature coefficient). Updateded
7-22-10
I tested a dual pole, 10.7 MHz ceramic filter (Murata), with a 320 KHz
BW. It has a negative temperature coefficient: -500 Hz / 1 deg
F. The phase change is -.57 deg / 1 deg F. Coaxial Cavity Filter:
Temperature variations of this filter will change Insertion Loss
(Magnitude variation) and Center Frequency
(Phase variation). The characteristics of this filter depend
solely on
its physical dimensions. Since copper and brass expand (and contract)
over temperature, it is natural for this filter to be affected by
temperature. The diameter of the inner center stubs and the
diameter of the outer cavity will increase with temperature (although I
don't know how much). These dimensions will not alter the resonant
frequency. The length (height) of the center stubs will increase with
temperature, causing the resonant frequency to decrease. The length
(height) of the outer cavity will increase with temperature, but should
have no affect on resonant frequency. However, since its height
increases with temperature, the tuning screws will move further away
from the top of the stubs. This will cause the resonant frequency to
increase. The combination of these factors create a temperature
coefficient that is unpredictable.
The coaxial cavity filter in the
Verification MSA has a positive Phase vs. Temperature coefficient of
1.0 deg/1 deg F (5.56 KHz / 1 deg F). The coaxial cavity
filter in the Original MSA has a negative
Phase vs. Temperature coefficient of -.20 deg/1 deg F (-1.11 KHz / 1
deg
F). This proves "unpredictability".
Temperature Characterization of the MSA
Even though the individual componentss of the MSA
can
characterized for temperature variations, the integrated MSA can only
be
characterized as a single unit. This is because the temperatures
of the individual SLIMs and components will vary widely. "Hot spots"
are common in integrated equipment. When characterizing a
temperature coefficient for an integrated system, the question is then
"what temperature is used as a reference". I suggest that the
exhaust air of the MSA be used as the Reference
Temperature. This is equivalent to the average temperature of the
internal components and SLIMs.
If the MSA is not well ventillated, the
internal air temperature
will stagnate, allowing the temperature of the individual SLIMs to
equalize to each other. However, internal temperature stabilization
will take a very long time and the difference between outside ambient
temperature and internal stabilization temperature will be quite large.
If the MSA is well ventillated, the internal
temperature will stabilize in a shorter period of time and be only a
few
degrees higher than the ambient temperature. The
Verification MSA has a small muffin fan, and the inside air temperature
(exhaust temperature) is about 6 degrees (F) higher than the outside
ambient temperature.
I characterized the Verification MSA by testing
Stabilization Time, Frequency variation, and Magnitude/Phase variation
(VNA) vs.
temperature.
Stabilization vs.
temperature (and time)
The purpose of this test is to determine how long it takes for the
MSA to "warm up" before measurements will be valid. It also
characterizes the Verification MSA for changes in Magnitude
and Phase vs. Temperature (both Ambient and internal) from turn-on to
stabilization. The following sweep took 66.7 minutes.
The
Tracking Generator output is connected to the MSA input
through two 10 dB attenuators with 6 inches of RG-188 coaxial
cable.
The MSA is commanded to 12 MHz (in the VNA-Transmission Mode), but any
frequency or power level could be used. "Ambient" is the air
temperature entering the MSA (room air). "Exhaust" is the
air temperature exiting the MSA.
It takes about 40 minutes from turn-on to stabilization (from
first data point to Marker 5)
MarkerTimeAmbientExhaustNotes See
Graph for actual Magnitude and Phase.
L 0
min 81.5 F 81.5 F
R 9
81.6
86.9
1 11
81.6
87.2
2 18
81.8
87.6
3 25
82.0
87.9
4 28
82.2
88.1
5 40
82.2
87.9 Stabilization
6 51
82.2
87.9
The ambient temperature is rising because I turned off the home air
conditioner to prevent sudden temperature changes in the ambient
surroundings. After stabilization, the Ambient-Exhaust
temperature differential is 5.7 degrees F.
The Magnitude shows very little change from start to stabilization,
Marker 2, 18 minutes. Phase has stabilized by Marker 5, 40 minutes. If
the user is satisfied with a Phase error of 1 degree (Marker
2, 18 minutes) he could begin taking data after 18 minutes of warm up.
The following is an example of the Verification MSA with
poor or no ventillation. It is a continuation of the previous
sweep, except I partially blocked the entrance and exhaust holes from
time 0 to Marker 1. If allowed to continue, I believe the Phase
would stabilize at about 22.5 degrees in another 30 minutes. This was
enough test time to tell me that poor ventillation is unacceptable.
From Marker 1, the holes were totally blocked, to simulate an MSA with
no ventillation. In both sections of the
sweep, the internal muffin fan is circulating the internal air. The
ambient temperature was rising since the home air conditioner was off,
but then it started raining outside and the ambient temperature began
to decrease. TimeAmbientExhaustPhaseMarkerNotes
0 min 83.1F 88.7
F 17.68
3 83.8
89.0 17.27
8 84.3
93.0 17.91
L Magnitude
is stable
10 84.5
90.6 18.50
15 84.9
91.2 19.84
22 85.2
91.2 20.87
27 85.2
91.2 21.34
29 85.1
92.1 21.60
32 85.2
92.8 21.76
1 Cover vents,
the "exhaust" temp is internal stale air temp
35 84.9
93.5 22.65
Started raining outside, ambient temp is
decreasing
40 84.3
94.4
23.90 R
44 84.2
94.8 24.76
53 83.8
95.1 26.19
62 83.8
95.3 27.09
64 83.8
95.7 27.28
67 83.8
95.7 27.46
120 84.0
96.9 28.94
NA Final
stabilization, not shown on this graph. See next graph.
Notice that the ending Ambient-Internal
temperature differential is 12.9 degrees F. This is over twice the
differential of normal operation. Also notice, the Magnitude has
changed
only +.05 dB. This is excellent Magnitude stability for such a
large internal temperature change.
The following will test to see how long it takes for the MSA to
stabilize from a
very high internal temperature. After the previous test, I removed the
blockages from the vents and allowed the MSA to stabilize with normal
air flow (muffin fan). TimeAmbientExhaustPhaseMarkerNotes
0 min 84.0 F 96.9 F
28.94
This is the final stabilization from the
previous test.
1 84.0
96.9
28.91 L
Removed the blockages from the vents
17 84.0
90.1
21.02 R
27
19.73
1 Phase is well within 1
degree of final stabilization
40
19.43
2 Stabilization
67 84.2
89.7
19.42 3
Notice that the final data point (Marker 3) temperatures are
about 2 degrees higher than the first graph. This is due to the room
temperature changing over a period of 2 hours. However, the
Ambient-Exhaust temperature differential has not changed much (5.5
degrees F).
Magnitude/Phase variation
(VNA) vs. Temperature
We can use the previously taken data to
characterize the Magnitude and Phase variations vs.
Temperature. Here is data taken two hours apart, with
a small temperature change:
Exhaust
Phase Magnitude Notes
87.9 F 16.63 deg
-30.59 dBm 89.7 F
19.42 deg -30.60 dBm
Data after 2 hours
+1.8
+2.79 -
.01 dB Delta changes
Exhaust temperature change = 1.8 deg F; average phase change = 2.79
degrees
Phase vs. Temp Coefficient = 2.79/1.8 = 1.55 deg / 1 deg F
This would suggest that the Verification VNA phase could be expected to
drift +1.55 degrees for every +1 degree F temperature change.
This would seem to be a significant drift, but any phase or magnitude
drift can be "calibrated out" when the VNA is "line calibrated" before
a
critical data measurement. Measurements usually take less than a
minute, and significant temperature changes inside the MSA are unlikely
to occur in
such a short period of time (unless, of course, your wife turns up the
furnace 10 degrees and the outlet is blowing directly on your MSA).
The Magnitude vs. Temperature coefficient is negligible. It is less
than .01 dB / 1
deg F.
An interesting set of data was taken with the Final Xtal
Filter (Resolution Filter) removed and replaced with a short
coax. Using the MSA's exhaust air temperature as a reference, the
phase is measured at two different temperatures:
At 89.9 deg F, the phase measured 163.91 degrees;
at 92.9 deg F, the phase measured 167.78 degrees
This is
Phase vs. Temp Coefficient = 3.87 deg/3.0 deg F = 1.29 deg / 1 deg F
Since the Final Xtal Filter is removed and replaced with a short piece
of coax cable, I expected the coaxial filter phase change would have
been the only contributor to the MSA phase change (any phase change
caused by a frequency change is insignificant to total phase).
Subtracting the 1.0 deg / 1 deg F of the cavity filter from
the total MSA of 1.29 deg / 1 deg F would be 0.29
deg / 1 deg F. This has to be the contribution of components
other than the coaxial cavity filter and the final crystal filter. It
is probably the phase contributions of the Log Detector, and the I.F.
Amplifier. Updateded
7-22-10 I performed a test on the Original
MSA, since removing the Coaxial Cavity Filter in the Verification MSA
is inconvenient. I tested the Phase change vs. Temperature of the
Original MSA with both the Cavity Filter and Final Crystal Filter
removed (and replaced with short coaxes). It showed a positive
Phase vs. Temp Coefficient: .2 deg / 1 deg F. This is quite close
to the calculated drift of the Verification MSA (.29 deg / 1
deg F), with its cavity filter coefficient removed.
Frequency Variation vs. Temperature
The most obvious way to characterize the MSA's
Frequency/Temperature coefficient is to measure the frequency of the
Master Oscillator at two different temperatures and calculate it.
However, with the MSA completed and covers in place it is not possible
to access these frequency points inside the MSA. Since the spare
output of DDS 1 is accessable from the
outside of the MSA, I measured it with a frequency counter that is
accurate to .1 Hz.
It is extremely stable with ambient temperature changes. The
Verification MSA has been previously calibrated to WWV. The Master
Oscillator was exactly 63.999560 MHz at an ambient temperature I don't
remember. In this test, I am not interested in the exact frequency of
the Master Oscillator, I am only interested in how much it changes over
temperature.
Using the Special Tests Menu, I commanded DDS 1 for a frequency of 16.0
MHz, with DDS Clock at 63.999560. The following
are the data:
At 88.7 deg F the DDS 1 output measured 16,000,004.1 Hz. This is .256
parts per million high.
At 83.0 deg F the DDS 1 output measured 15,999,986.4 Hz.
This is .850 parts per million high.
The Frequency vs. Temperature coefficient is = 1.106 ppm/5.7 deg F = .194 ppm/1 deg F.
This states that the integrated Master Oscillator changes .194 ppm for
every degree F of exhaust temperature (average inside
temperature). This seems to conflict with data we already have
for the Master Oscillator Slim (.15 ppm/1 deg F). Actually, it just
means that the Master Oscillator changes temperature more than the
average temperature change of the MSA.
Even though there is no temperature probe
directly on the Master Oscillator, we can make a calculation as to what
the temperature changed at the Master Oscillator SLIM.
As an individual module, the Master Oscillator has a Freq/Temp
coefficient of .15 ppm/1 deg F.
The test showed that the MSA temperature variation of 1 degree F
results in a Master Oscillator deviation of .194 ppm. Comparing
.194 ppm/.15 ppm = 1.293. Therefore the Master Oscillator temperature
changed 1.293 times the average change of the MSA temperature (exhaust
temp).
The Master Oscillator temperature changed: 1.293 x 5.7 deg F =
7.37 deg F
Thermometer used for
temperature testing
I don't have any thermocouples or thermal probes so
I fashoned a system to make these Temperature Tests. I thought it was
worth publishing, as it was a very easy and can be copied by anyone.
I modified a commercial Indoor/Outdoor Digital
Wireless Thermometer (La Crosse). Unknown price since it was a
Christmas gift, but it can't be too expensive. Check Wal-Mart for
similar items. This one will also read in degrees C. It seems to be
accurate and reads to the tenth of a degree.
The (In)side "Master" unit is simple to disassemble
(4 small screws on back). It has its thermister mounted on the side of
its main pwb. I just clipped the leads and added two twisted wires to
exit the side of the unit's vent holes. The wire is 6 inches, but can
be any length. I installed some heat shrink tubing at the thermister to
prevent stress on the leads. This is what I used to measure Exhaust
Temperature.
The (Out)side "Remote" unit is a transmitter (915
MHz) that squirts out its data every 10 seconds. It has very slow
thermal response, so I cut the plastic area covering its thermister
(the little do-dad below the marked surface mount resistor).
This improved thermal response time but since the
themister is mounted directly on a pwb, the whole pwb must stabilize in
temperature before the reading is accurate (about 5 minutes). At a
future date, I will remove the surface mounted thermister
and use extension wires similar to the Master unit modification. I used
this "Remote" unit for the Ambient Temperature measurements.
Comments on Compensation. Added 8-09-10.
Let me add a few cents on the subject of temperature compensation
for Frequency and one cent for Phase.
There are three components in the MSA/VNA that are susceptable to
temperature changes:
Master Oscillator - Frequency Change vs. Temp
Coaxial Cavity Filter - Resonant Frequency Change vs. Temp (phase)
Final Resolution Filter - Resonant Frequency Change vs. Temp (phase)
All three have such a minor change in Magnitude vs. Temp, it is not
even worth discussing (unless the Resolution Filter is extremely
narrow).
The Master Oscillator
My original MSA concept did not have the VNA capability in mind.
For Spectrum Analyzer and Tracking Generator operation, a minor amount
of the Master Oscillator's Frequency drift is noticeable only if the
Final Resolution Filter is extremely narrow (less than a few hundred
Hz.). Of course, if the Tracking Generator is used as a Frequency
Source, its frequency is dependent on the accuracy of the Master
Oscillator. With a Master Oscillator stability of .15 ppm/1 deg F (9.6
Hz/1 deg F, as measured in the Verification MSA), this equates to a TG
error of about 150 Hz / deg F.
The Master Oscillator frequency of 64 MHz was chosen because this
is a "standard" that is available from multiple sources. The present
oscillator (Cash chose this one for price, availability, and stability)
is a good one. With a little redesign of the SLIM-MO-64, it could be
temperature compensated to much better than its .15 ppm/1 deg F.
For those who wish to use a precision frequency source as a
replacement to this oscillator, these are the requirements:
A. Minimal requirement - 6 MHz to 100 MHz in 2 MHz increments
(6,8,32,34,etc)
B. Nominal requirement - 12 MHz to 100 MHz in 4 MHz increments
1. For oscillator frequencies less than 22 MHz, the DDS's X4 multiplier
can be utilized. The maximum division ratio in the DDS must be less
than 2, due to Nyquist. Since the output of the DDS is appx. 10.7 MHz,
its minimum clock must be greater than 21.4 MHz. Utilizing its X4
Multiplier, the minimum clock input must be greater than 21.4/4 or 5.35
MHz.
2. PLO 2 uses the LMX 2326 pll and its minimum clock divider ratio is
3. Its phase/frequency detector should operate at a minimum of 2 MHz (4
MHz is optimal). Therefore, its minimum clock input (Master Oscillator)
must be 6 MHz (12 MHz is preferred to obtain the 4 MHz PDF for PLO 2).
3. The maximum frequency for the DDS is 128 MHz, but the maximum for
the LMX 2326 is 100 MHz. Therefore, the Master Oscillator can not be
greater than 100 MHz unless it is divided down.
C. Optional Requirement. Requirements A and B assume the MSA's 2nd LO
is 1024 MHz. However, this is not absolutely necessary. Any frequency
between 1010 MHz and 1030 MHz could be used. It is possible to use
almost any oscillator frequency between 6 and 100 MHz, but changes to
the MSA software would need to be performed.
The Filters
For VNA operation, the Cavity Filter and Final Resolution Filters
become the dominant phase drift factors for the MSA. If I had designed
an MSA for utilization as a VNA only, I would not have used a Cavity
Filter or "Resolution Filter". They are not necessary for VNA
operation. In fact, the final filter would not be called a "Resolution
Filter". It would just be called a noise limiting filter. A very simple
single pole filter could replace the Cavity Filter, so could a
resistive attenuator. Either would exhibit little or no phase drift
over temperature. Same goes for a wide band final noise filter.
To make a long story short, if you are extremely critical to Spectrum
Analyzer frequency measurements, use a precision Master Oscillator. If
you are critical for phase measurements in VNA mode, use a precision MO
and modify the filter requirements for the First IF and Final IF.
Scotty
