Function - Basic Spectrum Analyzer
(with optional Signal Generator)
 
Created Dec 16, 2009

    The Function of Basic Spectrum Analyzer is available for all construction Levels of the MSA.  All MSA Build Levels refer to this page when operating as a Basic Spectrum Analyzer.  For MSA's other than the Basic MSA, the Signal Generator is active and can be commanded to any frequency between 0 Hz and 3000 MHz.

Function - Basic Spectrum Analyzer
Band Configurations for MSA Hardware
Walk Through the MSA Software - Spectrum Analyzer Mode, 12/12/09.  Using the SA software without needing the MSA Hardware. By Sam Wetterlin.  See Initial Set-Up and Calibration of the MSA for software installation.
Spectrum Analyzer Operating Guide for the MSA, 12/12/09. Bench-top guide to SA operation. By Sam Wetterlin
Signal Generator Operation

Spurious
Input levels



Function - Basic Spectrum Analyzer
    The MSA, or any commercial Spectrum Analyzer, will measure the absolute power and frequency of an input signal.  But, it is also a tool to analyze the type of RF signal.  Almost all signals are associated with some form of modulation.  The MSA doesn't have the "smarts" to tell you what modulation is present.  But, with a keen eye and some experience using the MSA, you will be able to define a signal's characteristics.
    The measurement capability of the MSA is extremely accurate but only if the input signal is within the MSA's calibrated range.  The MSA is calibrated for frequencies within its Band of Operation and for power levels from about 0 dBm to -120 dBm.  It is important to remember that the MSA calibration was performed with an input signal with a 50 ohm source impedance.
    If an input signal to the MSA has a source impedance which is not 50 ohms, the absolute power measurement, in dBm, will be in error.  For such a case, the MSA power measurement will become a relative measurement, not an absolute measurement.  The frequency measurement will not be in error.  I note this reminder because the SA is quite often used with input signals that are not 50 ohms.
    Relative measurements can be used to obtain an absolute result.  For example, an oscilloscope probe may be connected to the MSA input, and used to test circuits.  Scope probes have varied characteristic impedances, but are usually consistant over their specified frequency range.  The absolute gain of an amplifier circuit can be quantified by placing the scope probe on its input, and noting the SA "relative" power level as a reference level, i.e., -37 dBm.  The scope probe is then moved to the output of the amplifier circuit and the SA "relative" level is measured to be -22 dBm.  Then, the gain of the amplifier would be the difference of the two "relative" power measurments (-22dBm - -37dBm = +15 dB).  Here, the result of +15 dB is an absolute product.  If the scope probe (and MSA) has a much higher impedance than the amplifier circuit, the circuit will not be affected.  The quantity of 15 dB will be accurate.
    Many types of input probes can be utilized with the MSA to give relative power measurements.  One of my favorites is the "sniffer".  This is a length of coaxial cable with an sma connector on one end, which is connected to the MSA input.  The other end of the coax has a small (1 inch) loop of wire connected from the center conductor to the shield.  The loop is brought into the proximity of a circuit and acts like a small recieving antenna.  I promise you, ALL CIRCUITS RADIATE.  Their components are tiny transmitter antennas.  A "sniffer" measurement is certainly not accurate, but it does tell you if a circuit is "active".  It is especially useful when trying to find circuits or modules that "leak" or "radiate".

Band Configurations for MSA Hardware
    The operating range of the Spectrum Analyzer is split into three frequency bands: 1G, 2G, and 3G Bands; (0-1000 MHz, 1000-2000 MHz, and 2000-3000 MHz).  Default is 1G Band.  The 1G or 3G Band use the same hardware paths within the MSA.  To use the 2G Band, the hardware paths are re-configured:
1G and 3G Configuration:
    Mixer 1 Output (J3) to Cavity Filter Input
    Mixer 2 Output (J2) to I.F. Amplifier Input (J3)
    PLO 3 output (J2) to Mixer 3 Input (J1)
    Signal Generator output is Mixer 3 Output (J2)
2G Configuration:
    Mixer 1 Output (J3) through Low Pass Filter to I.F. Amplifier Input (J3)
    Signal Generator output is PLO 3 output (J2)

Signal Generator Operation
msascreens/swpparamsa.gif
    "Sig Gen Freq" and Box. This control is not shown for the Basic MSA since it is available only Build Levels 2 or 3 (MSA/TG or VNA), and only when in Spectrum Analyzer Mode. This controls the CW Signal Generator Frequency that is available at the Tracking Generator output connector.  Any frequency between 0 MHz and 3000 MHz can be commanded. The change will take effect when the "Restart" is clicked.  Higher output power for frequencies between 1000 MHz and 2000 MHz can be obtained (+10 dBm) if the Path is re-configured, using the PLO 3 output (J2) as the Signal Generator output.

    Note: The Signal Generator in the MSA/TG or VNA cannot be disabled (turned off).  There will always be a frequency exiting the Tracking Generator port.  The output is not confined to a fundamental frequency.  There will be other product frequencies and harmonics generated within the MSA/TG.  Sometimes this can affect normal MSA operation.  When not used, it is advisable to "park" the Signal Generator to a frequency that will not affect spectrum analyzer operations.  I suggest "parking" the Signal Generator to a frequency above the selected sweep of the Spectrum Analyzer.  It is possible to attribute strange spurious to the Signal Generator.  If this occurs, move the frequency of the Sig Gen and see if the spurious move or disappear.

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