It has been far too long since my last update and so I decided to do a cursory analysis of an FM radio transmitter (the kind you would use to listen to your MP3 player in a vehicle which lacks an input port).
FM Transmitter Model: JH-CMWT104
The transmitter under test is one which I purchased many years ago and bears the model number: JH-CMWT104. It is a 12v device and so I am using my converted ATX PSU to power the unit with a clean (verified by oscilloscope) +12v. The test bench included a Rigol DSA815-TG spectrum analyzer and a DG1022 arbitrary function generator.
Test-bench establishing local background radio noise for signal comparison
The first step was to establish local RF background measurements / spectrogram for later comparison. As an antenna, the test bench used a 1m collapsible whip antenna connected via BNC-to-N adapter with a 90° elbow.
A quick look at the resulting spectrogram (50Mhz to 550MHz span, RBW/VBW 3kHz) shows the FM broadcast band indicated in the lower portion of the display.
FM transmitter 100MHz unmodulated showing multiple harmonics
With the background parameters established, the FM transmitter was powered on with no signal at the device’s input. The transmitter was set to 100MHz for ease of demonstration.
Shown in the spectrogram, it is clear that there are multiple harmonics of the 100MHz fundamental. As the harmonics will be n-integer values, they will land nicely on the graticule lines of the display.
FM transmitter 100MHz unmodulated showing multiple harmonics w/ peak table
Using peak markers, the offending harmonics are shown in the spectrogram, labeled as 1 through 4.
No harmonics were visible (amplitude > 1dB above background level) beyond 500MHz and so the span was intentionally set from 50MHz to 550MHz to optimize signal display.
FM transmitter 100MHz unmodulated showing 2nd harmonic at 200MHz -27dBc
The 3rd harmonic (300MHz, -60dB relative to fundamental) was observed and has approximately 35dB of suppression from the fundamental which is pretty good.
What is intriguing is the 2nd harmonic at 200MHz which is only 27dBc (dBc = relative to the carrier) weaker than the fundamental. Recall that 26dB expresses a difference ratio of 400 times.
26dB can be broken down as: 10dB + 10dB + 3dB + 3dBrecall that for each 3dB you have a doubling (2x) and for 10dB it is an increase in magnitude (10x) 26dB can be therefore be thought of as: (10 * 10 * 2 * 2) = 400x
Transmitter output of 100MHz FM “carrier”
A spectrogram of the FM carrier signal at 100MHz shows a series of sideband pairs (… Fc – Fa, Fc, Fc + Fa …)
The same carrier is seen at all harmonics and has precisely the same characteristics as the fundamental.
This means that if a radio were tuned to 200MHz and the FM transmitter was set to 100MHz (as in the test scenario), the signal could be interpreted.
Transmitter output 100MHz with 2.7kHz FM modulation
When presented with a 2.7kHz tone, the FM signal character changes as seen as displayed in the spectrogram.
Using a function generator, it is possible to make a number of measurements of the transmitter’s performance.
I did not bother to measure the deviation of the signal at this point, though I may do so in the near future for interest’s sake. Agilent has a document with some details on how to measure deviation using a spectrum analyzer (p17).
