3rd harmonics of fm stations in the 327 receiver

13jan06 Links to plots/images:
strength and azimuth dependence of the harmonics (.ps) (.pdf)
dynamic spectra of first azimuth spin(.gif)

Summary (top)

Third harmonics of fm radio stations are showing up in the 327 Mhz receiver.

They are not being created in the receiver since they remain when rf filters are placed in front of the first amp.

Some show a constant azimuth dependence, some are more variable in time.

Some of the harmonics are weaker because they fell at the edge of the 25 Mhz filter that we were using.

The fm stations:

The whip antenna and the spectrum analyzer was used to measure the strength of the fm stations at the observatory (the whip is not on the platform so there may be a difference). The spectrum analyzer was set to a span of 10 Mhz (about 300Khz rbw).


freq	strength on spec Analyzer (dbm)	3rd Harmonic
107.3	-33	321.9
102.9	-39.3	308.7
104.1	-42.8	312.3
100.3	-57	300.9 *
106.5	-65	319.5
101.9	-68	305.7 *
100.7	-68	302.1 *
105.7	-79	317.1

* these frequencies are outside the 327 band we measured.

The data:

The azimuth swing data from 28oct05 was used to check out the strength of the harmonics (see 327 az swings). There were 5 complete azimuth spins between -90 and +270 deg azimuth. The dome was at 19 deg. A 25 Mhz band was centered at 320 Mhz with 2048 channels. The azimuth moved at .3 deg/sec.

The plots show the strength and azimuth dependence of the harmonics (.ps) (.pdf):

Fig 1 has the average spectra for each az spin. Each azimuth spin is plotted in a different color. The top plot is the mean, the center plot is the median, and the bottom plot is a peak hold for each spin. The dashed lines are the 3rd harmonics of the fm stations. The vertical units are Tsys. The 308.7 birdie is small because it is on the edge of the 25 Mhz filter.
Fig 2 plots harmonic strength versus azimuth for all 4 signals. 20 channels about each fm harmonic were averaged and then plotted versus azimuth angle. Each color is a different azimuth spin. Each plot is a different fm station harmonic. The azimuth repeatability is not perfect (probably because of the time variation of the signal).
Fig 3: 308.7 (fm 102.9) strength vs az in 90 degree sections. This blows up the azimuth scale a bit.
Fig 4: 312.3 (fm 104.1) strength vs az in 90 degree sections.
Fig 5: 319.5 (fm 106.5) strength vs az in 90 degree sections.
Fig 6: 321.9 (fm 107.3) strength vs az in 90 degree sections.

The strongest harmonic is 312.3 from fm 104.1 followed by 321.9 (107.3), 308.7 (102.9), and then 319.5 (106.5). The dynamic spectra of the first az spin also shows that 312.3 is the strongest/most spread out signal

Some observations:

The 321.9 and 319.5 harmonics were still present when the hipass, bandpass filter were placed in front of the first amp (the other two were outside the filter band). So the birdies are coming from outside the receiver (most likely from the fm station).
We listened to the 321.9 Mhz and 312.3 Mhz birdie in the 2nd IF with a radio receiver. We could hear the fm station on the radio at the 3rd harmonic.
The dynamic spectra showed that occasionally a large number of birdies occurred together

frequencies:308.7 309.9 311.07 313.1 314.3 317.5 318.9.
These are spaced by 1.2 Mhz. This is the same difference as 104.1-102.9 (the 2nd and 3rd strongest fm stations.
308.7 is one of the fm harmonics.
At another time we heard the 107.3 fm station at 317.5 (it should be at 321.9). The difference (321.9-317.5) is 4.4 Mhz: the same difference between the fm stations fundamentals.

Conclusions/todo:

308.7, 312.3, 319.5, and 321.9 are 3rd harmonics of fm stations that are not created in the receiver.
We should do field measurements to see if the transmitters are within specs. We might want to think about buying them some filters. We should start with 312.3 (104.1 radio redentor whose transmitter is in utuado).
What's going on with the bursts of 7 frequencies at once. Why is one of them the 3rd harmonic of the fm station shifted by 4 Mhz. Did the filters make this go away are were we just unlucky to have them there on the 28oct05?

processing: Y05/051028/fm_harmonics.pro

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