5–8
FM30/FM100/FM250 User's Manual
5.10 Carrier Frequency
Carrier frequency is measured at the output frequency with a frequency monitor
or suitable frequency counter.
To adjust frequency, see section 5.3.3. (FCC tolerance +/– 2000 Hz per FCC Part
73.1540 and 73.1545.)
5.11 Output Power
The output power reading on the front panel display should be 90–105% of the
actual value. For a more precise measurement, use a watt meter in the RF output
line. See sections 5.4.1 and 5.4.2 for setting power.
5.12 RF Bandwidth and RF Harmonics
You can observe RF bandwidth and spurious emissions with an RF spectrum
analyzer.
In the Stereo mode, feed a 15.0–kHz audio signal into one channel to provide 85%
modulation as indicated on a monitor. Doing so produces 38% main, 38% stereo
subcarrier, and 9% pilot per FCC Part 2.1049. As an alternative, use pink noise
into one channel.
Using a spectrum analyzer, verify the following (per FCC 73.317):
1. Emissions more than 600 kHz from the carrier are at least 43 + 10log(power,
in watts) dB down (58 dB for 30 watts, 63 dB for 100 watts, 67 dB for 250
watts). The scan should include the tenth harmonic.
2. Emissions between 240 kHz and 600 kHz from the carrier are down at least
35 dB.
3. Emissions between 120 kHz and 240 kHz from the carrier are down at least
25 dB.
5.13 Pilot Frequency
The pilot frequency should be within 2 Hz of 19 kHz. (FCC Part 73.322.) Using a
frequency counter, measure 1.9 MHz at pin 1 of U209 on the Stereo Generator
board. A 200–Hz error here corresponds to a 2–Hz error at 19 kHz. If the
frequency is off by more than 50 Hz, you may change the value of C213. (Changing
C213 from 56 pF to 68 pF lowers the 1.9 MHz by about 35 Hz.)
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