TROUBLESHOOTING AND REPAIR
FREQUENCY AND PHASE MODULATION
6. Cover the FM oscillator (Q1, Q2 section only) with a metal cover.
7. Set the 8901A to Auto, FM, + peak, 300 Hz HP, 15 kHz LP. Connect the 8901A to
J3. Set the UUT to SPCL 909, 800 MHz, INT ACFM, 1 kHz mod rate, 400 kHz
FM Dev. Set 8840A to DC volts, autorange. Connect the 8840A to TP11. Adjust
L1 for 400-kHz FM Dev ± 2 kHz. The 8840A should read 7V DC ± 100 mV dc. Set
the UUT to 40-kHz FM Dev. Adjust C9 for 40-kHz ± .2-kHz dev. The 8840A
should read 15V DC ± 200 mV DC. Repeat these steps until both specs are met.
Remove the 8840A from TP11.
8. Set the 8840A to DC volts, autorange. Connect the 8840A to TP12. Set the UUT to
300-kHz dev, INT ACFM on, 1-kHz mod rate. Key SPCL 942 to set FM Steer
DAC to 2048. Adjust R39 for 0.0V DC ± 20 mV DC.
9. Set the UUT to 200-kHz dev ACFM, INT ACFM on, 1-kHz mod rate. Set the FM
Steer DAC to 2048. Set the 8840A to DC volts, autorange. Connect the 8840A to
TP12. Adjust R35 for 0.0V DC ± 20 mV DC.
10. Set the UUT to INT ACFM, 10-kHz mod rate. Connect oscilloscope channel 1 to
TP8 and channel 2 to TP6. Set the UUT for 200-kHz ACFM. Adjust R63 until one
pulse on channel 1 is exactly in the middle of two pulses on channel 2. This
represents a 50% alignment of the pulses on TP6 and TP8. Set the UUT to 10-kHz
dev ACFM. Check for a pulse alignment of 22/78% ± 3%. Remove scope probes.
11. Set the UUT to INT ACFM, 5-rad øM Dev, 5 kHz mod rate. Set the 8901A to FM,
AVE, 300 Hz HP, 15 kHz LP, %. Note the 8901A reads 100%. Press the kHz button
on the UUT. Adjust R104 for a reading of 102% to 102.2% on the 8901 A.
12. Set the UUT to 800 MHz, 50-kHz dev, 5-kHz mod rate, INT ACFM. Set the
8901A for +peak, % off, >20-kHz filter, and all other filters off. Adjust R107 for
equal plus and minus readings around 50 kHz. Plus and minus readings must be
within .5 kHz of each other. This is a distortion check.
13. Set the UUT to SPCL 909, 800 MHz, 50-kHz dev, 70-Hz mod rate, INT ACFM.
Set the 8901A to +peak, >20-kHz filter. Adjust R102 for a 50-kHz reading on the
8901A.
14. Set the UUT to SPCL 909, 800 MHz, 100-kHz dev, 70-Hz mod rate, INT ACFM.
Set 8901A to +peak, >20-kHz filter, all other filters off. Adjust R94 for a 100-kHz
reading on the 8901A.
15. Set the UUT to 50-kHz dev, 5-kHz mod freq, INT ACFM. Set the 8901A to %. The
8901A should read 100%. Check the mod rates in Table 6E-5 to determine if the
UUT is within specification.
Table 6E-5. FM - Mod Rate Specifications
6E-15
MOD RATE (Hz)
1000
500
200
100
50
SPECIFICATION (kHz)
100 +2% -2%
100 +2% -2%
100 +2% -2%
100 +2% -2%
100
+1.5% -1.5%
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