Model 8640B TM 9-4935-601-14-7&P
SETTING MODULATION
Frequency Modulation
a. Set Meter Function (1) to FM.
b. To use the internal modulation oscillator, set FM (15) to INT. Set MODULATION FREQUENCY (18) to the
desired frequency, and set the peak deviation with the PEAK DEVIATION switch and vernier (16).
NOTE
The REDUCE PEAK DEVIATION annunciator (4) lights whenever the PEAK DEVIATION switch
setting is too high for the selected frequency range. When it lights, reduce PEAK DEVIATION.
c. Peak frequency deviation is indicated by the meter (3), and the meter is read in conjunction with the PEAK
DEVIATION switch (16) (e.g., with PEAK DEVIATION set to 320 kHz, a meter reading of 2.8 indicates that peak
frequency deviation is 280 kHz).
d. With FM (15) set to INT, the internal modulation oscillator signal is present at the FM OUTPUT jack (17) (600
ohm source impedance). Its level is set by AUDIO OUTPUT LEVEL (21).
e. To use an external modulation signal, set FM (15) to AC (or DC if modulation signal is less than 20 Hz). Apply
the signal to the FM INPUT jack (17) (600 ohm load impedance). The Signal Generator requires 1 Vpk (0.7071
Vrms) for full peak deviation. The PEAK DEVIATION controls (16) and the meter (3) are used the same as when
using the internal modulation oscillator signal.
f. To calibrate the external input, set the FM switch (15) to DC (with no signal applied to FM input) and read the
frequency of the RF Output. Set FM to CAL and, using the PEAK DEVIATION switch and vernier (16), offset the
frequency at RF OUTPUT an amount equal to the desired peak deviation. Set FM to DC or AC; a 1 Vpk (0.7071
Vrms) signal applied to FM INPUT will now produce the desired peak deviation. (Do not use FM CAL when
phase locked.)
Figure 3-6. Setting the Modulation Controls (3 of 4)
3-17
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