Specifications

5–4
FM500 User's Manual
5.3 Frequency Synthesizer Adjustments
5.3.1 Frequency (Channel) Selection
Refer to section 2.3.
5.3.2 Modulation Compensator
Refer to section 2.3.
5.3.3 Frequency Measurement and Adjustment
Next to the 10.24–MHz crystal on the RF Exciter board is a 1–11 pF piston trimmer
capacitor (C3). Use C3 to set the frequency of the 10.24–MHz crystal while
observing the output frequency of the synthesizer.
Use one of these methods for checking frequency:
❑ Use an FM frequency monitor.
❑ Couple a frequency counter of known accuracy to the output of the synthesizer
and observe the operating frequency.
(Do not connect to the 10.24–MHz clock
circuit.)
5.3.4 FSK Balance Control
An FSK signal (used for automatic identification of FM repeaters) shifts the
frequencies of the 10.24–MHz crystal reference oscillator and the VCO.
Use an oscilloscope to observe the cathode end of D4. With no program, the pulse
will be less than 1 µsec wide. With an FSK input (a 20–Hz square wave at the FSK
input will work), set trim pot R45 for minimum pulse width.
The setting will vary slightly with operating frequency.