Specifications
4–6
FM500 User's Manual
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4.4 RF Exciter Circuit Board
This board is also known as the Frequency Synthesizer board. The entire
component side of the board is a ground plane. Rotary switches along the front
edge of the board establish the operating frequency. The VCO (voltage-controlled
oscillator) circuitry is inside a shielded cover.
Illustration 6–7 and accompanying schematic can be used as reference in this
discussion. The following theory may apply to previous versions of the exciter
board, but it is typical of the operation of the current board which has the latest
technological improvements.
VCO, VCO61, operates at the synthesizer output frequency of 87 MHz to 108 MHz.
The frequency is controlled by voltage-variable capacitors DV71 and DV72. U7A
and U7B form an active filter to supply clean DC to the drain of Q71. They also
serve as a common-base RF amplifier for Q71. A71 and A1 are hybrid RF amplifiers
to provide buffering and gain.
A sample of the RF from the VCO goes to the input of A2. A2 amplifies the signal
and feeds it back to the synthesizer IC, U6. This signal, available at pin 4 of U6,
may be used with a high frequency receiver for deviation and frequency
measurements. (See sections 5.2 and 5.3.)
U6 is a phase-locked-loop frequency synthesizer IC. The 10.24 MHz from the
crystal oscillator is used by U6, along with ICs U1 through U3 and the frequency
selector switches, to generate the frequencies of the transmitter.
Illustration 4–4 RF Exciter Board
RF Exciter
FM500
FM BROADCAST TRANSMITTER
®
Power
Carrier
Modulation
Fault
Stereo
Mono RF Output
ProcessingInput Gain
Wide Band
High Band
Audio Input
High
Low
2
-6
-12
-18
2
+6 dB
+12 dB
10
20
Expand
Compress
RF Power
SWR
ALC
PA DC Volts
PA DC Amps
PA Temperature
Supply DC Volts
Voltmeter
SWR
Lock
Input
PA DC
PA Temp
Over
100
90
80
70
60
50
40
30
20
Pilot