Theory of Operation GPH-CMD VHF Radio
Page 4-2 BK RADIO
RF signals from the Front End enter a Mixer that converts them to 16.9 MHz. The 16.9 MHz IF signal
passes through two crystal filters that provide adjacent channel selectivity. The IF amplifier then amplifies
the signal and couples it to the 455 kHz IF integrated circuit. The 455 kHz IF IC consists of a Mixer,
limiter, demodulator, and squelch circuit. The 16.9 MHz signal enters the Mixer and is converted to a 455
kHz IF signal. A ceramic filter at 455 kHz provides more adjacent channel selectivity. The signal is then
amplified by the limiter and demodulated. Audio processing is then done on the options board. Filtered
audio noise is used to provide a squelch indication.
Figure 4-1 Receiver Block Diagram
A. RF Preselectors
The Preselectors provide greater than 75 dB of attenuation at the image frequency. Both bandpass
filters are varactor tuned, which allows a wide frequency spread. Overall, the preselectors exhibit a
bandwidth of 4.5 MHz. The preselectors consist of L2, L3, L4, L5, and associated circuitry.
B. RF Preamplifier
The RF Amplifier is a single-IC amplifier providing 22 dB of gain.
C. Mixer
The balanced diode Mixer converts the RF frequency to 16.9 MHz.
D. 1
st
IF Filters and IF Amplifier
The Crystal Filters are centered at 16.9 MHz and provide attenuation to frequencies adjacent to
the receive frequency. There are two sets of 1
st
IF Filters - one for wideband (25/30 kHz) mode,
and the other for narrowband (12.5/15 kHz) mode. The IF Amplifier provides 26 dB of gain at 16.9
MHz.
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