GPH-CMD VHF Radio Theory of Operation
BK RADIO Page 4-3
E. FM IF IC
The FM IF IC provides a second Mixer, a high-gain limiter, a demodulator, an OP-AMP, and a
Schmitt trigger. The Mixer converts the 16.9 MHz signal to 455 kHz. The local oscillator for the
Mixer consists of a 17.355 MHz crystal and associated circuitry. The 455 kHz signal is filtered by a
ceramic filter. A limiter provides most of the gain for the Receiver. The FM signal is demodulated
by the demodulator, the ceramic discriminator, and associated circuitry.
F. Noise Squelch
The demodulated audio is bandpass filtered with an active filter consisting of the internal OP-AMP
of the FM IF IC and external circuitry. Squelch gain is controlled by the Microprocessor. The
squelch noise is detected by the internal Schmitt trigger of the FM IF IC. The Microprocessor
samples the Schmitt trigger output to determine signal level and squelch information.
4.3.2 TRANSMITTER
The Transmitter consists of four major blocks (see figure below):
A. Power Amplifier
B. Harmonic Filter
C. Antenna Switch
D. Power Control Circuit
Figure 4-2 Transmitter Block Diagram
The Power Amplifier provides the necessary gain to amplify the VCO signal to a level of 5 Watts.
The Harmonic Filter is a seven-pole low-pass filter that provides rejection of the harmonics of the
transmit frequency.
The Antenna Switch routes the RF signal from the Transmitter to the antenna port while in Transmit
Mode. In Receive Mode, the antenna port is routed to the Receiver.
The Power Control circuit uses feedback to level the RF power from the Transmitter.
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