Specifications

to the base of Q601 serving as an RF amplifier. Diode D601 serves as protection from static RF

overload from nearby transmitters. The output of Q601 is then coupled to a second band pass

filter consisting of C611 through C623 and L606 through L609 at UHF and C607 through C601,

L603 through L601 at VHF. The output of the front-end block is then coupled to the double

balanced diode mixer D6. The Front-end block is pre-tuned at factory and no more adjustment

is required

FIRST MIXER

The Double balanced diode mixer consists of D9, T1 and T2 and generates the 45.1 MHz

intermediate frequency output from RF and local frequency. The filtered frequency from the

front-end module is coupled to T1and the local frequency from RX VCO is coupled to T2. The

45.1 MHz IF output is matched with the input of the 2-pole monolithic filter by L12, L13, C65 and

C66. The crystal filter provides a bandwidth of ±7.5 kHz at the operating frequency for a high

degree of spurious and inter-modulation protection. The IF filter provides additional attenuation

for the image frequency of the second mixer. The output impedance of the filter is matched with

the base of the post amplifier Q16 by C67 and C70.

SECOND OSCILLATOR MIXER LIMITER AND FM DETECTOR

The output of the post amplifier, Q16, is coupled via C71 to the input of IC6 (TA31136FN). IC6

is a monolithic single conversion FM transceiver, containing a mixer, the second local oscillator,

limiter and quadrature detector. Crystal X1, 44.645 MHz, is used to provide resultant 455kHz

signal from the output of the second mixer. The mixer output is then routed to CF1 (455F) or

CF2(455HT). These ceramic filters provide the adjacent channel selectivity of 25 kHz or 12.5

kHz bandwidth. After that, filtered signal is fed to the limiter and then audio is derived from the

limited signal at the quadrature detector.

RSSI ( RECEIVER SIGNAL STRENGTH INDICATOR )

The RSSI signal is output from IC6 on pin 12. The output is an analog DC voltage and varied as

much as the received signal strength. The signal which is filtered unwanted noise by the low

pass filter (IC4-B) in the RSSI signal is used for squelch system. Also, this signal is

compensated with a thermistor (TH3) at temperature.