Receive Signal Path
Incoming RF from the antenna jack is delivered to the RF Unit and passes through a
Signals within the frequency range of the transceiver enter a
Buffered output from the VCO is amplified by Q1026 (2SC5005) to provide a pure first local signal between
399.15and 439.15 MHz for injection to the first mixer
Q1029. The 50.85 MHz first mixer product then passes through monolithic crystal filter XF1001 (MFT50.85PT,
5.5kHz BW) to strip away unwanted mixer products, and is then amplified by Q1041 (2SC4215Y).
The amplified first IF signal is applied to FM IF sub- system IC Q1048 (TA31136FN), which contains the sec- ond mixer, second local oscillator, limited amplifier, noise amplifier, and RSSI amplifier.
The second local signal is produced from the PLL ref- erence/second local oscillator of X1002 (TCXO
The second IF then passes through the ceramic filter CF1001 (ELFC450G on “Narrow” channels) or CF1002 (ELFY450E on “Wide” and “Narrow” channels) to strip away all but the desired signal, and is then applied to the limiter amplifier in Q1048, which removes amplitude variations in the 450 kHz IF, before detection of the speech by the ceramic discriminator CD1001 (ECDA450C24).
Squelch Control
The squelch circuitry consists of a noise amplifier,
When no carrier is received, noise at the output of the detector stage in Q1048 is amplified and
Circuit Description
The resulting DC squelch control voltage is passed to pin 19 of the microprocessor Q1047 (LC87F74C8A). If no carrier is received, this signal causes pin 19 of Q1047 to go high and pin 30 to go high. Pin 35 signals Q1006 (2SC4154E) to disable the supply voltage to the audio amplifier Q1005, while pin 30 holds the green (Busy) half of the LED off, when pin 35 is high and pin 30 is high.
Thus, the microprocessor blocks output from the au- dio amplifier, and silences the receiver, while no signal is being received (and during transmission, as well).
When a carrier appears at the discriminator, noise is removed from the output, causing pin 19 of Q1047 to go low and the microprocessor to activate the “Busy” LED via Q1047.
The microprocessor then checks for CTCSS or CDCSS code squelch information, if enabled. If not transmitting and CTCSS or CDCSS is not activated, or if the received tone or code matches that programmed, audio is allowed to pass through the audio amplifier Q1005 (NJM2070M) to the loudspeaker by the enabling of the supply voltage to it via Q1048.
Transmit Signal Path
Speech input from the microphone MC1001 passes through the audio amplifier Q1021 (AK2342B) to Q1015 (M62364FP) which adjusts the microphone gain. The ad- justed audio is applied to the compander Q1010 (LA8630M) which compresses the speech signal accord- ing to a control command from the microprocessor Q1047 (LC87F74C8A).
The compressed speech signal passes through the dummy unit and
The output from Q1021 is applied to the AF mute gate Q1055 (2SK3541), then returns to Q1021, which con- tains the limiter amplifier, splatter filter and audio amplifier.
The filtered audio signal is applied to Q1015 (M62364FP) which is adjusts the audio level, then is ap- plied to varactor diode D1017 (HVD350B), which fre- quency modulates the VCO Q1033 (2SK508). A portion of the audio signal from Q1015 is applied to TCXO X1002 (TTS05VS).
The processed audio may then be mixed with a CTCSS tone generated by Q1047 (LC87F74C8A) for frequency modulation of the PLL carrier (up to ±5 kHz from the unmodulated carrier) at the transmitting frequency.
If a CDCSS code is enabled for transmission, the code is generated by microprocessor Q1047 and delivered to X1002 (TCXO TTS05VS) for CDCSS modulating.
The modulated signal from the VCO Q1033 (2SK508- K52) is buffered by Q1026 (2SC5005). The
13
