EFJohnson 7780 Squelch Circuit IC4A/B, IC5A, Transmitter Description, Amplifier Q9, Driver Q8

CIRCUIT DESCRIPTION

input signal to these transistors is high for narrow band channels. Therefore, with narrow band chan- nels, D14/D16 are forward biased and D13/D15 reverse biased and vice versa for wideband channels.

The 450 kHz signal from these filters is then fed back into IC3 on pin 5 and applied to a limiter- amplifier stage. From the limiter the signal is fed to the quadrature detector. An external phase-shift network connected to pins 10 and 11 shifts the phase of one of the detector inputs by 90° at 450 kHz (the other inputs are unshifted in phase). This network consists of C17 and ceramic discriminator X1. The detector is adjusted for wide and narrow band chan- nels by Q3, R8, and C16. The detected audio signal is then amplified and fed out of IC3 on pin 9.

4.3.3 SQUELCH CIRCUIT (IC4A/B, IC5A)

The audio output signal on pin 9 of IC3 contains audio and also high-frequency noise. This signal is fed to the receive audio and data processing circuits on the logic board (Section 4.6.1) and also to the squelch circuit. The amount of noise in this signal tends to be inversely proportional to signal strength (the higher the noise level, the lower the signal strength and vice versa). Therefore, it can be used for squelch control.

A bandpass filter that passes only noise frequen- cies occurring above the audio band is formed by an op amp in IC3 and external components. The input to this filter is pin 8 and frequency determining compo- nents (R17-R19, C20, C21) are connected to pin 7. Other components on the input such as Q4 provide frequency and wide/narrow band compensation, and thermistors R14/R26 provide temperature compensa- tion. The input audio/noise level is adjusted by R13. The detected noise output on pin 13 of IC3 is applied to pin 32 of the microprocessor in the logic unit.

4.4 TRANSMITTER DESCRIPTION

NOTE: A block diagram of the RF and logic units is located on page 8-3.

4.4.1 AMPLIFIER (Q9), DRIVER (Q8)

In the transmit mode, the VCO output signal is the transmit frequency. It is switched to the transmitter

by the switching circuit consisting of D7 and other components (see Section 4.2.2) and applied to ampli- fier Q9. Power to this stage and also Q8 is the 7.5-volt battery supply switched by Q16 and Q17. This switch is controlled by the switched T5V supply. Therefore, when the transmit mode is selected, the TX5V supply goes high which causes the output of Q17 to go low and turn switch Q16 on.

The output signal from Q9 is fed to driver Q8 which provides additional amplification. Impedance matching with IC1 is provided by L15, C62, and a resistive pad consisting of R41-R43. Power amplifier module IC1 provides up to 3 watts of RF power at the antenna jack. It contains three internal amplifier stages, each of which has a separate supply voltage (VCC1-VCC3). The supply voltage to the first stage (VCC1) is from the power control circuit, and the supply voltage to the last two stages (VCC2/VCC3) is the unswitched battery supply.

4.4.2ANTENNA SWITCH AND LOW-PASS FILTER

The antenna is switched between the receiver and transmitter by an antenna switch consisting of D11, D1-D3, and several other components. In the transmit mode, the TMUT input on pin 2 of Q7 is high. The T5V supply on pin 4 then appears on pin 3 and current flows through R20, L14, D11, D3, and L1/D1 and L2/D2. Since D11 and D3 are forward biased, the transmit signal has a low impedance path to the antenna.

L14/C31, L1/C2/D1, and L2/D2/C17 each form discrete quarter-wave line that is AC grounded at one end. When one end of a quarter-wave line is AC grounded, the other end presents a high impedance to the quarter-wave frequency. Therefore, L14/C31 isolate the 5-volt battery supply from transmitter RF, and the other two circuits isolate the receiver. In the receive mode, D11 and D1-D3 are no longer forward biased. Therefore, the receive signal has a high- impedance path into the transmitter and a low-imped- ance path into the receiver. From the antenna switch the transmit signal is fed to low-pass filter FI7. This filter attenuates harmonic frequencies occurring above the transmit band.

November 1999
Part No. 001-7780-500	4-4