Second Buffer; K9.4 V Switch - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

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Doubler-biasing differs between receive mode and transmit mode. For receive, R9677, R9678, and
R9676 (in parallel to dissipate power) plus R9679 and R9680 bias the base of Q9675 to 0.7-V
potential, if NO input RF power is applied to the base. For transmit mode, keyed 9.4 V is fed through
CR9694 and another parallel resistor network R9674 and R9675. This raises the current to the
collector of Q9675 via L9678, producing more power out.

3.5.3.10 Second Buffer

The second buffer circuit is Q9676 with a 460 MHz trap, made up of L9682 and C9686, on the
collector. The signal is coupled by series LC network of L9683 and C9687. For the receive mode,
Q9676's gain is approximately 1 to 4 dB; in transmit, its gain is approximately 7 dB.
In receive mode, K9.4-volts is off so that the base voltage of Q9692 is controlled by voltage divider,
R9694 an R9695. With temperature changes, the emitter-base junction of Q9691 tracks that of
Q9692's, stabilizing the collector current and collector voltage of Q9676. R9690, R9691, and R9692
set the current level to the collector of Q9676 in receive.
In transmit mode, K9.4-volts is applied to CR9693 and through R9697, R9699, and R9693,
increasing the current flow to Q9676. K9.4-volts on the anode of CR9690 increases the voltage on
the base of Q9692. This increases voltage at Q9692's emitter and Q9676's collector. In the transmit
mode, the buffer draws approximately 60 mA.

3.5.3.11 K9.4 V Switch

In the receive mode, K9.4-volts is off. CR9691 is reverse-biased, CR9692 is forward-biased;
therefore Q9693 conducts to produce 9.4 V on the collector. This forward-biases CR9678 and
CR9677, allowing RF to pass through C9688. R9687, R9688, and R9689 drop the 12 dBm signal on
the anode of CR9678 down to 0 to 5 dBm. This is the receiver injection signal which is applied to the
first mixer in the front end of the radio.
In the transmit mode, K9.4-volts is on. Q9693 turns off, reverse biasing CR9678 and CR9677.
However, CR9675 and CR9676 are forward-biased, allowing the RF signal to pass through C9689.
The signal (approximately 20 dBm) at the junction of CR9675 and CR9677, is attenuated about 1 dB
across the diodes. The transmit signal, at approximately 18 to 23 dBm, is applied to the power
amplifier via C9689.
September 5, 2008
Theory of Operation: Voltage Control Oscillator
6881076C25-E