Receive Mode-Aux 1* And Aux 2* High; Transmit Mode-Aux 1* High; Aux 2* Low; Talkaround Mode-Aux 1* Low; Aux 2* Low; Vco Buffer - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

Theory of Operation: Voltage Control Oscillator
The positive steering line connects to the cathodes of the four varactors and the negative steering
line connects to the anodes. The negative line should be -4.0 ±0.3 V and the positive line can go as
high as 9 V, allowing a difference of 15 to 16 V between the two. Normally, at room temperature, the
positive steering line will be between 1.5 and 5.5 V and will fluctuate with temperature change in the
radio. Modulation is connected to the negative steering line via R9651 and C9651.
When the radio is transmitting, the oscillator's frequency will be in the 403 to 412 MHz range. When
receiving, the frequency will be between 370.675 and 379.675 MHz. If the radio is in the TalkAround
mode, the frequency will be between 425.5 and 434.5 MHz. The transmit and TalkAround ranges are
produced by coupling an additional length of transmission line to the main transmission line and is
done by a high or low on the AUX 1* or AUX 2* input lines.

3.5.3.4 Receive Mode-AUX 1* and AUX 2* High

When AUX 1* is HIGH, 8.6 V is applied to the cathode of CR9646. Q9643 is turned off and Q9647 is
turned on placing approximately -6.2 V at the anode of CR9646 reverse biasing it. Likewise with AUX
2* high the same occurs except CR9647 is reversed biased with Q9644 off and Q9646 on. This
isolates the TRANSMIT/TALKAROUND transmission line from the MAIN transmission line.

3.5.3.5 Transmit Mode-AUX 1* High; AUX 2* Low

When AUX 1* is high, the same occurs as mentioned above, however, with AUX 2* low, CR9647 is
forward-biased, connecting the TRANSMIT/TALKAROUND transmission line through C9658 and
C9657 to the MAIN transmission line.

3.5.3.6 TalkAround Mode-AUX 1* Low; AUX 2* Low

With AUX 1* and AUX 2* low, CR9647 and CR9646 are forward-biased, connecting the TRANSMIT/
TALKAROUND transmission line through C9656, C9655, C9657, and C9658 to the MAIN
transmission line.

3.5.3.7 VCO Buffer

Q9642 amplifies and provides reverse isolation to the oscillator. The frequency is then applied to the
buffer-doubler-buffer section of the VCO carrier board.

3.5.3.8 First Buffer Circuit

The VCO output is coupled to the first buffer section through C9677. Q9660 amplifies and provides
additional isolation between the doubler and the VCO.

3.5.3.9 Doubler

The first buffer output is coupled to the doubler section through C9662 and a lowpass input match
circuitry (C9675, L9675, C9676, and L9676), which serves two purposes: it matches the input of the
doubler to 50 ohms, and improves isolation between the VCO and doubler. It also keeps the desired
doubler output frequency from getting to the synthesizer. The synthesizer feedback frequency is via
C9674 and R9669.
Q9675 doubles the frequency applied to its base. The components on the collector are built so that a
400 MHz signal is effectively shorted to ground, while the 800 MHz signal sees high impedance to
ground. The doubler is coupled to the buffer through C9681, and into a 50-ohm matching network
made up of C9683 and L9680.
6881076C25-E
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September 5, 2008