Motorola MTM700, Detailed Service Manual

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THEORY OF OPERATION (400 MHz) MTM700 Mobile Radio / Detailed Service Manual 4.1 - 15
Transmit Frequency Translation Loop
The Transmit Frequency Translation Loop comprises the following components:
• transmit Voltage Controlled Oscillator (VCO) and
• the ESCORT Near Unity Divider (NUD) Phase Locked Loop IC.
The operating frequency of Voltage Controlled Oscillator Q5601 is determined by resonator U5601
in conjunction with C5603, C5604, C5606, C5607, L5603, and varactors D5601-5604. The operat-
ing frequency of the oscillator is varied by means of a tuning voltage applied to the varactors.
The translation loop works as follows:
• A sample of the VCO operating frequency is buffered by Q5631 and applied to the FREQA
input of the NUD IC U5702, where it is divided by a programmable divider set to 1.25, and
phase compared to a reference frequency supplied to U5702 FREQB pin 9 from the main
synthesizer.
• The result of this comparison is a VCO tuning signal, buffered by charge pump transistors
Q5651, filtered by loop filter components C5653, R5652, and C5654, and applied to the VCO
varactors.
• The action of the feedback loop is to maintain the operating frequency of the Transmit VCO at
1.25 times the programmed operating frequency of the main synthesizer.
• The loop bandwidth of the transmit frequency translation loop is set to a high value in order for
the corrective feedback action of the loop to suppress re-modulation of the VCO as a result of
leakage of the modulated transmitter signal from the transmitter Power Amplifier output back
into the VCO.