Transmit Frequency Translation Loop - Motorola MTM800 User Manual

THEORY OF OPERATION (TRANSCEIVER)

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 with active element Q5601 is determined
by resonator W5601 in conjunction with C5603, C5604, C5606, C5607, L5603, and varactors
D5601-5604. The operating 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 pin 16 , where it is divided by a programmable divider set to
1.0625, 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
Q5604, Q5605, and Q5653, filtered by loop filter components C5653, R5652, C5654, C5655,
and L5607and applied to the VCO varactors.
• The action of the feedback loop is to maintain the operating frequency of the Transmit VCO at
1.0625 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.
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