E.F. Johnson Company 5100 ES, Service Manual

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5-12 5100 ES Series Portable Radio Service Manual January 2007
Circuit Description
5.3.2 Synthesizer
5.3.2.1 PLL IC
A CX72301 sigma-delta modulated PLL (U29) is used as the main receive and transmit
synthesizer. This PLL chip provides exceptional phase noise capabilities to reduce
adjacent channel interference and quick switching between the receive and transmit
modes.
In the receive mode the PLL is programmed to a Local Oscillator frequency that is 64.455
MHz from the receive frequency. For the 700 MHz receive band, the LO frequency is
higher than the receive frequency, and for the 800 MHz band, it is lower than the receive
frequency. In transmit mode, the PLL is programmed directly to select the desired transmit
frequency.
5.3.2.2 Reference Oscillator
A 16.8 MHz oscillator (Y1) provides the frequency reference for the synthesizer and
receiver backend IC. The center frequency of this oscillator is corrected using a DC tuning
voltage from the digital board during receive and it is modulated with voice or data during
transmit.
5.3.2.3 Analog Switches and PLL Loop Filters
Analog switches U16, U17, and U18 provide faster channel switching by changing the
time constant of the PLL loop filters.
5.3.2.4 VCOs
Three different VCOs are used. VCO U1 is used strictly in transmit mode. It's tuning range
covers the entire transmit and receive frequency bands. Transmit modulation is provided
to this oscillator's modulation port from the digital board through R9.
Two receive VCOs (U5 and U6) are required to meet the phase noise requirements. Each
VCO functions for only one of the two receive bands. An RF2361 buffer amplifier (U40)
provides the required level of drive for the receiver mixer's local oscillator signal as
discussed above.