CIRCUIT DESCRIPTION

UHF SYNTHESIZER DESCRIPTION

Part of the VCO output signal is fed out of the VCO on pin 2 and applied to a second harmonic filter formed by C842 and L802. A 50-ohm load is provided by R855, with C841 providing impedance matching.

Part of the VCO signal is also fed out of the VCO on pin 1 and applied to a 50-ohm, 3 dB pad formed by R842, R839, and R843. The signal is then fed to buffer amplifier Q801. Impedance matching is provided by C822, C826, and L801, and temperature-compensated bias is provided by R830, R833, R844, and CR801. Transistor Q801 provides isolation and also amplifies the signal to produce an output level of approximately 0 dBm. Capacitors C817 and C813 decouple RF signals, and L800, C818, C827, R836, R837, and R847 provide impedance matching and the proper signal levels to the receiver and exciter. R832 lowers the Q of L800 to make it less frequency selective.

3.7.4 VCO AND TCXO MODULATION

NOTE: If the wideband data input is used, the external device must provide FCC-approved modulation limiting and splatter filter circuitry and a stable 2.5 VDC reference level.

Both the reference oscillator and VCO are modulated in order to provide the required frequency response. If only the VCO was modulated, the phase detector in U804 would sense the frequency change and change the control voltage to counteract the change, especially at the lower audio frequencies. If only the reference oscillator was modulated, the VCO frequency would not change fast enough, especially at the higher audio frequencies. By modulating both, a relatively flat response is provided to all modulation frequencies.

Separate audio, data, and wideband data modulation signals are applied to the synthesizer on J201, pins 18, 17, and 16, respectively. The data signal includes LTR and Call Guard signaling, while the wideband data signal comes directly from an external device such as a modem without passing through the limiting and filtering circuitry.

The levels of the audio and data signals are set by digitally controlled variable resistors in U802. These resistors are adjusted in 256 steps by serial data from the microcontroller (see Section 3.3.1). The transmit

audio signal is applied to pins 2 and 4 which are the wiper and one leg of a potentiometer. It is fed out on pin 3 which is the other leg of the potentiometer (see Figure 3-4). Likewise, the data signal is fed in on pins 22 and 24 and out on pin 23.

These variable resistors set the modulation level of the audio and data signals and also allow the micro- controller to provide frequency compensation. This compensation is required because modulation tends to increase as the VCO frequency increases (tank circuit capacitance decreases).

The output signals on pins 23 and 3 of U802 are then combined with the wideband data signal and applied to summing amplifier U803. The output signal from U803 is then applied to the reference oscillator on pin 1 and also to a potentiometer on pin 19 of U802. The output on pin 18 of U802 is applied to the VCO on pin 5. This modulates both the reference oscillator and VCO, and the potentiometer in U802 adjusts the balance of these signals.

3.7.5SYNTHESIZER INTEGRATED CIRCUIT (U804)

Introduction

A block diagram of synthesizer IC U804 is shown in Figure 3-5.This integrated circuit contains the following stages. The basic operation of U804 is described in Section 3.7.1.

•Reference divider

•Main divider

•Prescaler (÷64/65)

•Phase and lock detectors

•Charge pump and divider programming circuitry.

Channel Programming

Channels are selected by programming the main divider in U804 to divide by a certain number. This programming is performed by the microcontroller over the SPI serial data bus which consists of CLOCK, DATA, and STROBE lines (see Section 3.3.1). As previously described, this divider is programmed so that when the VCO is oscillating on the correct frequency, the fR and fV inputs to the phase detector are the same frequency.

 

February 2001

3-14

Part No. 001-9800-001