CIRCUIT DESCRIPTION
UHF SYNTHESIZER DESCRIPTIONset the VCO to the center of the operating band when the control voltage is at its midpoint.
The output signal on the collector of Q102 is coupled by C108 to a cascode buffer amplifier formed by Q100 and Q101. This is a
The output signal on the collector of Q101 is directly coupled to the emitter of Q100. Impedance matching on the output of Q100 is provided by L100, C102, and C103. Resistor R102 lowers the Q of L100 to make is less frequency selective. The VCO signal is then fed to buffer Q801 and synthesizer chip U804 on the RF board.
VCO Frequency Shifting
In a particular UHF band, the VCO must be capable of producing frequencies from the receiver first injection frequency for the lowest channel up to the transmit frequency for the highest channel. Since the first injection frequency is 45 MHz below the receive frequency and the frequency band could be up to approximately 42 MHz wide, this results in a required VCO frequency spread of up to 87 MHz. If this large frequency shift was achieved only by varying the VCO control voltage, the VCO gain would be undesirably high. Instead, capacitance is switched in and out of the tank circuit to provide a coarse shift in frequency and fine shift is provided by the control voltage.
This switching is provided by PIN diodes CR104 and CR105 and controlled by a logic signal from the Q0 and Q1 outputs (pins 4 and 5) of shift register U800. When a PIN diode is forward biased, it presents a very low impedance to RF signals. Conversely, when it is reverse biased, it presents a very high impedance to RF signals.
Forward biasing one of these PIN diodes adds capacitance to the tank circuit which lowers its resonant frequency. Capacitance is added to the circuit when the control signal is low. Therefore, the lowest
frequency is selected when both control lines are low, and the highest frequency is selected when both are high.
For example, when Shift 1 goes low, CR104 is forward biased by current flowing through R103 and L105. Capacitor C111, which is part of the tank circuit, is then effectively AC grounded through CR104 and C107/C123. The control line is isolated from tank circuit RF by choke L105 and decoupling capacitor C104. The Shift 1 and Shift 2 logic signals for each band segment are listed in Section 4.3.3.
Frequency Control and Modulation
Fine VCO frequency control is performed by varying the DC voltage across varactor diodes CR103 and CR106 (coarse control is provided as described in the preceding description). As the DC voltage applied to a
The VCO is frequency modulated in a similar manner. Another capacitance leg of the tank circuit is formed by C116 and CR101. The audio and data modulation signal is applied across varactor diode CR101, and a fixed bias from a voltage divider formed by R853 and R854 is applied through R851 to pin 5. Isolation and filtering of this DC supply is provided by C838, C839, C840, and R852. Refer to Section 3.7.4 for more information on modulation.
3.7.3ACTIVE FILTER (Q800), BUFFER AMPLIFIER (Q801)
Q800 functions as a capacitance multiplier to provide a filtered
| February 2001 |
Part No. |