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4 CHANNEL and TONE FREQUENCY PROGRAMMING
4
Channel and Tone Fr equency Pr ogr amming
Channel and tone frequency programming is most easily accomplished with RF
Technology TecHelp Software or the Service Monitor 2000 software. This software
can be run on an IBM compatible PC and provides a number of additional useful
facilities. DOS and 32-bit versions are available.
TechHelp allows setting of the adaptive noise squelch threshold, provides a simple
means of calibrating the forward and reverse power detectors, setting the power alarm
preset levels, and enabling transmitter hang time and timeout time limits.
TecHelp/Service Monitor can be supplied by your dealer, distributor or by contacting
RF Technology directly.
5
Cir cuit Descr iption
The following descriptions should be read as an aid to understanding the block and
schematic diagrams given in the appendix of this manual.
5.1
VCO Section
The Voltage Controlled Oscillator uses a junction FET which oscillates at the required
transmitter output frequency. A varactor diode is used by the PLL circuit to keep the
oscillator on the desired frequency. Transistor Q20 is used as an active filter to reduce
the noise on the oscillator supply voltage.
microcontroller through Q10. It is keyed ON when any of the PTT inputs are active and
OFF at all other times.
The VCO output is amplified and buffered by monolithic amplifiers MA2 and MA3
before being fed to the PLL IC U6.
Amplifiers MA1, MA4 and MA5 increase the VCO output to approximately 4 mW to
drive the power amplifier. MA1 is not switched on until the PLL has locked and had
time to settle.
This prevents any momentary off channel transmission when the
transmitter is keyed.
5.2
PLL Section
Temperature compensated crystal oscillator XO1 is the frequency reference source for
the PLL Synthesizer. The frequency stability of XO1 is better than 1 ppm and it can be
synchronized to an external reference for improved stability. External reference option
board 11/9119 is required when using an external reference.
XO1 is frequency modulated by the processed transmit audio signal from U7b. This
extends the modulation capability down to a few Hz for sub-audible tones and digital
squelch codes. A two point modulation scheme is used with the audio also being fed to
the VCO to modulate the higher audio frequencies.
RF Technology T220
The VCO is keyed ON by the
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