User's Manual

___________________________________________________________________________
RF Technology T800 Page 13
5 CIRCUIT DESCRIPTION 5.1 VCO Section
5 Circuit Description
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 bipolar junction FET Q19
which oscillates at the
required transmitter output frequency. Varactor diodes D25 and D26 are used by the PLL
circuit to keep the oscillator on the desired frequency. A second varactor diode D3 is used to
frequency
modulate the VCO. Transistor Q20 is used as an active filter to reduce the noise
on the oscillator supply voltage.
The VCO is keyed ON by the 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 10 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
The frequency reference for the synthesiser is a crystal oscillator using transistors Q26 and
Q27 and crystal Y3. The temperature stability is better than 5 ppm and it can be synchronised
to an external reference for improved stability. External reference option board 11/9119 is
required when using an external reference.
A positive temperature coefficient thermistor, XH1, is used in versions intended for operation
down to -30 degrees Celsius. The thermistor heats the crystal's case to maintain its
temperature above -10 degrees thus extending the oscillator stability of 5 ppm down to -30
degrees ambient.
Varactor diodes D27-30
are used to frequency modulate the oscillator. The processed transmit
audio signal from U7b varies the diodes bias voltage to modulate the reference frequency.
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.
The 12.8 MHz output of Q27 is amplified by Q28 and Q29 to drive the reference input of the
PLL synthesiser IC U6. This IC is a single chip synthesiser which includes a 1.1 GHz pre-
scaler, programmable divider, reference divider and phase/frequency detector. The frequency
data for U6 is supplied via a serial data link by the microcontroller.