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Page 12 RF Technology T70/T150

5.1 VCO Section 5 CIRCUIT DESCRIPTION

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 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. 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 monolythic 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.

The phase detector output signals of U6 are used to control two switched current

sources. The output of the positive and negative sources Q3 and Q6, produce the

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RF Technology R150 Betriebsanleitung Seite 12