Frequency Synthesis
Introduction/Theory of Operation
3.1-23
10.0
Frequency Synthesis
The complete synthesizer subsystem consists of the Reference Oscillator (Y5701 or U5702), the
Fractional-N synthesizer (U5701), the Voltage Controlled Oscillator (Q5741), the RX and TX buffer
stages (Q5751, Q5771, Q5781) and the feedback amplifier (Q5791).
10.1
Reference Oscillator
The Reference Oscillator (Y5702) contains a temperature compensated crystal oscillator with a
frequency of 16.8 MHz. An Analogue to Digital (A/D) converter internal to U5701 (FRAC-N) and
controlled by the microprocessor via serial interface (SRL) sets the voltage at the warp output of
U5701 pin 16 to set the frequency of the oscillator. The output of the oscillator (pin 2 of Y5702) is
applied to pin 14 (XTAL1) of U5701 via a RC series combination.
Some models use the Reference Oscillator U5702 instead of Y5702. The Reference Oscillator
(U5702) contains a temperature compensated crystal oscillator with a frequency of 16.8 MHz. This
oscillator is tuned by a temperature referenced 5 bit Analogue to Digital (A/D) converter. The output
of the oscillator (pin 10 of U5702) is applied to pin 14 (XTAL1) of U5701 via a RC series
combination. The serial interface (SRL) for control is connected to the
µ
P via the data line SPI DATA
(U5702-25), clock line SPI CLK (U5702-22), and chip enable line PEND CE (U5702-24).
In applications were less frequency stability is required the oscillator inside U5701 is used along with
an external crystal Y5701, the varactor diode D5702, C5708, C5710 and R5704. The crystal may
not be replaced in case of failure. Instead of the crystal, the reference oscillator Y5702 must be
soldered in along with C5706, C5707, R5703. Components Y5701, C5708, C5710, R5704, D5702
must be removed and the value of C5709 must be changed. Afterwards the radio must be retuned.
10.2
Fractional-N Synthesizer (U5701)
The FRAC-N synthesizer IC (U5701) consists of a pre-scaler, a programmable loop divider, control
divider logic, a phase detector, a charge pump, an A/D converter for low frequency digital
modulation, a balance attenuator to balance the high frequency analogue modulation and low
frequency digital modulation, a 13V positive voltage multiplier, a serial interface for control, and
finally a super filter for the regulated 9.3 volts.
A voltage of 9.3V applied to the super filter input (U5701 pin 19) supplies an output voltage of 8.6
VDC at pin 18. It supplies the VCO (Q5741), VCO modulation bias circuit (via R5714) and the
synthesizer charge pump resistor network (R5723, R5724, R5726). The synthesizer supply voltage
is provided by the 5V regulator U5801.
In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin
VCP (U5701-32), a voltage of 13 VDC is being generated by the positive voltage multiplier circuitry
(D5701-1-3, C5716, C5717). This voltage multiplier is basically a diode capacitor network driven by
two (1.05MHz) 180 degrees out of phase signals (U5701-9 and -10).
Output LOCK (U5701-2) provides information about the lock status of the synthesizer loop. A high
level at this output indicates a stable loop. IC U5701 divides the 16.8 MHz reference frequency down
to 2.1 MHz and provides it at pin 11. This signal is used as clock signal by the controller.
The serial interface (SRL) is connected to the microprocessor via the data line SPI DATA (U5701-5),
clock line SPI CLK (U5701-6), and chip enable line FRACN CE (U5701-7).
Summary of Contents for GM1200E
Page 2: ...ii ...
Page 4: ...Cautions and Warnings iv ...
Page 6: ...Cautions and Warnings vi ...
Page 8: ...Contents viii Detailed Service Manual ...
Page 10: ...Table of Contents 1 ii Introduction ...
Page 14: ...Table of Contents 2 ii Control Head Level 3 Information ...
Page 16: ...Table of Contents 2 1 ii Introduction Theory of Operation ...
Page 24: ...Table of Contents 2 2 ii PCB Schematic Diagram and Parts List ...
Page 26: ...Table of Contents 3 ii UHF VHF Radio Level 3 Information ...
Page 30: ...Table of Contents 3 1 iv Introduction Theory of Operation ...