Service manual

July 1, 2002 68P81076C25-C
3-6 Theory of Operation: RF Board
3.1.2.7 Second VCO
The second VCO is a grounded-gate, FET Colpitts oscillator. The resonator consists of a fixed
inductor and a varactor. A potentiometer, R634, adjusts the negative voltage to the varactor. This
adjustment is performed at board test to bring the phase detector output to the center of its linear
region; that is approximately 2.25 V. (For HRN6014D, HRN6020C, HRN6019C, HRN4009D,
HRN4010C and later RF board kits, a voltage divider consisting of R633 and R635 brings the phase
voltage detector output to the center of its linear region (2.25 V), eliminating the adjustment at board
test.) The negative voltage is filtered by R611 and C612. The oscillator output is coupled into the IF
IC (U301) as a second injection source. It is also fed back to the prescaler (U601, pin 26) for phase
locking.
The prescaler contains a programmable, single modulus, divide-by-N circuit, and a charge pump
phase detector. The reference frequency (F
r
) is 300 kHz and comes in on U601, pin 31. The low-side
injection oscillator runs at 109.2 MHz and is divided by 364 inside the IC. The phase detector in the
chip compares the divided signal to F
r
and either sources or sinks current, as necessary, in order to
maintain frequency control.
The phase detector output is routed to the varactor via decoupling choke L604. A divide-by-N test
point is also provided from U601, pin 29.
3.1.2.8 Power Distribution
The command board provides all power to the synthesizer in the form of 9.6 Vdc. The prescaler has
onboard voltage regulators for 5 V and super filter 8.6 V. The 5-V regulator drives the external series
pass device Q602; the super filter’s pass device is Q603.
3.1.3 Receiver Back-End
This section discusses the receiver back-end components and detailed theory of operation.
3.1.3.1 First IF
The 109.65 MHz IF signal reaches the RF board via a connector J350. Transistor Q350 amplifies the
signal approximately 9dB and supplies the proper impedance for crystal filter Y350. (For HRN6014D,
HRN6020C, HRN6019C, HRN4009D, HRN4010C and later RF board kits, amplification circuitry
consisting of transistors Q350 and Q354 amplifies the signal approximately 9dB and supplies the
proper impedance for crystal filters FL350 and FL351.)
Transistor Q351 supplies filtered A+ for powering Q350 and the receiver front-end. Transistor Q352
switches the filtered A+ supply by reducing the base current from Q351.
NOTE: Since there is 12.5 Vdc on J350, it is important to use a DC block when connecting J350 to
an external source.
Y350 is a 4-pole crystal filter, consisting of two independent 2-pole crystal filters contained in a single
package. The filter package has a polarization mark located on the top to ensure proper installation.
Y350 supplies the 109.65 MHz IF selectivity and its output passes through a matching network and
then goes to ABACUS II IC (U301) pin 30.
(For HRN6014D, HRN6020C, HRN6019C, HRN4009D, HRN4010C and later RF board kits, FL350
and FL351 are 2-pole crystal filters which supply the 109.65 IF selectivity. The output passes through
a matching network and goes to the ABACUS II IC (U301), pin 30.)