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CIRCUIT DESCRIPTION
6-11
March 1999
Part No. 001-2009-600
The synthesizer contains the R (reference), N,
and A counters, phase and lock detectors and counter
programming circuitry.
One input signal to the phase detector in U403 is
the reference frequency (f
R). This frequency is the
17.5 MHz TCXO frequency divided by the reference
counter to the channel spacing or 12.5 kHz. The other
input signal (f
V) is the VCO frequency divided by the
prescaler and the "N" counter in U403. The counters
are programmed through the synthesizer data line on
J401, pin 20. Each channel is programmed by a divide
number so the phase detector input is identical to the
reference frequency (f
R) when the VCO is locked on
the correct frequency.
Frequencies are selected by programming the
three counters in U403 to divide by assigned number.
The programming of these counters is performed by
circuitry in the Main Processor Card (MPC), buffered
and latched through the Interface Alarm Card (IAC)
and fed in to the synthesizer on J401, pin 20 to Data
input port U403, pin 19.
Data is loaded into U403 serially on the Data
input port U403, pin 19 when U403, pin 17 is low.
Data is clocked into the shift registers a bit at a time by
a low to high transition on the Clock input port U403,
pin 18. The Clock pulses come from the MPC via the
IAC to J401, pin 19.
As previously stated, the counter divide numbers
are chosen so that when the VCO is operating on the
correct frequency, the VCO-derived input to the phase
detector (f
V) is the same frequency as the TCXO-
derived input (f
R). The fR input is produced by divid-
ing the 17.5 MHz TCXO frequency by 1187. The R
counter divides by 1187 regardless of the channel
number. This produces a reference frequency (f
R) of
12.5 kHz. Since the VCO is on frequency and no mul-
tiplication is used, channel frequencies change in
12.5 kHz steps. The reference frequency is 12.5 kHz
for all channels selected by this Exciter.
The f
V input is produced by dividing the VCO
frequency using the prescaler and N counter in U403.
The prescaler divides by 64 or 65. The divide number
of the prescaler is controlled by the N and A counters
in U403. The N and A counters function as follows:
Both the N and A counters begin counting down
from their programmed number. When the A counter
reaches zero, it halts until the N counter reaches zero.
Both counters then reset and the cycle repeats. The A
counter is always programmed with a smaller number
than the N counter. While the A counter is counting
down, the prescaler divides by 65. Then when the A
counter is halted, the prescaler divides by 64.
Example: To illustrate the operation of these
counters, assume a transmit frequency of 937.5000
MHz (channel 200). Since the VCO is on the channel
transmit frequency, this frequency is used. To produce
this frequency, the N and A counters are programmed
as follows:
N = 1171 A = 56
To determine the overall divide number of the
prescaler and N counter, the number of VCO output
pulses required to produce one N counter output pulse
can be counted. In this example, the prescaler divides
by 65 for 65 x 56 or 3640 input pulses. It then divides
by 64 for 64 x (1171 - 56) or 71,360 input pulses. The
overall divide number K is therefore (71,360 + 3640)
or 75,000. The VCO frequency of 937.5000 MHz
divided by 75,000 equals 12.5 kHz which is the f
R
input to the phase detector. The overall divide number
K can also be determined by the following formula:
K = 64N + A
Where,
N = N counter divide number and
A = A counter divide number.
NOTE: Section 8.2.5 describes how the N and A
counter numbers can be calculated for other channels.
6.2.9 BUFFER AMPLIFIER
A cascode amplifier formed by Q403 and Q404
provides amplification and also isolation between the
TCXO and Synthesizer U403. A cascode amplifier is
used because it provides high gain, high reverse isola-
tion and consumes only a small amount of power. The
input signal to this amplifier is coupled from TCXO
Y401, pin 5 by C420. C420 also provides DC block-
ing. Bias for the amplifier is provided by R430, R431,
R432, R433 and R428. L402 is an RF choke.