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CIRCUIT DESCRIPTION
The VCO frequency of A401 is controlled by a
DC voltage produced by integrating the phase detec-
tor output pulses of U401. The phase detector senses
the phase and frequency of the two input signals (f
and f
) and causes the VCO control voltage to increase
R
or decrease if they are not the same. When the fre-
quencies are the same the VCO is "locked" on fre-
quency.
One input signal to the phase detector in U401 is
the reference frequency (f
TCXO frequency divided by the R (reference) counter
to the channel spacing or 6.25 kHz.
The other input to the phase detector in U401 is
from the VCO frequency divided down by the "N"
counter and prescaler in synthesizer U401 to
6.25 kHz. The "N" counter is programmed through
the synthesizer data line on J201, pin 20. U401 is pro-
grammed so that the phase detector input (f
cal to the reference frequency (f
VCO is locked on the correct frequency.
The synthesizer contains the R (reference), N,
and A counters, phase and lock detectors and counter
programming circuitry. Frequencies are selected by
programming the three counters in U401 to divide by
assigned numbers. The programming of these
counters is performed by circuitry in the Third Party
Interface Card (TPI), which is buffered and latched
through the Interface Alarm Card (IAC) and fed into
the synthesizer on J201, pin 20 to Data input port
U401, pin 19.
Data is loaded into U401 serially on the Data
input port U401, pin 19. Data is clocked into the shift
registers a bit at a time by a low to high transition on
the Clock input port U401, pin 18. The Clock pulses
come from the MPC via the IAC to J201, pin 18.
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
) is the same frequency as the TCXO-
V
derived input (f
) which is 6.25 kHz.
R
The f
input is produced by dividing the
R
17.5 MHz TCXO frequency by 2800. This division is
done by the "R" counter in U401. The counter always
divides by 2800 regardless of the channel frequency.
This produces a reference frequency (f
August 2000
Part No. 001-2001-200
V
). This is the 17.5 MHz
R
) is identi-
V
) (6.25 kHz) when the
R
) of
R
6.25 kHz. Since the VCO is on frequency (receive fre-
quency plus 52.95 MHz) and no multiplication is used,
the channel frequencies change in 6.25 kHz steps and
the reference frequency (f
quencies selected by this receiver.
The f
input is produced by dividing the VCO
V
frequency using the prescaler and N counter in U401.
The prescaler divides by 64 or 65. The divide number
of the prescaler is controlled by the N and A counters
in U401.
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:
Assume a receive frequency of 150.025 MHz.
Since the VCO is 52.95 MHz above the receive fre-
quency it must be 202.975 MHz. To produce this fre-
quency, the N and A counters are programmed as fol-
lows:
N = 507
A = 28
NOTE: Section 8.2.5 describes how the N and A
counter numbers can be calculated for other channels.
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 28 or 1,820 input pulses. It then divides
by 64 for 64 x (507 - 28) or 30,656 input pulses. The
overall divide number K is therefore (30,656 + 1,820)
or 32,476. The VCO frequency of 202.975 MHz
divided by 80,476 equals 6.25 kHz which is the f
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.
6-6
) is 6.25 kHz for all fre-
R
R
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