WJ-8611 DIGITAL VHF/UHF RECEIVER
CIRCUIT DESCRIPTIONS
5-11
The Step Loop Synthesizer provides coarse tuning of the 2nd LO VCO to
set the output to within 500 kHz of the desired frequency. It provides a
1325.5 to 1327.5 MHz signal, with a 500 kHz tuning resolution. This
signal is mixed with a sample of the 2nd LO output frequency in the
synthesizer's feedback path. The down-converted output frequency is then
applied to a phase detector for frequency control. The frequency of the
feedback signal ranges from 2.25 MHz to 2.749 MHz. The 2nd LO
Resolution Loop Synthesizer controls the fine tuning that provides the 2nd
LO VCO’s 1 kHz resolution. The Resolution Loop VCO provides an
output frequency ranging from 180.0 to 220.0 MHz, tunable in 80 kHz
increments. Its output is divided by a factor of 80 to provide a 2.25 to
2.749 MHz output to the 2nd LO Output VCO’s phase detector. The
tuning resolution is 1 kHz, due to the frequency division. At the phase
detector, the feedback signal is compared with the Resolution Loop output
to develop the tuning voltage that locks the 2nd LO VCO to the desired
frequency.
Table 5-3
lists the relationship between the Step Loop,
Resolution Loop, and the 2nd LO Output frequencies. The frequencies of
the Step Loop and Resolution Loop are determined by the following
formulas:
FREQ
STEP
= [(INT(FREQ
LO2
- 1327.75/0.5)) x 0.5] + 1325.5
FREQ
RES
= (FREQ
LO2 -
FREQ
STEP
) x 80
Where:
FREQ
LO2
is the 2nd LO Output Frequency, in MHz
FREQ
STEP
is the 2nd LO Step Loop frequency, in MHz
FREQ
RES
is the 2nd LO Resolution Loop frequency, in MHz
Table 5-3. 2nd LO Synthesizer Tuning Frequencies
Step Loop
(MHz)Res =
500 kHz
Resolution
Loop
(MHz) Res = 80 kHz
Resolution
Loop
÷
80
(MHz) Res = 1 kHz
2nd LO
Output
(MHz) Res = 1 kHz
1325.50
180.0 - 220.0
2.250 - 2.749
1327.750 - 1328.249
1326.00
180.0 - 220.0
2.250 - 2.749
1328.250 - 1328.749
1326.50
180.0 - 220.0
2.250 - 2.749
1328.750 - 1329.249
1327.00
180.0 - 220.0
2.250 - 2.749
1329.250 - 1329.749
1327.50
180.0 - 220.0
2.250 - 2.749
1329.750 - 1330.249
In a normally operating 2nd LO Synthesizer, the 2nd LO output VCO is
controlled by a tuning voltage that ranges from +5 to +15 V at test point
TP6. The individual tuning voltages for the Step and Resolution Loop
VCOs each range from +1.5 to +15 V at test points TP9 and TP56,
respectively. Each of the phase-locked-loop circuits provide a lock
indication to the Digital Control PC Assembly (A2) to provide an
indication of the operating status of the 2nd LO circuit. The LOCK2
Courtesy of http://BlackRadios.terryo.org
Summary of Contents for WJ-8611
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Page 119: ...5 i SECTION 5 CIRCUIT DESCRIPTIONS Courtesy of http BlackRadios terryo org...
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