Pll Circuits; Apc Circuit; Vhf Pll Circuit (Pll Unit) - Icom IC-821H Service Manual

When the FORA/FORB voltage increased, the output from
the difference amplifier will be decrease to reduce the IF
amplifier gain. This adjusts the RF output power until the
FORA/FORB and POCO voltages are well-balanced.
4-2-8 APC CIRCUIT
(PA-A, PA-B AND MAIN UNITS)
The APC (Automatic Power Control) circuit protects the
power amplifiers on the PA-A and PA-B units from exces-
sive current.
Current drain of power amplifiers is detected by voltage
drops at resistors (PA-A unit R1, R12 and PA-B unit R1,
R16). The original voltage (IC1) and dropped voltage (AIC2
or BIC2) are applied to the APC difference amplifier (MAIN
unit, IC4a or IC4b).
The signal output from the difference amplifier reduces IF
amplifier gain until these voltages are well-balanced.
4-3 PLL CIRCUITS
IC-821 H contains 2 PLL circuits and 1 local oscillator. The
VHF and UHF PLL circuits adopt "I com's original I-loop
PLL" to obtain very fast lock up times .
• PLL circuits
ICl
IC2a
4-3-1 VHF PLL CIRCUIT (PLL UNIT)
The VHF PLL circuit generates the 1 st LO frequency, and
the signals are applied to the 1 st mixer circuits in the RF-A
unit. The PLL circuit consists of a VCO, prescaler and DDS
circuits.
The signal generated at the VHF VCO circuit (01, D1-D3,
D8) is amplified at the buffer-amplifiers (02, IC5), then
applied to the prescaler circuit (IC4). The prescaler circuit
divides the applied signal, and outputs it to the VHF DDS
circuit (lC1) via the buffer-amplifier (04). The VHF DDS
circuit generates digital signals using the applied signal as a
clock frequency. The phase detector section in ICl com-
pares it's phase with the reference frequency which is
generated at the reference oscillator (Xl). IC1 outputs
off-phase components as pulse signals via pins 52, 53.
The output pulses are converted into DC voltage at the loop
filter circuit (IC2a) and then applied to the VHF VCO circuit.
The D/A converter (R15-R38), low-pass filter (L2-L4,
C4-C10) and buffer-amplifier (IC14) circuits are connected
to the DDS output to convert the digital oscillated signals
into smooth analog signals.
The output DC voltage from the loop filter circuit is also
applied to the RF-A unit to control the tuned bandpass filter.
IC2b
> - - -
To RF-A board
01,01-03,08
I--_-_l,r-~
. .
~··~,'l-------A-l-L-O-
To VHF 1 st mixer circuit
T
\..:l}
(RF-A board, 011, 012)
Xl: 30.2 MHz
L...-_--' ' •.....•....... _;
Prescaler I---C
1/8
IC4 IC5 VHF PLL
circuit
r---------------------------
_________________________ ..J
IC7b
UHF PLL
circuit
- : > - - _
To RF·B board
IC6
IC7a
05,04,010
I----+---:--~I
Phase
. - - ' - - -__ 1 detector 1-'---1
Loop
filter
1----4 _ _
i:r-~
..
~
. .
~ ; : '
___
--..~B:....l~L:....O_
To UHF 1 st mixer circuit
T
\..:l}
(RF-B board, 014, 015)
'------'
' ..............
_;
Prescaler I---C
1/8
08 IC9 09
014
B2LO T UHF d ' . .
1 - - - - - - - - - - -_ _ _ _ _ _ _ _
0
2n mixer CircUit
(RF-B board, 011)
> - - - - - - - - - -
To BFO circuits (MAIN unit, ICl 01, IC103)
024
4-6