Ramsey Electronics FX-440 Manual page 106

An easy illustration of how simple assembly error causes an unlocked or
mistuned PLL can be seen in the fact that the Reference Frequency • R"
counter is externally programmable by grounding various combinations of
pins 4,5,6, permitting 8 possible divider values.
Let's say that pin 6 of the IC or socket is bent and does not make
contact. This would cause the • R" divider to divide by 1160 rather than
1024, for a Reference Frequency of 11.034 KHz rather than 12.5 KHz!
Thus, the VCO output would become a multiple of 11. 034 KHz. If "N • is
programmed as 35,680 for 446.00 MHz. Simplex, the single bent IC pin
would cause the output frequency to be 35,680 X 11.034 KHz, which is
393.69 MHz! Because the VCO cannot tune this low, the loop becomes
unlocked.
In addition to looking over the following block diagram of the internal
circuits of U6, it is worthwhile to study the block diagram and the
schematic of the whole FX- transceiver circuit. The more you work to
understand it, the more confident you' II become in using, maintaining,
programming and customizing your FM transceiver.
MO ---ll---:C12;-:"";, 8;-orROM;:;";'RITT;EFE'°"REN;:;MCE,.---,
RA1 DECODER
RA2
- l l - - - - r - - - '
OSCool - - - - - - . ,
12
"'
NOTE:
f\kl tirO!.ll;h
N9,
NJ !lrculJi
M, .,d RM
f'lroui;li
RA2
h ... e pul~ re11s1or1 nol sh~n
MC145152 BLOCK DIAGRAM
FX- TRANSCEIVER PLL OVERVIEW:
r,,
V;;i
Vdd
RAO
"''
RA<
OR
ov
' .
N1
N2
N3
'.
" LO
' 27
osc.
3 26
OSCout
'
25 M
5
,.
"'
'
23
M
7 22
"'
.
21
A1
NS
N4
The reference oscillator is internal to U6, governed by Y2. The precision
of the 12. 80 MHz reference oscillator can be adjusted by trimmer C35.
The R divider feeds 12.5 KHz to the phase detector section of U6 (12800
divided by 1024).
The output of the UPB569,
+
64/65 prescaler U3 is AC coupled via CS?
to pin 1. IC U3 is a "dual-modulus" prescaler, controlled by pin 9 of U6.
The pre-scaled output of the VCO is fed to the A and N counters. The
"N" number programmed on the diode matrix is pre-determined to divide
this frequency down to 12.5 KHz for phase comparison with the 12.5 KHz
output of the crystal controlled reference divider.
Assm. 46
FX-440