Ramsey Electronics FX-440 Manual page 25

frequency down to 12.5 KHz for phase comparison with the 12.5 KHz
output of the crystal controlled reference divider. Maximum "N" is 65,535,
achieved by switching on all 16 parallel inputs.
Unlike simpler PLL IC's, U6's phase detector has TWO outputs at pins 7
and 8. These outputs go through very simple low pass filters (R44-C68,
RS~C91)
to cut back the 12.5 KHz whine sound of U6 at work. Op amp
US:A sums together the phase detector outputs and the output of US:A is
passed through a network of 2. 2 uf electrolytic capacitors (C67, 70, 90, 92)
to smooth out the phase detector pulses to clean DC for controlling the
vco.
R48 and
cas
form yet another low pass filter to ensure that any 12.5 KHz
"whine• will not get into the VCO. Because the DC charge developed in
C85 (.1 uf) would slow down the PLL during major frequency swings, '
such as just going from transmit to receive, D8 and D10 are· set up
back-to-back across voltage dropping R48. Whenever there is a major
frequency shift (which means a significant VCO control voltage change),
one way or the other, one diode or the other is switched on to short out
R48 and discharge C85. This lets the PLL relock instantly; CBS recharges
and the diodes become no factor in the circuit.
The lock detect output (pin 28) gives a strong series of pulses when the
PLL is unlocked. When the PLL is locked, only a tiny sawtooth wave
appears at pin 28. The "lock detect" voltage is watched by US:B. If
•unlock" pulses appear, they are integrated through R90 and C96 as a
fairly clean DC voltage charge built up in C96. If this charge causes USS
to swing low, bias is removed from Transmit Buffer Q10, preventing
transmitter damage and unwanted emissions.
Stage
H:
The Diode Matrix and
PLL
Synthesizer Programming
There are two diode-matrix programming areas on the PC board. The
obviously larger area is for frequency channel programming. The second
space is for offset programming added in by U7-U10.
The 19 1 OOK resistors at the frequency programming matrix and the 13
1 OOK resistors at the offset matrix are • pulldown resistors,• to ensure
positive logic switching action of U6.
Q15 and its associated switching diodes ensure that the desired offset is
switched in during transmit, that offset programming does r.ot interfere
· when simplex is desired and that the offsets do not interfere with receiver
operation ... and that receiver programming (21.4 MHz lower) does not
interfere with transmit operation.
A variety of techniques are possible for binary programming of U6's 16
paralel inputs. We focus on the diode programming approach with some
brief suggestions on externally-controlled switching. It is very intentional
on our
part
to leave innovative programming schemes up to FX
transceiver users, because there's no single best way to do it for
everybody.
FX-440
Ref. 23