FX 146
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24
R48 and C85 form yet another low pass filter to ensure that any 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; C85 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 U5: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 U5B 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 100K resistors at the frequency programming matrix and the 14
100K 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 not 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.
FREQ.
N =
BINARY PROGRAMMING VALUES
144.000
28,800
148.000
29,600
0111
0000
1000
0000
0111
0011
1010
0000
146520 (KHz)
5 (KHz)
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123
assures that the otherwise-silent PTT circuitry is indeed functioning and
obviously has nothing to do with the RF output of the transmitter.
The PTT circuit may also be activated at pin 3 of the Packet I/O jack. A
direct short to ground is not necessary. The author noted very positive PTT
action with resistance as high as 100K from pin 3 to ground. This is a highly
reliable and efficient PTT system provided that careful attention is given to
correct selection and installation of all parts: resistor values, PNP transistors,
zener diode orientation, U4 installation, correct wiring of microphone jack.
Stage M: ASSEMBLY PROCEDURE:
Install the following parts:
?
M1: C86, .001 uf (marked 102 or .001 or 1nf).
?
M2: C83, .001 uf.
?
M3: C88: .001 uf.
?
M4: C77, .001 uf.
?
M5: C23, .001 uf.
?
M6: R57, 2.2K (red-red-red).
?
M7: R59, 100K (brown-black-yellow).
?
M8: R58, 270 ohms (red-violet -brown).
?
M9a: R50, 10K (brown-black-orange).
?
M9b: R49, 10K (brown-black-orange).
?
M10: R51, 47K (yellow-violet-orange).
?
M11: R56: 47K (yellow-violet-orange).
?
M12: R61: 2.2K (red-red-red).
?
M13: R37: 100K (brown-black-yellow).
?
M14: C93: 4.7 or 10 uf. (Watch polarity!)
?
M15: C89: .001 uf.
?
M16: C24: .001 uf.
?
M17. Install U4, a 14 pin DIP IC, type LM324, which contains all 4 op
amp sections of this circuit. Orient the notched end as shown on the
board. If you elect to provide a DIP socket, use the same care as if
soldering the IC itself.
