FX 146
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work together for channel programming.
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The Phase Detector (or "phase corrector") which compares the 5
KHz Reference Frequency with the "intended" 5 KHz output of the
divide by N counter and sends correcting pulses to the VCO to
keep the output of the N divider right at 5 KHz.
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A "lock detect signal" circuit.
An easy illustration of how simple assembly error causes an unlocked or
mistuned PLL can be seen in the fact that theReference 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 2048, for a
Reference Frequency of 8.8276 KHz rather than 5 KHz! Thus, the VCO
output would become a multiple of 8.8276 KHz. If "N" is programmed as
29,304 for 146.52 MHz Simplex, the single bent IC pin would cause the
output frequency to be 29,304 X 8.8276 KHz, which is 258.684 MHz!
Because the VCO cannot tune this high, 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'll become in using, maintaining, programming and customizing
your FM transceiver.
FX- TRANSCEIVER PLL OVERVIEW:
The reference oscillator is internal to U6, governed by Y2. The precision of
the 10.240 MHz reference oscillator can be adjusted by trimmer C81. The R
divider feeds 5 KHz to the phase detector section of U6 (10.240 divided by
2048).
The output of the 12017 ÷64/65 prescaler U3 is AC coupled via C57 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 5 KHz for phase comparison with the 5 KHz output of the crystal
controlled reference divider.
The minimum "N" number is 1. The maximum can be stated in a variety of
ways, some more exact than others. We could just say "64K less 1" or 2 to
an n-th power less one. We could say the maximum is what you get when
you install diodes in all 16 positions of the parallel programming matrix. We
could express it as "2 x 32768 - 1" or just say that it is 65,535!
The details of N divider programming will be covered in the next section.
We'll even see that the roles of the offset matrix and the binary adders (U7-
U10) are clear, brief and simple to explain.
FX-146
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magic number (decimal) to understand in FX binary frequency programming.
N = the frequency in KHz divided by 5 KHz.
NBFM
= "Narrow-Band FM." NE- = prefix to transistors or IC's made by
Signetics, Inc.
OFFSET
= The distance in KHz or MHz of the transmitted signal from the
primary or received signal. The "TX Offset" may be higher or lower,
depending on the repeater setup or other application.
OP-AMP
= "Operational Amplifier," a high performance linear amplifier with
an Inverting input, a Non-inverting Input and one Output.
ORIENT, orientation
= Yes, the Orient is Marco Polo's Far East where they
build many FM radios and where we buy many common parts. When we
ask you to "orient" a kit part one way or the other, we're just asking you to
get it RIGHT -- per all the "orientation" illustrations that we provide.
PACKET
= Rapid transmission of intelligible computer generated data in
groups or bursts of a fixed length called "packets."
PHASE DETECTOR
= In a frequency synthesizer, the phase detector
compares the signal from a reference oscillator to the signal from a
programmable counter and sends a corrected control voltage to the L-C
section of a VCO. Thus, it can be considered a phase "corrector." See also:
VCO, PLL, VARACTOR.
PLATED-THROUGH
= Refers to a style of PC board manufacturing in which
traces on both sides of the board are interconnected by a coating of solder
around the inside of each hole.
POLARITY
= Refers to the "+" and "-" sides of batteries, power supplies,
power cables, electrolytic capacitors, etc.
PLL
= "Phase-Locked Loop," a frequency generator circuit in which an
oscillator output is analyzed by counters and a phase detector which
controls and corrects the voltage supplied to varactor diode(s) which
determine oscillator frequency.
POT
= jargon for "potentiometer", a variable resistor, either a panel control
or a "mini" device on a PC-board.
PRESCALE, Prescaler
= a circuit or device which divides an incoming
frequency down to a lower frequency so that it can be handled more easily
by later digital counting circuitry or devices.
PTT
= "push to talk", referring to all switches and circuitry involved in turning
a transmitter on and off from a switch usually built into a microphone. This
switch may also be a relay or transistor in a packet TNC, etc.
PUFF
= No, not the magic dragon. It's an easier way to say "picofarads" in
the Ramsey warehouse and other high tech temples.
QUADRATURE
= Refers to the Phase Angle (90 degrees) considered by
the FM discriminator or detector.
REFERENCE FREQUENCY
= a precise, known frequency, usually crystal-
