FX 146
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110
Stage H: FREQUENCY PROGRAMMING:
Understanding and Building the Diode Matrix
The labeling of the 16 parallel programming inputs of U6 should have a
familiar ring to anyone with at least some understanding of computer
principles, which should include all of us by now. Even though our desired
"N" number is a five-digit decimal number, it is programmed as a "16 bit"
BINARY number.
Binary numbers can be as big as you like, but we get there by counting on a
base of two, either 1 or 0, yes or no, on or off. This is, of course, the
foundation for all digital circuitry.
The programming inputs of your FX transceiver synthesizer can be set for
any frequency in its range, using the correct "N" number, by means of the
diode programming provided with your kit, or with simple switches, or by
digital switching circuitry, or by a dedicated microprocessor circuit, or by a
control circuit controlled by the same computer you use for packet, etc.
We will cover only 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. Our job was to
break the price barrier on a practical, state-of-the-art VHF transceiver and
make it highly useful for most operating patterns.
You have easy front-panel selection of ANY 12 frequency pairs and never
need to buy a crystal. Nor should you ever need factory service. THAT is
what this transceiver is all about!
There are several methods for quickly finding the required binary code for a
particular frequency and its "N" number:
1.
Descending Subtraction (see Programming Worksheet)
2.
Printed reference lists (see Popular 2 Meter Band Pairs)
3.
Computer programs (see our sample BASIC program)
We recommend strongly that you fully understand how to make the
"attempted descending subtraction" calculation yourself, because that is
your ONLY means for checking the accuracy of printed information,
computer programs or the operation of experimental programming circuits.
Let's walk through the programming of 146.52 MHz, which is the national
Simplex Calling Frequency and is also the demonstration and alignment
standard for the FX-146 model. You'll see exactly what we mean by
"descending attempted subtraction." Also, this is how the model BASIC
program for diode programming included in this book is structured.
FX-146
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37
EXAMPLE 2: RECEIVER OSCILLATOR FREQUENCY
We know from previous circuit discussion that the PLL synthesizer must run
21.4 MHz lower when in receive mode. Two things must be done to do this;
first, we switch out varactor diode D3 to allow the VCO L-C circuitry to tune
21.4 MHz lower, and secondly, program in a 'minus' 21.4 MHz offset to the
synthesizer. This offset is permanently programmed into the matrix because
the 21.4 MHz 1st IF is integral to the FX receiver design. Look closely at the
Receive offset diode row and see why the diodes are installed the way they
are.
N = 21400 ÷ 5 KHz = 4,280
Binary code for N=4,280
Now, invert all the bits:
1 0 1 1 1 1 0 1 0 0 0
Add 1:
1 0 1 1 1 1 0 1 0 0 1
You'll see this is the number programmed into the Receive offset matrix line
for a minus 21.4 MHz offset.
EXAMPLE 3. TWO'S COMPLEMENT WITH CARRY
For illustration purposes, we'll pick an odd-ball offset such as 640 KHz. In
this case, N = 640 ÷ 5 KHz = 128.
Binary code for N=128
8192 4096 2048 1024 512
256
128
64
32
16
8
0
1
0
0
0
0
1
0
1
1
1
8192 4096 2048 1024 512
256
128
64
32
16
8
0
0
0
0
0
0
1
0
0
0
0
Invert all bits:
1 1 1 1
1 1 0 1 1
1 1
Add 1:
+ 1
Sum at '8' bit position, carry 1
1 0
Sum at '16' bit position, carry 1
1
0 0
Sum at '32' bit position, carry 1
1 0
0 0
Sum at '64' bit position, carry 1
1 0 0
0 0
Sum at '128', no carry needed
1 0 0
0 0
Final result:
1 1 1 1
1 1 1 0 0
0 0
