Ramsey Electronics FX-440 Manual page 39

subtraction process! This may sound complicated, but it really isn't. We
simply find 'N' as usual, take its 'two's complement' and program the
result into the offset matrix. We'll show you three detailed examples:
Don't be alarmed. If all you need is the standard 440 M.Hz band offsets,
just install the diodes in the offset matrix as clearly illus\rated on your PC
board. Understanding the following information is essential ONLY for
those who need to program non-standard Minus or AUX offsets.
First we need to understand some rules for binary addition:
1) 0+0=0
2) 1+0= 1
3) 1 + 1 = 0 and CARRY 1
Carrying is done from right to left
EXAMPLE 1: -5 MHz TX Offset
As for the + 5 MHz offset, N = 5000+ 12.5 KHz = 400. However this is
the last of Decimal numbers in this work. We' II have to work with Binary
numbers now. In brief, we set up the complement, or exact inverse of N
in binary, and then add 1. This will give us our two's complement that is
needed for programming into our offset matrix.
Binary code for N = 400
8192 4096 2048 1024 512 256 128 64 32 16 8 4 2
0 0 0 0 0 1 1 001 000
Now, just INVERT all of the above 'bits':
1 1 1 1 1 0 0 1 1 0 1 1 1
Then, ADD 1:
1 1 1 1 1 0 0 1 1 1 0 0 0
This is our two's complement value which we program into our offset
matrix. You'll see these diodes in the position for - 5 MHz TX offset.
Even though it appears that we are adding a huge N number to the
original transmitter frequency, the binary adders will follow the rules of
Binary addition and provide the proper programming information to the
PLL synthesizer.
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 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.
FX-440 Ref. 37