Pete KE9OA wrote:

starman wrote in message


How are you designing the PC board(s) for this receiver? Do you use some
CAD software?

I do use a CAD program. For the prototyping, I print out the artwork on an
Inkjet transparency, and use boards that have a photosensitized resist. Our
jobber wants 250 dollars for each prototype run, so it is cheaper for me to
do it myself.
Once I get everything designed, we will go to our board house, and have some
real boards made up. It is quite a bit of work, spotfacing all of the holes
on the ground plane, and soldering feedthrough wires, to connect the top
ground plane to the copper flood on the bottom side of the board, but it is
the only way to get a board with a nice low impedance RF ground.
Another interesting thing.................it is a good idea not to lay down
your ground vias on a fixed grid; instead, drop them around the board in a
pseudo-random fashion. This way, you can minimize the chances of having
resonances in the structure.
I remember one project that I was working on a few years back. The designer
decided to lay down all of the ground vias on a 50 mil grid. This was a
900MHz hybrid synthesizer, that used a mixing scheme to translate the tuning
range. Anyway, the board had a very sharp resonant peak right in the middle
of the image band. The engineer that I was working with didn't believe that
this was the case, until we started drilling out the vias with a Dremel
tool.
A good way to check a PC board for undesired resonances is to take the
unpopulated board, and connect an SMA launch at each end of the board (input
and output). Connect a network analyzer, and you should see a flat noise
spectrum, if the board was properly designed.
Another trick of the trade for checking VCOs is to connect a network
analyzer to the inpur of the VCO. Set up the analyzer for a Smith Chart type
of display. You will know if you have your feedback capacitors optimized for
the tuning range of interest, if you are centered in the maximum magnitude
region of negative resistance. This was a pretty common technique at
Rockwell. When I mentioned this to the folks that I was working with in my
department at Motorola, they had never heard of this method.

Pete

That's really interesting. I've studied how Drake designed the PC boards
for their R8 series of receivers. I'm going experiment with the
grounding system to see how it affects the synthesizer noise which shows
up at certain frequencies, mostly in the higher HF range. It's much less
on the 'B' model than earlier versions but I think there's still room
for improvement.

Thanks for the reply.


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