Anytime! Anyway, it is a good idea to bring in the power supply return
(ground) wire to the noisest part of the circuit board; this way, the ground
is "cleaner", the further you get away from that section of the board. When
you are laying out the board, make sure that you locate the low level RF
circuitry as far away as possible from the noisy circuitry, and NEVER
connect the ground return wiring from the power supply to the low level RF
section of the board. If this is done, the return currents of the noisy
circuitry will flow through the ground plane in the low level section. This
noise will modulate the RF section, and be superimposed on the RF output,
possibly causing undesirable spurs, high phase noise in the VCO, etc.
A decoupling network applied in this scenario won't be effective, because
you will be bypassing to a noisy ground system.
Another approach to a quiet design is to implement the topology described
above, but to take it to another level, by breaking up the ground plane,
having the segmented planes connected either by a thin trace, an inductor,
or a ferrite bead.
It all begins at the board layout level.............once you realize that
you can control the DC and AC (RF) current flow on the board, it becomes
much easier to come up with a good 1st or 2nd pass design.
If you do have the power supply components on the same circuit board as the
rest of the circuitry, ALWAYS locate them in the area of the noisy circuitry
On another note.........I did some field testing on the MW receiver today.
Hooked up to a 75 foot antenna, no overload was noted on any of the 50kW
broadcasters in my area. I can tune 20kHz from WBBM 780, WGN 720, and WMAQ
670, and no desense is noted. Part of the secret here is to have your AGC
voltage detected after the final selectivity determining element. I've
noticed with both my Drake SPR-4 and my SW8, that when you are within 10kHz
of a strong station, the modulation sidebands from that station capture the
AGC system. It doesn't have to be this way. Once I get this receiver project
completed, I will look into a way of cleaning up those AGC loops. I do
notice that my TR7 does not exhibit this symptom, to I need to compare the
differences.
starman wrote in message
...
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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