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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We will have a company fabricate the display, since there will be a spectrum
type function, possibly along with some soft-keys. As far as backlighting,
we need to see what options the manufacturer offers.

Pete

starman wrote in message
...
Pete KE9OA wrote:

Yes, Marvin.................we will be marketing this receiver. Our
software
guy still has to write the programming code for the synthesizer, along
with
the code for the graphics LCD. I should have all of the RF design
completed
by the end of the month, so the ball will be in his corner. I am hoping
for
a March or April ship date, but you know how schedules can slide. As far
as
style, I am thinking of making it about the size of the Drake SW8. Let
me
know what you are looking for, folks!

Pete

Did you have to outsource the LCD display or did you find an off the
shelf one that's suitable? How will you backlight the LCD?


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Thank you! That is exactly what I am shooting for in this design. The
prototype is just using an LM386, and the audio is quite good, but for a
marketable version, I wouldn't dream of using this chip. At the very least,
there will be separate bass and treble controls. The handle sounds like a
good idea, but I still have to work out the mechanical details for the
tiltable, rotatable loopstick.
As we speak, I am working on a homodyne detector, using a 1350 as a limiting
amplifier, feeding an NE602. I am not sure how it will work out, but it
looks promising. I have also decided to have an envelope detector in the
design, for people like myself that like that old time feeling of fading
signals!

Pete

grumpus wrote in message
om...
Pete, best of luck with your endeavor. I'm sure that whatever you
come up with will be more than worthy, but here's my two cents
anyway:) What about the audio? If you could duplicate the deep bass
response and crystalline highs of the Grundig Satellit 600 and 650 you
would have a radio for the ages. That's what we all want, the best
listeners' radio which is also a superb dxing machine. A handle would
be nice also.

Bon Chance,

Grumpus


"Pete KE9OA" wrote in message
...
Yes, Marvin.................we will be marketing this receiver. Our
software
guy still has to write the programming code for the synthesizer, along
with
the code for the graphics LCD. I should have all of the RF design
completed
by the end of the month, so the ball will be in his corner. I am hoping
for
a March or April ship date, but you know how schedules can slide. As far
as
style, I am thinking of making it about the size of the Drake SW8. Let
me
know what you are looking for, folks!

Pete

Marvin Kroll wrote in message
...
Is there any other information available about this receiver?
Is it meant to produced commercially?

"Pete KE9OA" wrote in message
...
The MW receiver is progressing pretty well.................I have
completed
the synthesizer, and am now doing the final pass on the receiver
board
itself. MDS measures at .2uV right now, with strong signal handling
up
to
about 900mV.
Next week, I plan to design the sync detector, but there are a
couple of
things to keep in mind. If there are two closely spaced signals, the
stronger signal will capture the system. For this reason, I may also
include
the envelope detector function.
The initial results are very encouraging....................in the
"RF
Alley" that I live in, with three 50kW MW broadcasters, no overload
problems
were noted. I will keep all of you posted on the progress of the
design.

Pete

Pete,

When you get the time, take a look at the PC board layout of an R8B,
especially the top RF board. I guess the lowest board with the
synthesizer should be considered the noisiest one. It's located under
the chassis, apparently for shielding purposes. The SW8 is similar. I'm
going to revisit my R8B with your PC board grounding suggestions in
mind. Thanks again.

Pete KE9OA wrote:

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.

snipped


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In article
,
"Pete KE9OA" wrote:

I so 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.

This sounds to me like the problem resonance was just moved to a
different frequency by removing vias. The solution should have been to
add more ground vias.

I¹m assuming the situation you are painting is a continuous ground
plane on the bottom with circuit features on the top of the board with
additional ground plane ³flood² on the top in a bid to provide more
isolation between circuit paths or just improving ground on the board.

To get patches of ground plane on the top of the board to behave the
same electrically as ground plane on the bottom the impedance must
remain low relative to the frequency of operation. To accomplish this a
number of vias must connect the patches or areas of ground plane on top
to the continuous ground plane on the bottom. The rule of thumb I use
is a 1/4 wave of the highest frequency of operation. The reason for the
1/4 wave is this is the minimum feature size that is likely to resonate
inadvertently in the design so for 900 MHz that would be about ~ 278 ps
for a 1/4 wave and at ~ 145 ps an inch for a FR4 type dielectric that
would be ~ 1.9 inches to propagate on the board. You don¹t want any
ground plane features on the board top to be any longer than 1.9 inches
without a via to the ground plane below.

For example lets say you pick a via spacing of 1 inch to be safe and
you have two circuit traces going two a mixer on the board. These two
traces start several inches apart on the board and gradually come to
about .5 inches of each other as they approach the mixer. If you put
ground plane between them it will look like a finger pointed at the
mixer and with 1-inch regular grid placement of the vias none might
have connected this finger to the ground plane below. This finger can
then behave as a 1/4-wave stub if it is 1.9 inches long. This can be
fixed by adding (at least) one via at the end of the finger to the
ground plane below lowering the impedance next to the mixer so it can¹t
move electrically.

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.


I never thought of doing this. Thanks for the idea.

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.

I think I understand what you are describing here but I need more detail
to be sure.

--
Telamon
Ventura, California

I don't think I have the service manual for the "B" model; I am not sure if
the original R8 is similar.

Pete

starman wrote in message
...
Pete,

When you get the time, take a look at the PC board layout of an R8B,
especially the top RF board. I guess the lowest board with the
synthesizer should be considered the noisiest one. It's located under
the chassis, apparently for shielding purposes. The SW8 is similar. I'm
going to revisit my R8B with your PC board grounding suggestions in
mind. Thanks again.

Pete KE9OA wrote:

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.

snipped


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I usually go for less than one tenth of a wavelength for maximum spacing
between vias. I never lay out the vias on a grid. This is one of the things
I learned at one of the EMI/EMC classes I took at when I was working at
Rockwell-Collins. I understand that different folks have different
approaches to board design, and these different approaches do work well, my
approach has been ok, with boards I have been designing well up to 5GHz. I
do need to state that I am not the foremost expert in this field; I am just
a simple soul that is scratching the surface of the RF realm!


This sounds to me like the problem resonance was just moved to a
different frequency by removing vias. The solution should have been to
add more ground vias.

We did redo the board layout without increasing the number of vias. We
merely replaced them in a pseudo-random fashion, and the board didn't show
any resonances until we got up to 13GHz, which was well above the 1.8GHz
image band.
Originally, the vias were dropped on a 50 mil grid; 1/20 of a wavelength in
the 1.8GHz image band is 328 mils, so the distance between vias was well
within the desired window.


I¹m assuming the situation you are painting is a continuous ground
plane on the bottom with circuit features on the top of the board with
additional ground plane ³flood² on the top in a bid to provide more
isolation between circuit paths or just improving ground on the board.

To get patches of ground plane on the top of the board to behave the
same electrically as ground plane on the bottom the impedance must
remain low relative to the frequency of operation. To accomplish this a
number of vias must connect the patches or areas of ground plane on top
to the continuous ground plane on the bottom. The rule of thumb I use
is a 1/4 wave of the highest frequency of operation. The reason for the
1/4 wave is this is the minimum feature size that is likely to resonate
inadvertently in the design so for 900 MHz that would be about ~ 278 ps
for a 1/4 wave and at ~ 145 ps an inch for a FR4 type dielectric that
would be ~ 1.9 inches to propagate on the board. You don¹t want any
ground plane features on the board top to be any longer than 1.9 inches
without a via to the ground plane below.

I agree..............to do any less than this will result in unassociated
ground flood, resulting in a sympathetic radiator.


For example lets say you pick a via spacing of 1 inch to be safe and
you have two circuit traces going two a mixer on the board. These two
traces start several inches apart on the board and gradually come to
about .5 inches of each other as they approach the mixer. If you put
ground plane between them it will look like a finger pointed at the
mixer and with 1-inch regular grid placement of the vias none might
have connected this finger to the ground plane below. This finger can
then behave as a 1/4-wave stub if it is 1.9 inches long. This can be
fixed by adding (at least) one via at the end of the finger to the
ground plane below lowering the impedance next to the mixer so it can¹t
move electrically.

At high frequencies, another good technique is to drop at least four vias on
the ground return leads of mixers, MMICs, etc
It looks like you have been in the industry for awhile!


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.


I never thought of doing this. Thanks for the idea.

Anytime! This takes some of the guesswork out of the characterization. I
have even run into improperly designed boards, where the company was too
cheap to add another layer in the form of a power plane, and the power
distribution traces formed a resonant circuit. The designer thought that he
could drop some decoupling capacitors along the power traces, not realizing
that he was creating a transmission line filter up in the microwave region.


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.

I think I understand what you are describing here but I need more detail
to be sure.

What this consists of is removing the resonator from the oscillator circuit,
leaving only the feedback capacitors (collpits circuit)
intact. Next, you connect a network analyzer to this input poing of the
circuit, and set it up for a Smith Chart response, viewing the S11
parameters. The trace you are interested in is the Unity Gain Circle. In the
frequency region where the circuit will function as an oscillator, you will
see a bit of negative resistance. As you adjust the value of Cequiv of the
feedback capacitors, you will see this region mover around. In this way, you
can optimize the circuit, seeing the changes in the imaginary terms.
Another cool thing about this technique is that you predict whether or not
the circuit will have a monotonic response (VCOs)
This negative resistance should be very smooth; if there a small squiggly
loops in the response, the response will not be monotonic. In other words,
if you were working with a VCO, and you had a tuning voltage of 2 to 5V, as
you increse the voltage from 2 to 5V, the frequency of the VCO should
increase at a rate determined by its KV characteristic.
If this isn't the case, for example, suppose you start out with a tuning
voltage of 2V; you will be starting at frequency F. As you increase the tune
voltage, the frequency should now be (F+X), but what can happen at some
tuning voltages is that you actually see the frequency decrease slightly,
only to increase again as you continue to increase the tuning voltage. In
other words, you can have two different tuning voltages that can invoke the
same frequency from the VCO!
Can you imaging trying to design a predictable PLL when this happens?
Oh, one more thing........................about those board resonances that
we were talking about. There was one microwave synthesizer board that I was
characterizing for spurs several years back. All of the spurs were
below -70dBc, but as soon as the unit was installed into the enclosure, the
3rd harmonic rose to -30dBc. This board was mounted on bosses in about 15
different places. I discovered that when I loosened one of the mounting
screws in the middle of the PC board, and adjusted the tension on the screw,
I could use it like a trimmer to null the harmonic down to the original
level. I never did figure out what was going on, and we eventually decided
to place a piece of Kapton tape on the underside of the board, and use a
nylon screw in this location. I did try that RF absorbing foam, and even
that didn't work.
I do realize that this really wasn't a cure..............an old friend of
mine put it perfectly; a problem board is like a water ballon. If you push
into the balloon at one point, it bulges out in another direction. In like
manner, a simple change to change a resonance in one point of the board can
cause another resonance in another part of the board, if the board isn't
designed properly.
Unfortunately, sometimes these problems don't show up until it is too late.
Thanks for the input!

Pete

--
Telamon
Ventura, California

Pete KE9OA wrote:

[...]


As far as
style, I am thinking of making it about the size of the Drake SW8. Let me
know what you are looking for, folks!


Sounds like a cool radio you're developing.
The SW8 size would be perfect. Please put a handle on it, not a stupid shoulder
strap like some portables have. I think a good noise limiter is a must for MW.
I see you're attempting synchronous detection. If you can make it
sideband-selectable with a passband offset control (a-la Drake R8B), you'll
have a winner! If you can't get the synch detector to work well, consider
adding SSB mode and an ANALOG fine tuning knob for manual ECSS instead. Unless
the synch detector works really well, it's pretty much useless (a-la the ICOM
R75). Oh, and be careful that the digital display doesn't radiate digital hash
that can be picked up by the radio's loop antenna.

Other than that, if possible, give it superb audio. And make it a cool color
like red or blue. I'm sick of gray and black radios!

Good luck with your project!





------------------------------------------------------------
Nations are like leaves; they change color before they fall.
------------------------------------------------------------

Pete KE9OA wrote:

I usually go for less than one tenth of a wavelength for maximum spacing
between vias. I never lay out the vias on a grid. This is one of the things
I learned at one of the EMI/EMC classes I took at when I was working at
Rockwell-Collins. I understand that different folks have different
approaches to board design, and these different approaches do work well, my
approach has been ok, with boards I have been designing well up to 5GHz. I
do need to state that I am not the foremost expert in this field; I am just
a simple soul that is scratching the surface of the RF realm!

Given that the highest HF frequency is 30-Mhz, then 1/10 wavelength
would be about 1-meter. This is much larger than the circuit boards in a
radio like the R8, so how important would it be to adhere to the 1/10
wavelength rule for grounding an HF board?


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In article
,
"Pete KE9OA" wrote:

snip

I think I understand what you are describing here but I need more
detail to be sure.

What this consists of is removing the resonator from the oscillator
circuit, leaving only the feedback capacitors (collpits circuit)
intact. Next, you connect a network analyzer to this input poing of
the circuit, and set it up for a Smith Chart response, viewing the
S11 parameters. The trace you are interested in is the Unity Gain
Circle. In the frequency region where the circuit will function as an
oscillator, you will see a bit of negative resistance. As you adjust
the value of Cequiv of the feedback capacitors, you will see this
region mover around. In this way, you can optimize the circuit,
seeing the changes in the imaginary terms. Another cool thing about
this technique is that you predict whether or not the circuit will
have a monotonic response (VCOs) This negative resistance should be
very smooth; if there a small squiggly loops in the response, the
response will not be monotonic. In other words, if you were working
with a VCO, and you had a tuning voltage of 2 to 5V, as you increse
the voltage from 2 to 5V, the frequency of the VCO should increase at
a rate determined by its KV characteristic. If this isn't the case,
for example, suppose you start out with a tuning voltage of 2V; you
will be starting at frequency F. As you increase the tune voltage,
the frequency should now be (F+X), but what can happen at some tuning
voltages is that you actually see the frequency decrease slightly,
only to increase again as you continue to increase the tuning
voltage. In other words, you can have two different tuning voltages
that can invoke the same frequency from the VCO! Can you imaging
trying to design a predictable PLL when this happens?

Using the network analyzer to measure the reactance of the feedback
circuit looks like a good way to characterize its response.

If the VCO described above was used as part of a PLL it would lead to
jitter problems.

Oh, one more thing........................about those board
resonances that we were talking about. There was one microwave
synthesizer board that I was characterizing for spurs several years
back. All of the spurs were below -70dBc, but as soon as the unit was
installed into the enclosure, the 3rd harmonic rose to -30dBc. This
board was mounted on bosses in about 15 different places. I
discovered that when I loosened one of the mounting screws in the
middle of the PC board, and adjusted the tension on the screw, I
could use it like a trimmer to null the harmonic down to the original
level. I never did figure out what was going on, and we eventually
decided to place a piece of Kapton tape on the underside of the
board, and use a nylon screw in this location. I did try that RF
absorbing foam, and even that didn't work. I do realize that this
really wasn't a cure..............an old friend of mine put it
perfectly; a problem board is like a water ballon. If you push into
the balloon at one point, it bulges out in another direction. In like
manner, a simple change to change a resonance in one point of the
board can cause another resonance in another part of the board, if
the board isn't designed properly. Unfortunately, sometimes these
problems don't show up until it is too late. Thanks for the input!

You had the board working by itself and you then screwed it to a metal
frame, which provided additional ground paths between different parts of
the board. Apparently that middle spot was either a noisy part of the
board or the sensitive part of the board.

You changed the impedance of the path by adjusting the screw. I¹ll bet
the spur got worse as the screw was tightened lowering the impedance of
the new problem path.

The problem was the new conductive path not radiated which is why the
lossey foam did not help.

Using the insulated screw is similar to dividing power or return planes
in a board. You are directing noise currents so they aren¹t a problem.
I¹ve seen notches in posts and plates from world-class manufactures of
test equipment for the same reason. Sometimes it¹s the only thing you
can do to solve a coupling problem.

--
Telamon
Ventura, California