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