Re: AM RF Signal Generation

Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

Hello Mike,

So on to the next topic of frequency pulling, which as far as I know
has never been run in SPICE due to the long startup and settling
time. This is now trivial since the new method is roughly 2,500 times
faster (please see the update in the paper if you haven't seen it -
Estimating Startup Time.)

If you ever want to look at circuits where that is exploited to
extremes check the vintage SSB transceiver IC-202 from ICOM. It uses
pulled crystals, one per 200kHz segment (plus some margin) of the
144-148MHz band. Very stable, very nice spectral cleanliness, low
power consumption versus a synthesizer. I used that set for years
before giving it to my father.

That really is an extreme example. That is 0.2/144=0.00138, or 1,388ppm.
The crystal probably can't be running in overtone mode, but has to be
multiplied up from the fundamental. Even then, normal crystals won't go
that far. There's a nice comparison of the pulling capability of overtone
vs fundamental in figure 2 of

http://www.horizonxtal.com/info_04.htm

Equation 3 gives the pulling amount as a function of the crystal
capacitances. I betcha the capacitances don't work out to realistic values,
so there is probably some other circuit trick, such as an inductor in
series with the crystal. This may work, but it probably degrades the phase
noise and frequency stability, as is often mentioned in rrah whenever the
topic of pulling comes up. But it may be ok for casual use.

Also, I have definitely come to the conclusion that the frequency
calculation in the paper is incomplete. I need to add the series trim
capacitor to the equations to bring the frequency down slightly to the
expected value. This means redoing all the figures in the paper to reflect
the new component values, which will take time. But I'll try to get to it
this weekend.

After setting the open loop frequency to exactly 1MHz, it will be
interesting to see the actual frequency that Transient Analysis predicts it
will run at. And how much it changes with different operating conditions.

Since the .IC command starts the oscillation in a known phase and
steady-state amplitude, it is easy to go out several hundred cycles and
look at the phase of the oscillator and compare it to the calculated
frequency.

These kinds of investigations were difficult or impossible to do before due
to the long startup and settling time when running the crystal at full Q.

But using the .IC command makes them easy now, so there is a lot of work to
do in confirming all the various modes and circuit parameters that everyone
had to take for granted before.

Regards, Joerg

Regards,

Mike Monett
.

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