[time-nuts] Re: pulling some crystals

[Richard Karlquist]

When I was working for Zeta Labs, circa 1978, I was in charge of a huge
order for 5th overtone VCXO's.  My boss told me to use the "standard
Zeta VCXO design".  It had various problems, and I quickly decided to
invent a new design that actually worked right, and quietly put it into
production without alerting my boss.  The new design was wildly
successful and I used it for many years.  It was successful because
there were no adjustments or "factory select" components and every one
we built simply "worked", with any crystal.  There were no "reject"
crystals.  There was nothing special about the crystals, we ordered them
right out of Croven's catalog. 

Start by making a so-called "free running" Colpitts LC oscillator using
a grounded emitter transistor with capacitors from base to ground and
collector to ground and a feedback inductor from collector to base.  I
used a 2N5179 transistor which was "state of the art" in those days, and
would suggest you use an equivalent part in a plastic SMT package.  
Don't use a transistor with a higher fT.  Less is more as the saying
goes. Start by making the base capacitor 10 times the value of the
collector capacitor.  If the oscillator has spurious sidebands, make the
base capacitor larger.  Choose the feedback inductor to make the LC
oscillator operate at the crystal frequency.  Construct the collector
capacitor from several standard value (E12) capacitors in parallel, so
you can hit the crystal frequency within a few percent.  Use 1%
capacitors and 5% inductors.  I have only described the RF aspects.  You
can figure out biasing, such as a resistor in series with the emitter,
etc. 

When you are happy with this foundation oscillator, disconnect the
emitter from ground and instead connect it ground by using what I will
call the crystal tuning network:  consisting of an inductor in series
with a varactor in series with the crystal.  The emitter goes to the
inductor and the other end of the crystal goes to ground.  Add yet
another inductor, this time in parallel with the crystal.  Choose this
inductor to be parallel resonant with the holder capacitance (C-zero) of
the crystal.  The other inductor should be series resonant (at the
crystal frequency) with the varactor when the tuning voltage is in the
middle of its range.  You need to take care of emitter DC current return
to ground with a a series inductor resistor circuit from the emitter to
ground. 

I suggest you start without the varactor network and optimize the LC
oscillator using just the crystal in series with the emitter.  It should
oscillate near the series resonant frequency of the crystal.  Back in
those days, I used an HP4815 vector impedance meter to measure how well
the inductor that resonates the holder capacitor was tuned.  Now a days,
I use any network analyzer and (important) connect the parallel
combination of crystal and inductor from port 1 to port 2, NOT from port
1 to ground.  The goal is the smallest possible value of s21 at the
crystal frequency. 

You'll have to empirically determine what hyperabrupt varactor to use
and how much inductance in series with it.  Fundamentally, you need
enough tuning range to take into account crystal calibration error
(typically 10 ppm) plus any temperature drift plus any aging.  You
should be able to reliably get 50 to 100 ppm pulling range.  All other
things being equal, a 5th OT design has 1/25 the pulling range of a
fundamental. 

Be sure to check for spurious UHF oscillations, that you may not see on
your scope or spec an.  The 2N5179 was famous for oscillating at 1 GHz. 
You can fix this by putting a resistor in series with the collector. 
The HP 10811 oscillator fixed this by having a 62 pF capacitor with
short leads from the collector to somewhere (see the schematic). 

So that's all there is to it.  It always worked for me. 

 ---

Rick Karlquist
N6RK 

On 2023-12-04 18:42, glen english LIST via time-nuts wrote:

> After some commentary, please:
> 
> I built some 98.304 MHz 5th overtone oscillators, the crystals have a 60 deg C turning point, they're kept close to that temp and varactors  pull it around.
> 
> It's a reference for a  UHF PLL. However I am having difficulty with a well reproducable production configuration and pulling. Pulling too much and I see spurious modes.