[GPS_Standard] VE2ZAZ

[Dave Platt]

grahamh at austin.rr.com wrote:
> Dave:
> 
> You might want to pull a data sheet on the CERMET trimmer you
> are using for course set.  The sample spec sheet I pulled shows
> resistance tolerance of +/- 100 ppM per degree C !

The one I chose looked to be a good deal better than that, if
I recall correctly.  It's a 20-turn precision pot. not just
a small trimmer.

Something I could do, I suppose, is to coarse-adjust the pot to
where I like it, measure it, and then replace it with a voltage-
divider pair of low-tempco fixed resistors.  I've got some Vishay
bulk-metal-foil resistors sitting around and could probably find
a pair which made close to the right ratio.  That would eliminate
at least this one point of potential temperature sensitivity.

> Also review the voltage tolerance versus temperature of the
> single chip regulators.  They are typically not good enough for controlling
> a disciplined oscillator.  You may want to consider precision voltage
> sources, rather than common single chip regulators.
> 
> Consider all voltage tolerances in terms of the frequency control sensitivity
> of the device you are controlling.  You will find that you are dealing
> with control voltages that need to be stable in the range of tens of microvolts
> DC.

Yup... and that's a bit difficult for practical reasons.

In the case of the VE2ZAZ design (as I've implemented it) there seem
to be several points of vulnerability.  The control output voltage from
the VE2ZAZ board is based on the +5 supply on that board (since it
comes from the PIC's PWM output) and can drift.  Switching the
filter op amp to use a separate, more precise set of rails wouldn't
help.  The +5 I feed to my coarse-adjust pot could drift.

Based on my experiments with the Efratom oscillator, I rather strongly
suspect that the +12 being fed to the oscillator itself may be
equally critical.  The oscillator's effective pull-range (when the
control voltage is adjusted from 0 to +5) seems to depend significantly
on the oscillator input voltage... if I reduce the oscillator power to
+9, the adjustment range decreases.  This makes me think that the
internal varicap may be referred to the oscillator's positive rail
voltage... and a drift in this voltage might have the same effect as
drift on the control input.  I couldn't find any sign that the
Efratom provides a "reference voltage" output... I should re-check this.

> What you are fighting might not be the oscillator, but the design
> of the course expander and control circuits you built.

You're quite right.  I tried to be careful in the design and
selection of parts, but it's entirely possible that it's acting
as the performance limiter at the moment.  Warming up various
components on the board with a hot-air pen might tell me if this
is indeed the case.

I wonder, though... would things necessarily be any better if I
bypassed the whole coarse-adjust system, and fed the VE2ZAZ
control-voltage output to the oscillator?  Doing so might simply
make the problem move around... I'd eliminate sensitivity to thermal
drift from the cermet 20-turn pot and the 78L05, but would multiply
by a large factor (5:1 or 10:1) the sensitivity to any drift from the
7805 which powers the VE2ZAZ board.

Thanks for your suggestions and ideas - all good and worth following
up!

73,

	Dave