I don't recall seeing that anyone ever reported the official temperature
setpoint for the outer oven. Most of the info on the Z3801A still
resides on realhamradio.com.

I found in my notes a paper printout of a webpage apparently put up
someone named Shegnauer, circa 2006, showing the schematic, and some
info and estimates for the temperature range, and a spice model of the
control system operation. It's a nice document. I tried to find it
online now, but it seems to have disappeared over the years. The same
link showed up in old messages on time nuts and other lists, but it
appears dead now. It must have been pretty well known, so it's likely
still floating around out there at some URL or in someone's files. If
you don't have it already, you should try to find it. I only have the
paper copy.

Anyway, it was suggested that the outer oven should run around 60-65 deg
C, while the thermistor is around 16.2 k ohms there. Personally, I
believe this is too high, and should be around 50-55 deg C or maybe a
bit more - right near the maximum ambient rating for the Z3801A,
neglecting any internal box temperature rise. If the performance specs
are relaxed toward the higher end of operating temperature range, then
the outer oven may not have to run so high. I don't recall, but think
that may be the case.

Ed

Hi Bob,

Can you please be more specific about this? I have a Z3801A project, and
have always assumed the double-oven is a good thing. If it's actually
detrimental, then I'd like to know more. I searched a bit in this
direction, but found nothing to support this view. If it's actually bad,
then I can delete it with a pull of a connector. It's a great
simplification if true.

Reference message:

"On a true double oven. The outer oven improves stability. It is set up
along with the inner oven
and the design is optimized for both ovens. That’s not at all the case
with this oven. It’s just a
warmup heater. It actually makes the stability of the device worse in
most cases. Yes that sounds
crazy. For all the deep math and examples of just why this is true
…. head over to Rick’s set
of papers. It’s all in there ….."

Ed

I would like to figure out for sure whether the outer oven in the Z3801A
OCXO is a benefit or not. Does anyone know of actual experiments,
studies, correspondence, hypotheses, or other documents, or personal
experience/knowledge specifically pertaining to this unit, indicating
problems with the thermal stability or uselessness of the outer oven? I
have seen all sorts of commentary on this over the years, but is it
conjecture, or fact?

I have done some searching around over the years, and recently, and
found nothing specific about it. I may not know best where or how to
look, but I think I'm doing it OK - it appears sparse so far. I'd
appreciate if anyone can post up any links to such info.

I have studied some more general papers found, like Rick's ones about
designing better OCXOs. I know that the Z3801A's OCXO is far from
perfect, and the inner and outer ovens etc are not optimally arranged,
so there may be possible weaknesses, but I'm looking for actual known or
discovered problems in actual use.

If there is a fatal flaw in the design or application of the outer oven,
then I doubt it's worth trying to improve it much - so many cycles of
taking apart and reassembly would be risky to the guts.

If it actually can't be blamed for anything, then I think it should be
beneficial to the thermal stability of the OCXO inside. I think it
should help especially in holdover mode, against temperature fluctuations.

I would think that someone with a Z3801A and the right analysis gear
could do some experiments to look for thermal instabilities
(oscillations etc due to interaction of the control loops) under at
least some typical conditions (but not exhaustive, unfortunately).

Also, it should be possible to see if it helps or not in normal and
holdover modes, with the outer oven working or disabled, while subject
to certain temperature variations. I have a Z3801A, but none of the
other stuff needed to see what's actually going on.

If someone has done some of the above tests (or similar) in the past, a
report on it is a perfect example of the kind of documents I'd like to see.

Ed

Ok, late to the party … lurking around from time to time.

Note that these same web pages have a new home at Stefan Hegnauer's Homepage - Electronics (selfhost.eu) . No new or additional content to what is archived at the links above, though (and thanks for archiving it in the first place!). However, links should point to sensible content.

Yes, I’m the original author of this, long time ago. Feel free to contact me if you have any questions.

-Stefan

Hi Bob,

I think what may be missing is the unknown thermal resistance of the
outermost insulation surface to the ambient air. This can be very messy,
but could be estimated experimentally on an actual thing, including the
conduction, radiation, and convection effects. If there's a draft in the
room, then it's a different deal.

Deleting the foam, say, takes out its 0.04 deg C/W of resistance, but
doesn't short it to ambient temperature. If it were in a water bath or
heavy metal outer box held at Ta, it would be a different story, but
here the air interface is the big part.

Ed

These midget flanged base style bulbs are extensively used in old gear.
I used to have a pretty big collection of salvaged old indicator lamp
fixtures and bulbs, but gradually used them up to now having only a few
of the really nice ones left.

I have done a lot of LED swap-ins with these, to get virtually permanent
bulb life in projects that I build. I gut the glass bulb and adhesive
out of the base, and mount a suitable LED, and design or modify the
electronics to work with the LED. The LEDs never look quite like the
original lamps - sometimes worse, sometimes better - so it usually takes
some fooling around with the jewel and LED colors and added diffusing
materials to be satisfactory.

Doing this for existing equipment almost always needs some internal
mods, which are typically quite easy. I have a fair number of certain
bulbs still good, saved for replacements in various gear. Once they run
out, it will be time for changing to LEDs. I save all the burned out
bulbs as husks for LED transplants.

Here's an interesting aside (I think). The Tek 7000 series scopes use
this style bulb (three of them) for graticule illumination. A few years
ago, I made and installed some "white" LED replacements to see how they
worked. It looked pretty good, and I was happy. Then I took some screen
shots with a phone/camera. The graticules showed up nearly invisible in
the pictures. The false spectrum of the LEDs fooled the eye OK, but the
camera's response was way different.

The lesson is to make sure the LED "color" actually works properly in
all of its applications. The LEDs I used back then were of some whitish
unknown type. I think the more modern high power ones built for lighting
should have a good spectrum for cameras too, but I haven't yet tried the
obvious experiment to find out.

Ed

Maybe the clock option is working just fine, but not applied properly.
It could be that the driver output is an open-collector, meant to pull a
stepper coil or such down to ground for each pulse. Without a load,
you'd see just some small collector voltage due to the base current in
the output transistor - the forward base-emitter voltage, minus the
forward base-collector junction voltage could be in the tens of mV into
high a impedance (like a scope) measurement.

So, try adding a pull-up resistor to draw at least a few mA, and see if
it looks different. Good luck.

Ed

As always on this subject type, I must comment on the utility of
managing ground loops with common-mode chokes on cables between
equipment. I think this is the simplest, most beneficial thing you can
do for good quality signal distribution.

Good cables, shielding, and grounding are all fine and good, but do not
address the ground loop currents that inevitably flow between any
interconnected pieces of gear. Adding CM chokes raises the common-mode
impedance of the cable(s), greatly reducing the loop currents. You can
see these parts all the time, inside equipment, or added or built into
cable assemblies and such - almost always to fix EMC issues. The
simplest are the clip-on type ferrites that are easily added to almost
any cable type.

These are mainly to suppress RF and HF emissions from SMPSs and digital
circuits and such, but they can also be effective even for mains
frequency (and harmonics) ground loop control, with the right cores and
some simple tricks. The reason is that the voltage levels and impedances
are quite low (millivolts and milliohms), so adding even a small
inductive reactance can be quite effective, percentage-wise.

If you have mains frequency ground loop problems, you can try cores with
as high an A sub L as possible, and even better, multiple turns to get
the squaring benefit. There are ferrites available that should do well,
depending on the situation. I have even used regular silicon steel wound
or laminated line frequency transformer cores as CM chokes in certain
situations where very large line rejection was needed.

For narrow-band distribution of 10 MHz for instance, another option is
to use transformer coupling of the signal, so the low frequency ground
current and interference is eliminated, and a high frequency CM choke
can take care of the rest.

Ed

A cold oven is nearly the only and best explanation for that much error.
The good news is that these can be readily opened up and worked on.
There's plenty of info out there.

Ed

Wow - that is scary-burnt. Maybe it's a good idea to have some form of
thermal cutout, even if a nuisance sometimes.

Ed

It's a bit tricky to use prescalers for arbitrary input signals. As
others have mentioned, they self-oscillate when there isn't a valid
input signal. This is not only a possible, but nearly certain
characteristic due to the (typically) ECL input amplifier being biased
right at the logic threshold, followed by lots of gain, then divided by
flip-flops. These were typically used in PLL synthesizers over limited
frequency ranges, with known, adequate power levels.

If you are counting strong signals at well defined DC levels, you can
use ECL directly with proper biasing. For a general purpose counter, you
want lots of sensitivity and wide frequency range, so the usual
prescaler with self-biasing and AC coupling is a simple way to go. You
just have to be aware of the limitations. You can eliminate the
oscillation tendency by giving up some sensitivity, but it's easiest to
just ignore it unless it causes trouble.

I'm quite familiar with the MB series and other prescalers, and have
used them in many projects. All the prescalers I have were salvaged from
old equipment, and I believe that most are long obsolete, since their
function has been mostly absorbed into the modern PLL chips.

Nowadays you can get 100-series ECL parts that can toggle to several
GHz, and in various gates, FFs, and counter types. You could build say,
a divide by 10 or other values with a programmable counter, I believe up
to a couple GHz. I don't recall all the parts and specs, but they're out
there. If you want to make it for arbitrary inputs, you'd first buffer
it with an AC coupled, self-biased line receiver cascade before
counting, and you'd be right back to a self-oscillating - but much more
versatile - prescaler.

So, if you can get this fully assembled, connectorized, ready to go
MB506 module for seven pounds, just do it - maybe get several - if the
specs are suitable for your needs. It beats the hell out a DIY one,
except for not having a nice convenient decade divide ratio. You should
look up the MB506 datasheet to see the actual capabilities - you can
usually extend the low end, for instance, by adding more input coupling
capacitance. Anyway, I don't know if the MB506 is even made anymore, but
if it is, I'd guess it alone would cost more than this module.

There used to be all sorts of prescaler ICs with various ratios,
including 10, so you might find NOS parts, but with the grief of
building something. There's also an old trick in doing counters with
binary prescalers, which is to change the counter clock frequency so the
net overall divide ratio is a nice decade value, but making the clock
and working the decimal points gets tricky. Also, if the binary divide
is big, the counter is correspondingly very slow.

Of course, if you want to get fancier, you can make a PLL/VCO based
converter that replicates the unknown input frequency at say 1/10th. But
you'll still likely have the same sort of prescaler deal involved,
inside of a PLL chip.

Ed

Looks like the classic MC12080 is still in production by Onsemi in the
USD7 range. It can divide by 10, 20, 40, or 80 up to 1.1 GHz spec and
likely can reach over 1.5 GHz. You'd have to build a circuit for it,
unless someone offers a module of some sort.

Ed