[time-nuts] hp5061A questions

Wed Jun 1 19:39:44 EDT 2005

I agree with Chuck.

Also keep in mind that not only does the A4 perform the function of
matching the internal CBT microwave cavity with the outside world with a
high degree of temperature compensation, the input side of the A4 used a SRD
to generate a hash of sidebands from the input 90 MHz, from which one and
only one sideband was selected. thus the name "Harmonic Generator".

It's a fascinating piece of machined metal and electronics, and is a
conversation piece when employed as a paperweight.

Dave Carlson

----- Original Message -----
From: "Frequency Standards & Services" <chuck at frequencystandards.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Wednesday, 01 June, 2005 12:31 PM
Subject: RE: [time-nuts] hp5061A questions

Hi David:
It isn't so much that the equipment is rare or hard to find, it is just
knowing how to tune them and what to look for when you do. One example of
getting one tuned wrong, you will still get a good signal out of it but the
temperature range that the unit will operate in will be drastically reduced.
You will find that small room changes in the room temperature will cause
serious changes in your stability and sometimes even make them break lock.
I have attended the class that HP used to give on tuning them and I have a
set up for completely rebuilding them. Unfortunately, anymore, it is almost
cheaper to just find someone with a cesium with a bad tube and buy the
Harmonic Generator from them.
On the other hand, if you just want to experiment with one and see what
happens, as long as you have a known good unit to put back in when you are
done, it probably wouldn't hurt anything. Just don't experiment with the
only one you have. As you probably recall from working with wave guide
screws, a tiny movement can make a huge difference in output, frequency,
etc..
Chuck Norton

Frequency Standards & Services
2727 E. Palmer Park, Ste. 100
Colorado Springs, CO. 80909-3032
719-228-0540  voice
719-228-9009   fax

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
Behalf Of David Kirkby
Sent: Wednesday, June 01, 2005 1:15 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] hp5061A questions

Dave Carlson wrote:

> Absolutely. Especially don't mess with any of the mechanical adjustments
on
> the A4 harmonic generator that is attached to the CBT. These are aligned
to
> resonate exactly to 9.192 GHz and it would be almost impossible to realign
> one without using a special waveguide tuning fixture. Only a few of the
> extrusions are intended for matching the A4 to each individual CBT.
>
> If you've already turned some of the knurled screws, nuts, knobs, and hex
> screws, you may never be able to capture the correct resonant peak, even
if
> you can get the cesium oven to start working.
>
> The best remedy to a mis-aligned harmonic generator is to find another one
> that has not been adjusted and attach it to your CBT, or send it to
someone
> with the correct waveguide fixture.
>
> Dave Carlson

As a matter of interest, from someone who knows next to zero about
cesium sources, but a reasonable amount about radio frequency and
microwave engineering, what does the waveguide tuning fixture do? What
is so special about it? Could an amateur fabricate one?

X-bade rectangular waveguide suitable for 9.192GHz is easy to come by
(known in the UK as WG16, but also as WR80 and RG52). I used that very
waveguide as a child in a 10GHz amateur radio system with a Gunn diode.

Tuning of waveguide components can be achieved by inserting screws in
the side - again not too hard to fabricate.

My guess (and note I have never seen a cesium source) is that the
waveguide fixture would temporarily be part of the cavity, so if the
dimensions of that fixture were known, it would not be hard to make it
oneself with access to a milling machine, or get a small engineering
company to do it.

I appreciate from reading what you have said that not touching the
assembly is the best idea, but if someone has "tweaked" one, I would not
have thought getting the cavity back on resonance would be hard, given a
few physical dimensions, and a microwave power meter. Waveguide to N
adapters are pretty easy to fabricate too, although again I suspect they
can be bought used for very little. And microwave power sensors with N
connectors are easy to come by, or again could be made easily.

Just more curious than anything else, as I don't have a cesium source,
and don't feel I will ever become a real "time-nut" and get one.

Soon I should have my bit of kit built which will have both a
10811-60111 and a Standford PRS10 both locked to GPS. I'm sure the two
oscillators will drift relative to each other, so I know I'm going to be
questioning which is the most stable, but I think I will stop before
buying a cesium source.

--
David Kirkby,
G8WRB

Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/

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