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Thread: [EE] Phase locking DRO's
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picon face BY : Robert Rolf email (remove spam text)



David Bengtson wrote:
> Well, you have a somewhat complex problem on your hand. You will need

Yes, it is that, but we're tackling it one piece at a time.
The hard/expensive part, getting big dishes, turned out to be the
easiest step, what with the DTH dishes displacing the BUDs.

> to end up building a Phase locked loop from the cheap DRO's that you
> have. You will also  need to come up with a very stable reference

Yes, that is the plan. Time and access to test equipment we have.
Thousands of dollars for commercial oscillators, we don't.
We also want others to be able to reproduce our work.

> oscillator. Here is a rough ASCII Diagram (Fixed Pitch Font, Please)
>
>   Ref--------Divider-------Phase-------Loop------DRO---------Stable RF
>   Osc.                     Detector    Filter         |
>                              |                        |
>                              |                        |
>                              |--- Divider-------------|
>
> This is the canonical block diagram of a Phase Locked loop. You have

Yes, I'm familiar with it. However, at microwave frequencies, the
output divider is exceedingly difficult to make. I'm looking at
some form of PIN diode sampler (like used in the old TEK 491 SA)
to do the phase detection. The hope is that the DRO's are stable
enough (100Mhz or so) that a good 100Mhz reference will hold them.

> the Section marked DRO, and you need to design a build the rest of it.

Yep, that is the challenge (and learning experience).
The key question I need to have an answer for it can the DRO's found
in consumer LNB's be voltage controlled over a sufficient range to
stay in lock over temperature (-40 to +70 like we get up here).

> Once you do that, you will be able to use this as a stable RF source
> for doing Radio Astronomy and Radio Interferometry. I have seen
> several web sites that talk about amateur radio astronomy, and a good
> web search will turn up several, including ones that talk about
> interferometry. I think that what you want to search for is Project
> Bambi. (I don't name them).

I am familiar with the various web sites. Most of the amateurs
are doing simple total power receivers or waveguide type
interferometers,
with relatively short (meters) baselines. We hope to do better, but
it's a long road yet. MIT has a very nice design built out of standard
parts, but they obviously have a -real- budget.


> GPS correletors are not the thing that you are looking for, although

That was a suggestion from a helpful PICLIST member.
However, the GPS RF chips are working around what would be our IF (since
the GPS industry makes the technology much cheaper to acquire). If one
supplied the 2nd dish signal as the 'chip', one -might- get a cheap
high frequency correlator. Obviously I'll have to look at the approach
in much more detail. Its just a wild thought at the moment.

> you could use a GPS receiver to help with the reference oscillator.

That is one option we are exploring. There is a nice design
(PIC based) that Brook Shera has put together to use the 1 PPS out of a
GPS. We're hoping to modify it a bit for WWVB reception (since it's
cheaper to build, [loopstick, bandpass crystal, op-amp and comparator]).

The plan is to come up with methods that amateurs can duplicate for
themselves without too much effort. Things like building your own
TCXO's with a 12Cxxx part as controller.

> The Accuracy of the reference oscillator determines the accuracy of
> the entire system.

Of course, although here it is more a case of STABILITY than accuracy.
We're not into sub megahertz spectral line analysis (yet), and the
DRO's will probably have too much jitter for that to work in any case.

Basic interferometry will work as long as both LOs are locked together,
but pumping 5150Mhz over 300 Meters (no waveguides) is no easy task. It
also doesn't allow for really wide baseline techniques to be developed.

> I would start with building one dish and then extending it to another
> one.

But one dish does not an interferometer make <G>.
I already do TPR radio astronomy twice a year, during sun-out season.


> This sounds like a fun project.

It's supposed to be. However, since I'm the only RF guy on the 'team'
I'm a bit overwhelmed by how much I have to do and yet learn, and
I need any help I can get to at get pointed in the right direction.
Why reinvent the wheel if someone has already done this sort of thing?

Thank you for your comments.

Anyone have any recommended texts/articles for 'real world', 'hands on'
microwave work? Something like "The Art of Electronics" at GHz.
What I've found to date has all be highly theoretical and not terribly
useful.

Robert.Rolf-at-ualberta.ca

<392A2D80.46B19E9A@UAlberta.ca> 7bit

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