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'[OT][EE] Phase locking DRO's'
2000\05\20@011942 by Robert Rolf

picon face
Given the diverse group on this list, perhaps someone can point
me in the right direction to solve this small problem: How to
Phase lock the cheap DROs (dielectric resonant oscillator) used in
consumer LNBs (C and Ku band) so that one can do interferometry?

Commercial PLL LNB's are way out of our price ranges ($1500 vs $50
for non-PLL) and the cheap versions have MUCH better noise figures.
I'm hoping that there is some simple way to control the on board
DRO so that a stable (and phase matched) LO can be achieved.
I've considered piezo electric (too slow). Varactors in proximity
to the cavity (doesn't seem to work but then I don't really know what
I'm doing). And plain old, change the VCC (again not fast enough).

Any suggestions on how one does a phase comparison at Ghz frequencies
(5150MHz) to a 100Mhz reference and then control a DRO would be
appreciated. I've been exploring a PIN diode sampler, but don't
seem to have the right configuration.
I have access to some decent tools, but they just don't
teach this kind of plumbing around here.

The other gotcha is how to ship a 1Ghz or so signal 300 meters (or 3 km)
without incurring huge losses (or outrageous cabling costs).
I assume that the same hardware that the cable people use for their
HFC plants would work. Know anyone with a couple of spare fiber
convertors
they'd like to donate to the cause <G>?


Some background:
With the large number of big dishes (BUDs) being dumped in
favor of the smaller DTH (LSDs), we thought it was a real waste to see
them hitting the dumpster, so we have been taking the great unwanted up
to the roof of our physics building to bootstrap an amateur radio
astronomy observatory. The physical construction is going well, but
it now falls upon me to come with with a decent RF section so that
we can doing meaningful science. This means using interferometry to get
sufficient sensitivity and resolution. (Widely separated dishes can
behave like a single big dish with suitable signal processing. You just
don't get the sensitivity (or the pointing headaches) of a single
large dish). Computers are cheap. Sound cards are cheap (and so are
3D video cards. Subvert the DSP and you've got one heck of a
correlator).

It is trivially easy to build a total power receiver (TPR) from surplus
C band hardware. Any of you who've experienced a sun-out (twice a year
the
sun passes behind the geosynchronous satellite and drowns out their
signals) have already done radio astronomy. It is much more difficult
to do interferometry since you need to either mix the signals from
two widely separated dishes at frequency (and the old LNA's had terrible
noise figures compared to today's LNBs), or have phase matched down
conversion and then mix the signal to get a correlation (and thereby
pull your extraterrestrial signal out of the noise). PLL LNB's are
just too expensive for an amateur group to afford, so I'm hoping to
be able to hack some cheap LNBs to do the job and get us started.

Thank you in advance for your time and suggestions.

spam_OUTRobert.RolfTakeThisOuTspamUAlberta.ca

2000\05\20@100326 by Alan B Pearce

face picon face
>or have phase matched down
>conversion and then mix the signal to get a correlation (and thereby
>pull your extraterrestrial signal out of the noise). PLL LNB's are
>just too expensive for an amateur group to afford, so I'm hoping to
>be able to hack some cheap LNBs to do the job and get us started.

Just a suggestion - I have no experience with doing this - but have a look at
the chips sets used in GPS receivers, especially the correlator control chips
that have up to 12 correlators in them to lock onto the satellites. The two
chipsets I looked at are normally supplied with no code in the micro contained
in the correlator chip. If you get real friendly with a rep, they may donate you
a sample chip.

2000\05\21@023202 by Robert Rolf

picon face
Alan B Pearce wrote:
>
> >or have phase matched down
> >conversion and then mix the signal to get a correlation (and thereby
> >pull your extraterrestrial signal out of the noise). PLL LNB's are
> >just too expensive for an amateur group to afford, so I'm hoping to
> >be able to hack some cheap LNBs to do the job and get us started.
>
> Just a suggestion - I have no experience with doing this - but have a look at
> the chips sets used in GPS receivers, especially the correlator control chips
> that have up to 12 correlators in them to lock onto the satellites. The two
> chipsets I looked at are normally supplied with no code in the micro contained
> in the correlator chip. If you get real friendly with a rep, they may donate you
> a sample chip.

Unfortunately these chipsets are very application specific.
Grossly simplified, they
look for a specific set of spread spectrum digital 'chips' {the pseudo
random spreading sequence unique to each satellite) at a very specific
rate. When they find a matching 'chip' they synchronize and
then provide a 50Hz recovered data frame rate which is subsequently
processed into pseudorange and ultimately location.

The Philips chipsets ARE available with microcode, for a price,
but the signals that are being extracted are so different from what
we look at that this approach would not work. Thanks for the suggestion
though.

Radio astronomy 'correlators' compare the signals from multiple
dishes and provide an output that is higher if the signal is correlated,
and lower is it is not (just random). In it's simplest form a
correlator is just an RF multiplier. Noise * noise= Noise, average
value=0. Signal * Signal = Signal ^2, with some average value.
[ever wonder why they're called 'square law' detectors?]

With noise+signal, the noise averages out to zero, but the -correlated-
noise (i.e. the same signal seen by two different dishes) is non zero.
This is what allows us (and NASA) to see a signal that is buried in
noise. In order for the multiplication to produce a result, the phase
relationship of the two signals must match, so the LO MUST be phase
locked (or you have to do the multiplication at frequency, and at 4Ghz
that's a bit difficult (not to mention the losses in bringing the two
signals together at a central correlator).

This description is a gross simplification of the process since one has
to compensate for many other phase shifts, but it all starts with a
phase locked LO in the downconvertor.

So? How do I phase lock a DRO? Pumping 5150Mhz around to the widely
separated dished just doesn't seem to be a viable alternative.

Robert.Rolf-at-ualberta.ca

2000\05\22@164937 by Peter L. Peres

picon face
Hi,

wrt locking DROs, I know that by tampering with the bias of the oscillator
transistor it is possible to pull the frequency a little bit (a few MHz
maximum at 10 GHz). Assuming that you can select from a number of DRO's,
find two that have the exact same (or near enough) frequency and try to
use them like this. The simplest form of tampering consists in changing
the supply voltage. The less simple one involves changes in the bias
circuit of the oscillator, but for the latter you need the schematics and
instruments. I do not know about the phase noise characteristics of
the DROs but I suspect that they are not very good. They should be
less good than a multiplier chain's followed by a DRO filter. Just an
opinion.

hope this helps,

       Peter

2000\05\22@223250 by David Bengtson

picon face
Well, you have a somewhat complex problem on your hand. You will need
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
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
the Section marked DRO, and you need to design a build the rest of it.
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).

GPS correletors are not the thing that you are looking for, although
you could use a GPS receiver to help with the reference oscillator.
The Accuracy of the reference oscillator determines the accuracy of
the entire system.

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

This sounds like a fun project.


Dave Bengtson

On Sun, 21 May 2000 00:30:07 -0600, in  you wrote:

{Quote hidden}

2000\05\23@030620 by Robert Rolf

picon face
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

2000\05\23@043310 by Alan B Pearce

face picon face
How about the following scenario for your phase locking - I am thinking along
the lines of TV studio camera genlocking.

If each dish can have a DRO as a local oscillator to down convert the signal to
an IF frequency, and each DRO can be phase locked to a stable source. The stable
source is fed by coax or other suitable means - exactly how is unimportant so
long as it is still a stable signal at each dish. The local phase locked loop at
the dish has a means of injecting a stable DC voltage into the loop to provide a
constant phase offset to the oscillator signal. The control of this voltage is
done from a central point to get the IF signal received back at the central
point phase correlated to one of the other dishes which will be used as a
reference signal.

The control point will need a means of correlating each IF signal to the one
being used as a reference, and sending a command out to the phase control
voltage of all the other dishes. If the reference dish has an identical phase
control loop to the other dishes, it may also be possible to use this to adjust
for ageing or some other parameter which may make some of the other loops go
outside control range at extremes of phase angle difference. The Shera
electronics with modified control software would probably be the way to go.

P.S. I just received a couple of D/A converters from Analogue Devices (AD1861)
which are the equivalent of the Burr Brown ones Brooks Shera used, as I want to
build a copy of his frequency standard. I managed to get them as samples N/C!

Also using PICs in the control system, you could drop the OT.

2000\05\23@052541 by Pavel Korensky

flavicon
face
At 01:04 23.5.2000 -0600, you wrote:
>
>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


I made it (with small modification - Lattice PLD chip instead the LSI
logic) and it works really good. I am using it as a master timebase
reference for my various instruments.

But oscillator is a real problem. I tried couple of TCXOs and results was
not good. Finally I bought the OCXO (expensive) and results are much much
better.
Currently, I am trying to make my own OCXO as a hobby project.

>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.

Look at http://www.karlquist.com  - Rick Karlquist was developping OCXOs for
Hewlett-Packard and he knows a lot about the whole problem. On his pages,
there is a very nice and detailed design of ultra high precision OCXO
(which I am trying to duplicate at home). The whole design is patented, but
I got the permission from Rick to use the design for non-profit projects
(because HP is not currently actively using this patent in any design).

The whole oscillator is a bit complex to make (you need lathe and mill),
but the result should be perfect. BTW, he is using PIC16C73 for the PID
control of the oven temperature.

Best regards

PavelK

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2000\05\23@161021 by Robert Rolf

picon face
Alan B Pearce wrote:
> constant phase offset to the oscillator signal. The control of this voltage is
> done from a central point to get the IF signal received back at the central
> point phase correlated to one of the other dishes which will be used as a
> reference signal.

As I understand it, this is approximately how it's done. One also has
to match the cable delays to within an IF wavelength. One also
introduces
an 'offset' in the LO to accommodate the look angle and it's effects
on path length.

> outside control range at extremes of phase angle difference. The Shera
> electronics with modified control software would probably be the way to go.

We're hoping its adaptable to the task. Why reinvent the wheel?

In theory one doesn't even need a D/A, just PWM and a LPF.

Robert.Rolf-at-ualberta.ca

2000\05\24@035232 by Alan B Pearce

face picon face
>One also has to match the cable delays to within an IF wavelength. One also
>introduces an 'offset' in the LO to accommodate the look angle and it's effects
>on path length.

Perhaps it did not come across clearly enough in what I wrote, but I envisaged
that these could be taken care of by the phase offset of the LO. I guess the
problem here is the cable length can be significantly more than 360 degrees at
the LO frequency! Certainly I expected to be able to adjust the look angle
factor with the LO, but I guess the physical spacing again becomes more than 360
degrees.

>In theory one doesn't even need a D/A, just PWM and a LPF.

I would have thought this could introduce a heap of unwanted sidebands on the LO
due to filtering difficulties. Also I figured that the D/A would have much finer
control steps for the LO, without going through the math of just how fine they
may need to be.

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