poor man's 21cm interferometer

Michiel Klaassen

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Nov 26, 2017, 8:25:34 AM11/26/17

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Hi All,

I am thinking of a test with multiple bucket antennas (like the one I described for Jon), each with an amp, wired together.

Each bucket cost 3.50 euro and a noisy in-line amplifier is 5 euro, a splitter 4 euro, or an F-type T-junction 1 euro.

BTW; there is a 47 ohm resistor in the input line of the amplifier which can be shorted.

Placed on a row or on a tilting plank and integrating again 5 minutes for each of the 288 result scans in 24H, could it give a fringe-less end result. (long axis parallel with galaxy plane)

Regards,

Michiel

DSC01616b.JPG

Marko Cebokli

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Nov 26, 2017, 12:09:17 PM11/26/17

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Hello Michiel,

you did not mention what kind of backend (bandwidth) you intend to use, but with an RTL dongle after the combiner, fringes from the Sun shouldn't be too hard - except that here are a lot of artificial signals on L band, from aircraft and satellites, which might jam you.

As for the amplifiers, I would prefer SPF5189 based ones from Ebay, I bought a few, at <10 euro each, and theay aren't bad. Of course, you coud stumble on a mine... :-)

LNA4ALL is also a good alternative. Much less noise and probably also more resistant to off-band overload. That 47ohm resistor at the input really doesn't give me a good feeling :-) Might provide a good (forced) match, albeit a very noisy one.

Marko Cebokli

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Marcus D. Leech

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Nov 26, 2017, 6:10:20 PM11/26/17

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On 11/26/2017 12:09 PM, Marko Cebokli wrote:

Hello Michiel,

you did not mention what kind of backend (bandwidth) you intend to use, but with an RTL dongle after the combiner, fringes from the Sun shouldn't be too hard - except that here are a lot of artificial signals on L band, from aircraft and satellites, which might jam you.

As for the amplifiers, I would prefer SPF5189 based ones from Ebay, I bought a few, at <10 euro each, and theay aren't bad. Of course, you coud stumble on a mine... :-)

LNA4ALL is also a good alternative. Much less noise and probably also more resistant to off-band overload. That 47ohm resistor at the input really doesn't give me a good feeling :-) Might provide a good (forced) match, albeit a very noisy one.

Marko Cebokli

A slightly-more-exotic approach using unsynchronized RTL-SDRs is shown here:

http://www.ccera.ca/files/DTP_RX.pdf

Marko Cebokli

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Nov 27, 2017, 10:18:24 AM11/27/17

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Nice idea!

Marko Cebokli

Michiel Klaassen

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Nov 27, 2017, 10:33:46 AM11/27/17

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Hi Marko and Marcus,
Thank you for your ideas, perhaps later I will upgrade to a higher level, because the idea now was to make a poor man's interferometer.
The in-line amp has a very robust casing (see picture) and can be dc fed through the coax cable but the SPF5189 LNA needs an additional casing and dc cabling.
Also sma connectors are more expensive, and the required coax idem.
As a Dicke kind of test I want to switch the in-line amp off and on; later.

Assuming that the half power beam width (HPBW) = 66*lambda/D, then with Lambda=21cm and D=15cm, HPBW=92 degrees.
If we want no negative fringes then the radio signals phase to each CAN may not differ more than 180 degrees or 1/2 lambda;=10cm over the half angle of the HPBW.
Then the distance between the two CAN's may not be more then 10/sin(90-92/2) = 14 cm. So basically placing the CAN's next to each other.

Michiel

2017-11-27 15:18 GMT+00:00 Marko Cebokli <s57...@hamradio.si>:

Nice idea!

Marko Cebokli

On Sunday, November 26, 2017 06:10:15 PM Marcus D. Leech wrote:

On 11/26/2017 12:09 PM, Marko Cebokli wrote:

Hello Michiel,

you did not mention what kind of backend (bandwidth) you intend to use, but with an RTL dongle after the combiner, fringes from the Sun shouldn't be too hard - except that here are a lot of artificial signals on L band, from aircraft and satellites, which might jam you.

As for the amplifiers, I would prefer SPF5189 based ones from Ebay, I bought a few, at <10 euro each, and theay aren't bad. Of course, you coud stumble on a mine... :-)

LNA4ALL is also a good alternative. Much less noise and probably also more resistant to off-band overload. That 47ohm resistor at the input really doesn't give me a good feeling :-) Might provide a good (forced) match, albeit a very noisy one.

Marko Cebokli

A slightly-more-exotic approach using unsynchronized RTL-SDRs is shown here:

http://www.ccera.ca/files/DTP_RX.pdf

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Marko Cebokli

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Nov 27, 2017, 12:45:25 PM11/27/17

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Hello Michiel,

if you'll put the cans together, you will get about one fringe per day, which will be hard to distinguish from thermal drifts etc.

I think about 10m would be a nice baseline, to get a decent fringe frequency.

Marko Cebokli

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Michiel Klaassen

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Nov 28, 2017, 4:03:30 AM11/28/17

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Hi Marko,

No it is not as simple as that. As I have said before I want to have no fringes, so actually I want to have a synthesized interferometer beam.
The principle is given here
http://www.cv.nrao.edu/course/astr534/Interferometers1.html

And att is a screen print.
With two antennas you get the well known fringes, but with 4 antennas you can synthesize a nice beam again.
How to calculate that, I have to find out, but in the article the 4 antenna ceperations are given as projected lengths; b6, 2b6, 3b6, 4b6, 5b6, and b.

Michiel

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Paul Oxley

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Nov 28, 2017, 9:58:59 AM11/28/17

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Michiel

We are considering a concept using the Lime SDR board that freezes the fringes by sampling over a very short period of time. This can be done with the Lime Board since the sample rate can be set to a maximum of 640 M Samples per Second. The concept is analogous to setting the shutter speed very high to stop action in a picture. With the digital cameras, it would be analogous to using a strobe light with a very short burst of light.

What you have left with the fringes frozen is the differences in wavelengths from the arrival time on different angles. You also need to remove the residual fringe peak/valley from the point in time where it was frozen. This appears to be able to be removed by averaging over a small number of adjacent samples.

We haven't built the hardware nor software yet. So we do not have any results to prove the concept.

Paul

From: Michiel Klaassen <vmin...@gmail.com>
To: sara...@googlegroups.com
Sent: Tuesday, November 28, 2017 4:03 AM
Subject: Re: [SARA] poor man's 21cm interferometer

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Jim Abshier

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Nov 28, 2017, 12:53:19 PM11/28/17

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Michael,

In 1997 or 1998 I did a one-dimensional aperture synthesis experiment that seems to be like what you are interested in doing. I collected 4 sets of fringe data with different baselines and combined them to produce a one-dimensional synthesized image of a point source (Cassiopeia A). The fringe data was collected with a 400 MHz phase switched interferometer that used helical antennas. A description of the experiment was published in the January/February 1998 issue of the SARA Journal. Back issues of the SARA Journal can be obtained from a CD that is available from SARA. I don't know if I still have a copy of the original submitted article, but I will look for it if you are interested.

Jim Abshier

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Michiel Klaassen

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Nov 28, 2017, 1:27:30 PM11/28/17

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Interesting project you are considering Paul.
If you want me to test your HW&Sw for this or other projects, please let me know
Regards,
Michiel

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Michiel Klaassen

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Nov 28, 2017, 1:30:02 PM11/28/17

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Hi Jim,

That sounds interesting; if you can find a copy, yes, please send it to me.

Regards,

Michiel

Paul Oxley

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Nov 28, 2017, 6:00:23 PM11/28/17

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Michiel

Thanks for the interest. We will keep you in mind when we get more done. The first step may be the opportunity to comment on a draft paper on the process.

Paul

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Jan Lustrup

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Nov 29, 2017, 10:54:13 AM11/29/17

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Hi all,

I just receive an AD8302 board from E-bay I bought for $15.-

Then I gave it a try instead of my trusty M1J double balanced mixer to see how it works for interferometry. The AD8032 has phase detection with low RF levels of only -60dBm. This much lower then what the M1J double balanced mixer needs for a multiplying d.c. output.

Here is first light image made of the sun on my new 21cm interferometry setup using AD8302 in “phase detector” mode.

Image under is first light of “Taurus A” with same setup except the coax ends are joined at a SMA “T” connector for “adder mode” and then feeding the AD8302 in “amplitude” mode.

The vertical displayed unit is “Voltage” and not “dB” as shown in the chart.

My Setup: Two antennas (2,3m dish and “Horn of plenty”), Vlna, 25meter LMR400 coax run, lna, 21cm band pass filter, Vlna, AD8302 phase detector, integrator/voltage amp, lab jack U3 to PC running Radioskypipe.

The Vlna’s are “G4DDK” type with 0.24dB NF and 39dB gain.

The lna’s are PSA3+ type with 20dB gain. (E-bay)

The 1420MHz band pass filter has 1MHz BW (used on E-bay and adjusted for 1420MHz)

The phase detector is AD8302 Amplitude Phase Detector module (E-bay).

The homemade integrator/DC amp is with a “TL082” dual op amp.

So far the AD8302 board look promising for use in radio astronomy.

sun with noise.png

Taurus A

1st light taurus a.jpg

image001.png

image004.jpg

image007.jpg

image010.jpg

Wolfgang Herrmann

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Nov 29, 2017, 11:00:10 AM11/29/17

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We are using the AD8302 successfully on a Ku-band interferometer.

Good device for that purpose.

Wolfgang

Von: sara...@googlegroups.com [mailto:sara...@googlegroups.com] Im Auftrag von Jan Lustrup
Gesendet: Mittwoch, 29. November 2017 16:54
An: sara...@googlegroups.com
Betreff: [SARA] A 21cm interferometer setup using an AD8302 Phase detector

So far the AD8302 board look promising for use in radio astronomy.

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Walter Clark

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Nov 29, 2017, 7:43:59 PM11/29/17

to 'Wolfgang Herrmann' via Society of Amateur Radio Astronomers

We are using the AD8302 successfully on a Ku-band interferometer.

Good device for that purpose.

Wolfgang

Wait.
I thought the AD8302 was go only up to 2.7 GHz?

Jim Sky

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Nov 30, 2017, 12:15:14 AM11/30/17

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Jan, that is fantastic news. I have been playing with the AD8302 for a while but never took it to the final test like you. Congratulations. Very nice.

Jim S

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Michiel Klaassen

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Nov 30, 2017, 11:13:55 AM11/30/17

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Hi All,
To my surprise I saw in another post again fringes, or to be more specific; dc fringes of a "21cm hydrogen line frequency".
In my opinion these fringes have nothing to do with the hydrogen signal; that is a deception.
I thought that was already explained in an older post.

And yes, then the commercial salesman kicks in immediately ; of course.

What I do find fantastic is the accomplishment of Jim Abshier.
Ten years ago he did a test to show the principle of beam synthesis.
The result is attached.
He did not know how long the lenghts of the sections had to be for an optimal resulting beam, but just tried and the result is very good.
Again; congratulations.

I hope that I can reach that level with a full 21cm hydrogen spectrum.

Regards,
Michiel

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Wolfgang Herrmann

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Nov 30, 2017, 2:25:47 PM11/30/17

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Sorry, I was a bit short in my comment. The AD8302 is working on the IF of the Ku-band LNBs which is in L-Band.

Wolfgang

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Michiel Klaassen

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Feb 11, 2018, 11:09:01 AM2/11/18

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Futher to this subject; let us say we have two antennas and that the sun passes the beams.

When you add the signals directly or via a reverse splitter, then, when the sun passes, you get fringes.

We do not want that, because the amplitude of the signal then varies between 0 and 2X signal. That looks nice, but you can do nothing with it.

To correct that, we have to add a delay in one of the antenna cables. That can be done with extra cable lengths.

When we choose a very course method, the minimum would be adding in/out; 1/2 lambda.

When we observe at 1420MHz we have to switch in/out; 21cm/2=10.5cm.

We have to do the switching-over every time during the time the source is visible in the beam.

The result will be the same as like a rectified sine in a diode bridge; result is better, but no joy yet.

When the distance of the antennas is increased, then we have to increase the switching rate.

If we want to continue during a 24H drift measurement, then we have to do the switching during this entire session.

Best is then, to make an automatic switch.

Of course the accuracy can be improved, like with correction every 1/4, 1/8 and 1/16 lambda or better. Then you need to switch between more sections etc.

When you add a third antenna then you have to switch delays for that antenna at a different time; further away from the central antenna means more frequent switching etc.

I found a nice schematic diagram by Iulian Rosu. See also PDF source.

The lenghts of the cable sections would be small resp; 1.31, 1.62, 5.24 and 10.48 cm. This could also be made as a stripline.

When you take a standard FR5 PCB then the Er=4.2, and then the track width would be 1.4mm for 75 Ohm.

If you take a double sided pcb, then the space gap between the track and ground has to be 3.5mm minimum.

So perhaps I can do that by cutting/peeling the top layer of the pcb.

The switching could b done by PIN diodes like Mouser 755-RN242CST2RA.

The length L1 can be a convenient length.

The rest of the circuit could be something like the "LED DICE with Slow Down" see;

http://www.talkingelectronics.com/projects/30%20LED%20Projects/30%20LED%20Projects.html

I have not built it yet; perhaps there are other proven solutions.

I hope this will generate some (better) ideas.

Regards,

Michiel

Phase_Shifters-Iulian Rosu.pdf

4 bit phase shifter05.JPG

Michiel Klaassen

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Feb 11, 2018, 1:51:50 PM2/11/18

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Or drive the shifter with a Velleman board type K8056 trough RS232. Can be extended with numerous boards/antennas and use python sw.

Dave Typinski

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Feb 11, 2018, 2:39:20 PM2/11/18

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Hi Michiel,

An antenna array set up for interferometry (which usually has element spacing >
lambda) is not ideal to act as a phased array radio telescope (which usually has
element spacing < lambda/2).

If one wants to record a specific target at 1420 MHz across time, then yes, the
fringes of an interferometer are bad. Steering the beam electrically (using
delay lines) is one solution, as you've described. One might also wish to move
the elements closer together (closer then lambda/2) to eliminate the grating
lobes and provide a greater main beam directivity.

A 1420 MHz array with a small number of elements is physically small and light
weight. One could mount the array on a motorized equatorial mount to keep the
target at the array's natural beam centerline continuously. This could be built
quite inexpensively and would avoid the discontinuous jumps associated with
delay line beam steering.

A third option would be to put the array flat on the ground and digitize &
record each element's signal separately, then do the phase shifting and power
combining (and thus the beam forming) in software as a post-processing operation
(like LOFAR and LWA). SDR dongles (one for each array element) are inexpensive;
but, their local oscillators must be tied together (identical frequency and
phase) for this approach to work. The advantage is that one can re-run the beam
forming operation on the same raw data as many times as required to form beams
toward different locations.
--
Dave

> <mailto:vmin...@gmail.com>>:

>
> Hi All,
> To my surprise I saw in another post again fringes, or to be more specific;
> dc fringes of a "21cm hydrogen line frequency".
> In my opinion these fringes have nothing to do with the hydrogen signal;
> that is a deception.
> I thought that was already explained in an older post.
>
> And yes, then the commercial salesman kicks in immediately ; of course.
>
> What I do find fantastic is the accomplishment of Jim Abshier.
> Ten years ago he did a test to show the principle of beam synthesis.
> The result is attached.
> He did not know how long the lenghts of the sections had to be for an
> optimal resulting beam, but just tried and the result is very good.
> Again; congratulations.
>
> I hope that I can reach that level with a full 21cm hydrogen spectrum.
>
> Regards,
> Michiel
>
>
>

> 2017-11-28 23:00 GMT+00:00 Paul Oxley <oxl...@att.net <mailto:oxl...@att.net>>:

>
> Michiel
>
> Thanks for the interest. We will keep you in mind when we get more done.
> The first step may be the opportunity to comment on a draft paper on the
> process.
>
> Paul
>
>

> --------------------------------------------------------------------------------
> *From:* Michiel Klaassen <vmin...@gmail.com <mailto:vmin...@gmail.com>>
> *To:* sara...@googlegroups.com <mailto:sara...@googlegroups.com>
> *Sent:* Tuesday, November 28, 2017 1:27 PM
>
> *Subject:* Re: [SARA] poor man's 21cm interferometer

>
> Interesting project you are considering Paul.
> If you want me to test your HW&Sw for this or other projects, please let
> me know
> Regards,
> Michiel
>
> 2017-11-28 14:57 GMT+00:00 Paul Oxley <oxl...@att.net

> <mailto:oxl...@att.net>>:

>
> Michiel
>
> We are considering a concept using the Lime SDR board that freezes
> the fringes by sampling over a very short period of time. This can
> be done with the Lime Board since the sample rate can be set to a
> maximum of 640 M Samples per Second. The concept is analogous to
> setting the shutter speed very high to stop action in a picture.
> With the digital cameras, it would be analogous to using a strobe
> light with a very short burst of light.
>
> What you have left with the fringes frozen is the differences in
> wavelengths from the arrival time on different angles. You also need
> to remove the residual fringe peak/valley from the point in time
> where it was frozen. This appears to be able to be removed by
> averaging over a small number of adjacent samples.
>
> We haven't built the hardware nor software yet. So we do not have
> any results to prove the concept.
>
> Paul
>
>

> --------------------------------------------------------------------------------
> *From:* Michiel Klaassen <vmin...@gmail.com
> <mailto:vmin...@gmail.com>>
> *To:* sara...@googlegroups.com <mailto:sara...@googlegroups.com>
> *Sent:* Tuesday, November 28, 2017 4:03 AM
> *Subject:* Re: [SARA] poor man's 21cm interferometer

>
> Hi Marko,
>
> No it is not as simple as that. As I have said before I want to have
> no fringes, so actually I want to have a synthesized interferometer
> beam.
> The principle is given here
> http://www.cv.nrao.edu/course/ astr534/Interferometers1.html

> <http://www.cv.nrao.edu/course/astr534/Interferometers1.html>

>
> And att is a screen print.
> With two antennas you get the well known fringes, but with 4
> antennas you can synthesize a nice beam again.
> How to calculate that, I have to find out, but in the article the 4
> antenna ceperations are given as projected lengths; b6, 2b6, 3b6,
> 4b6, 5b6, and b.
>
> Michiel
>
>
> 2017-11-27 17:45 GMT+00:00 Marko Cebokli <s57...@hamradio.si

> <mailto:s57...@hamradio.si>>:
>
> __

> <mailto:s57...@hamradio.si>>:

> Nice idea!
> Marko Cebokli
> On Sunday, November 26, 2017 06:10:15 PM Marcus D. Leech wrote:
> On 11/26/2017 12:09 PM, Marko Cebokli wrote:
> Hello Michiel,
> you did not mention what kind of backend (bandwidth) you intend
> to use, but with an RTL dongle after the combiner, fringes from
> the Sun shouldn't be too hard - except that here are a lot of
> artificial signals on L band, from aircraft and satellites,
> which might jam you.
> As for the amplifiers, I would prefer SPF5189 based ones from
> Ebay, I bought a few, at <10 euro each, and theay aren't bad. Of
> course, you coud stumble on a mine... :-)
> LNA4ALL is also a good alternative. Much less noise and probably
> also more resistant to off-band overload. That 47ohm resistor at
> the input really doesn't give me a good feeling :-) Might
> provide a good (forced) match, albeit a very noisy one.
> Marko Cebokli
> A slightly-more-exotic approach using unsynchronized RTL-SDRs is
> shown here:
>
> http://www.ccera.ca/files/DTP_ RX.pdf

> <http://www.ccera.ca/files/DTP_RX.pdf>

>
>
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Michiel Klaassen

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Feb 13, 2018, 5:28:02 AM2/13/18

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Hi Dave thank you for your remarks.

Yes I know; placing the antenna's closer together; see the nice result of Jim Abshier earlier on this subject.

But I want to try to get a ridiculous fine beam; so placing the antenna's as far as possible from each other; but no fringes.

At thesame time I am also experimenting indeed with more dongles on one pc; when executing they each use a complete core; freezing the dual pc; funny.

regards

Michiel

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Michiel Klaassen

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Apr 17, 2018, 1:47:18 PM4/17/18

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First results with a simple interferometer on 1420MHz.

I am still experimenting with a coherent dongle system.

Simply said; two RTL dongles with one xtal.

The idea is to record both data streams and multiply them in software.

Yes; connecting the two antennas directly by adding the signals in a 'splitter' is easy.

However the phase info from the two coherent radios get lost up to now.

A am trying to copy the result of David Lonard but now with a 'poor mans' solutions.

In the mean time I only have the direct interferometer result.

However, with this result we can also do some nice calculations.

Like; Calculate the actual E/W distance between the two dishes when it is not easy to get that with a measure or rope.

Second; Calculate the radio solar diameter.

The distance of the two dishes can be calculated by

Dcalc=lambda/cos(DEC)/sin(360/24/3600*fringe time)= 16.3m

The fringe time or frequency can also be calculated by fft of the fringe time graph.

Second, the diameter of the sun is calculated from a Taylor expansion;

Diam[minutes]=180/phi*60*(sqrt(6)/phi*lambda/Dcalc*sqrt(1-(frma-frmi)/(frma+frmi))

where frma is fringe max value and frmi is fringe minimum value.

Result; diameter radio sun=30 minutes; (nice, I think)

I have not seen these calculations on SARA yet, so that is the reason to mention it.

Later I will put it also a a project on www.parac.eu

regards

Michiel

2018-02-11 19:39 GMT+00:00 Dave Typinski <dav...@typnet.net>:

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##1601.06161.pdf

fringe04.PNG

David L, N5OIQ

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Apr 17, 2018, 5:56:04 PM4/17/18

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I wrote some code a while back to align sample streams from two SDRs that share a common clock but where the signal streams have a delay. It should work with saved data streams from two or more rtl-sdrs. This link provides some Python code that I used and describes the (tedious) process.

https://groups.google.com/forum/#!topic/amateur-radio-interferometry/k5rmMLVmsPI

More recently, I've got a LimeSDR and found that its two receiver channels are nice and coherent and fringes can be seen on-the-fly and I would suggest it as a nice upgrade to rtl-sdrs. They are not available yet, but I would look into this new SDR that is cheaper than the LimeSDR called XTRX.

https://www.crowdsupply.com/fairwaves/xtrx

I've pre-ordered a XTRX 'Octopack' which will have 16 coherent receivers. That should be an interesting product that can generate 120 baselines and can do some serious science.

David

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Marcus D. Leech

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Apr 17, 2018, 6:08:29 PM4/17/18

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On 04/17/2018 05:56 PM, David L, N5OIQ wrote:

I wrote some code a while back to align sample streams from two SDRs that share a common clock but where the signal streams have a delay. It should work with saved data streams from two or more rtl-sdrs. This link provides some Python code that I used and describes the (tedious) process.

https://groups.google.com/forum/#!topic/amateur-radio-interferometry/k5rmMLVmsPI

More recently, I've got a LimeSDR and found that its two receiver channels are nice and coherent and fringes can be seen on-the-fly and I would suggest it as a nice upgrade to rtl-sdrs. They are not available yet, but I would look into this new SDR that is cheaper than the LimeSDR called XTRX.

https://www.crowdsupply.com/fairwaves/xtrx

I've pre-ordered a XTRX 'Octopack' which will have 16 coherent receivers. That should be an interesting product that can generate 120 baselines and can do some serious science.

David

Be aware that while they're nicely coherent, you'll still need to calibrate-out the phase-offset between units both due the frac-N synthesis
paradigm, and the fact that there's no way (currently) to have them all start streaming at the same time with an external event like
a PPS pulse or similar.

David L, N5OIQ

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Apr 17, 2018, 7:17:54 PM4/17/18

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Here is their blurb on the Octopack.

The Octopack is based on a switch that routes between a single x4 PCIe 2.0 card and eight x1 PCIe 2.0 cards. This means you can’t simultaneously utilize the full bandwidth of all eight XTRX boards, but it’s still capable of running LTE-A and other applications. All eight XTRX boards on an Octopack can be synchronized using the included sync board, which has a more stable clock generator and connects via the included FPC cable to the first XTRX. Using external clock synchronization ports (CLK_IN/PPS_IN), it’s possible to synchronize multiple Octopacks, thus creating 32 x 32, 64 x 64, or even larger MIMO systems.

They seem to be claiming that it will synch on Clk_in and PPS-in. I suppose that I will have to find out when I've got the Octopack in my hands if the phase will be the same every time I fire it up. From my understanding and playing with closure phase, modeling is used to cope with any phase offsets so I think that this is not a big problem anyways. Complex phase visibility averaged data can be phase aligned at a post-processing step.

Marcus D. Leech

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Apr 17, 2018, 7:55:49 PM4/17/18

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On 04/17/2018 07:17 PM, David L, N5OIQ wrote:

Here is their blurb on the Octopack.

The Octopack is based on a switch that routes between a single x4 PCIe 2.0 card and eight x1 PCIe 2.0 cards. This means you can’t simultaneously utilize the full bandwidth of all eight XTRX boards, but it’s still capable of running LTE-A and other applications. All eight XTRX boards on an Octopack can be synchronized using the included sync board, which has a more stable clock generator and connects via the included FPC cable to the first XTRX. Using external clock synchronization ports (CLK_IN/PPS_IN), it’s possible to synchronize multiple Octopacks, thus creating 32 x 32, 64 x 64, or even larger MIMO systems.

MIMO systems only require ongoing phase-coherence, and they, like us, tend to have algorithms for calibrating-out phase offset.

The "ideal" case, of course, is that your receiver "array" has a shared LO across all mixers, rather than a shared clock to drive a PLL synthesizer.
The USRP X310 with a pair of TwinRX boards gives you 4 completely-coherent receivers when in "shared LO" mode. But the "pricing envelope"
is rather different.... :)

They seem to be claiming that it will synch on Clk_in and PPS-in. I suppose that I will have to find out when I've got the Octopack in my hands if the phase will be the same every time I fire it up. From my understanding and playing with closure phase, modeling is used to cope with any phase offsets so I think that this is not a big problem anyways. Complex phase visibility averaged data can be phase aligned at a post-processing step.

Indeed, you can "fix it in post" as they say in the movie industry. If fact, if you do the peak-finding and alignment of all your interferograms in
post-processing, then you can relax the coherence requirement to each *baseline* has to be processed by a two-channel receiver that is internally
coherent, but not necessarily coherent with any of the other receivers, so you form your baselines analog-wise, rather than in the sample-plane.
This changes the number of receivers required from N to N(N-1)/2 (the number of unique baselines), assuming, of course, that you want to do
all possible baselines at the same time, in real-time. For stationary sources, of course, you don't have to process all baselines at once, and you can
take *weeks* to fill-out the u,v plane with enough baselines to form images....

David L, N5OIQ

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Apr 17, 2018, 8:40:04 PM4/17/18

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From what I am reading, all 16 receivers share the same clock. All are driven by a single LO.

Marcus D. Leech

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Apr 17, 2018, 8:50:01 PM4/17/18

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On 04/17/2018 08:40 PM, David L, N5OIQ wrote:

From what I am reading, all 16 receivers share the same clock. All are driven by a single LO.

No, they are driven by a common *reference clock*, which each of the individual *synthesizers* use. Since said synthesizers operate in frac-N
mode, they will "lock" at an arbitrary phase position wrt to the 10MHz reference clock.

Near as I can tell, there is NO provision on the LMS7002M chip (which is what is used by the XTRX) for export/import of the actual LO signal.

So, all those synthesizers will "march together", but perhaps out-of-phase.

Now *SOME* frac-N synthesizers have a "phase reset" feature that can be leveraged to drag all of the LOs into precise phase alignment, but again,
the LMS7002M doesn't, as far as I can tell, have this feature.

These chips were designed for telecom applications, where the overwhelming functional requirement is for MIMO support, which requires phase-coherence,
but not necessarily zero mutual phase-offset.

Even with a fully-shared-LO system, phase-offsets tend to creep in, because getting equal phase-lengths on high-frequency LO signals is tricky at best, and
*maintaining* those phase-lengths over possibly differing physical conditions is near impossible without exotic closed-loop compensation systems.

Paul Oxley

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Apr 17, 2018, 9:04:12 PM4/17/18

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Marcus

From what I know about traditional Phase Lock Loops (PLL), the phase is far from being arbitrary. The Reference 10 MHz is counted down by the R counter to a common frequency which is compared to the LO frequency which is counted down to the same frequency by the N counter.

Fractional N PLLs may work a bit different, but I do not think that the phase error would be significant.

With common LOs, you can have differences in the phase of the LO by differences in the path length of the LO distribution. Noise is also a consideration since noise coupling could be different on different LO distribution paths. If there are any amplification stages in the LO paths, the noise would also be different.

Paul

From: Marcus D. Leech <patchv...@gmail.com>
To: sara...@googlegroups.com
Sent: Tuesday, April 17, 2018 8:50 PM
Subject: Re: [SARA] poor man's 21cm interferometer

Dcalc=lambda/cos(DEC)/sin(360/ 24/3600*fringe time)= 16.3m

Marcus D. Leech

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Apr 17, 2018, 9:21:24 PM4/17/18

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On 04/17/2018 09:03 PM, Paul Oxley wrote:

Marcus

From what I know about traditional Phase Lock Loops (PLL), the phase is far from being arbitrary. The Reference 10 MHz is counted down by the R counter to a common frequency which is compared to the LO frequency which is counted down to the same frequency by the N counter.

Fractional N PLLs may work a bit different, but I do not think that the phase error would be significant.

With common LOs, you can have differences in the phase of the LO by differences in the path length of the LO distribution. Noise is also a consideration since noise coupling could be different on different LO distribution paths. If there are any amplification stages in the LO paths, the noise would also be different.

Paul

I've been using correlator systems with frac-N PLLs for many years now. The offset from the ref-clock really *is* arbitrary. They are very different from
integer-N PLLs in this regard.

Another complicating factor is that the mixers on these devices (not sure if it's true for LMS7002M) may use a 2XLO phase-splitter. In that case, there's
no way to predict the "state" of the flip-flop based phase-splitter, and you can have a 180deg phase ambiguity from that--that is, the mixer is either
in-phase with its brethren, or exactly 180deg out-of-phase.

Paul Oxley

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Apr 17, 2018, 9:40:46 PM4/17/18

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Marcus

I agree that the best PLL solution uses integer counters. Fractional N PLL phase differences would be a small portion of the comparison frequency. The difference would be controlled by the hardware counters and be constant. (Not arbitrary.)

If you cannot use integer N PLLs, then use a very low frequency comparison to limit the phase differences.

Another consideration is the sample clock. It also needs to be in phase on each ADC.

For RA applications, I am not sure that the small phase differences is important. The phase difference would appear to be an error in the baseline spacing of the antennas which will also contain errors.

Paul

From: Marcus D. Leech <patchv...@gmail.com>
To: sara...@googlegroups.com

Sent: Tuesday, April 17, 2018 9:21 PM

Subject: Re: [SARA] poor man's 21cm interferometer

Marcus D. Leech

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Apr 17, 2018, 9:55:53 PM4/17/18

to sara...@googlegroups.com

On 04/17/2018 09:40 PM, Paul Oxley wrote:

Marcus

I agree that the best PLL solution uses integer counters. Fractional N PLL phase differences would be a small portion of the comparison frequency. The difference would be controlled by the hardware counters and be constant. (Not arbitrary.)

If you cannot use integer N PLLs, then use a very low frequency comparison to limit the phase differences.

Another consideration is the sample clock. It also needs to be in phase on each ADC.

For RA applications, I am not sure that the small phase differences is important. The phase difference would appear to be an error in the baseline spacing of the antennas which will also contain errors.

Paul

I'll just refer to this app-note, which discusses the differences in mutual phase coherence between integer-N and fractional-N PLL synthesizers, and why
a "phase resynch" feature was invented in the first place.

https://ez.analog.com/servlet/JiveServlet/download/2451-1-11986/ADF4350_Phase_Sync_RevC.pdf

David L, N5OIQ

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Apr 17, 2018, 9:58:41 PM4/17/18

to Society of Amateur Radio Astronomers

With the LimeSDR (LMS7002M), I have seen less than 1 ns delay between each receiver channel, so whatever issues there are between clock offsets between each channel are small so I see no new to fuss over their clocking. Sun light heating of the coax cables leading to each antenna cause greater phase errors in my experience.

Marcus D. Leech

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Apr 17, 2018, 10:10:28 PM4/17/18

to sara...@googlegroups.com

On 04/17/2018 09:58 PM, David L, N5OIQ wrote:

With the LimeSDR (LMS7002M), I have seen less than 1 ns delay between each receiver channel, so whatever issues there are between clock offsets between each channel are small so I see no new to fuss over their clocking. Sun light heating of the coax cables leading to each antenna cause greater phase errors in my experience.

The phase-offset within the LMS7002M should be small and well-bounded. After all, there's a single LO being feed to a pair of mixers--so the effects are
mostly temperature coupled.

But we're discussing multi-LMS7002M systems here, in which case, the Analog Devices app-note I just posted provides some enlightenment about what
I'm talking about.

The BEST-case scenario is that:

(A) The divider modulus (MOD) is small (let's say, under 10), in which case there are fewer possible phase offsets.
(B) The PLLs are programmed at exactly the same time

But in other situations, even if the synthesizer is programmed at exact same time, if MOD is larger, you end up with a much larger set of
possible mutual phase offsets, and the term "arbitrary" starts to actually be a reasonable phrase to use.

Let's say the modulus is 9, there are 9 possible phase-offsets between any two synthesizers, even if everything else is "perfect".

Now having said all that, it is usually the case that "array phase" tends to be calibrated at the start of every observing run anyway, so the
"apparently-near-random" phase-offset among your receivers may not be an issue.

Paul Oxley

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Apr 17, 2018, 10:24:24 PM4/17/18

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David

A agree. The coax characteristics is not constant over temperature.

Another factor that is always present is the pointing accuracy of the antennas. Wind can slightly change the direction of the antenna and result in a phase difference from the antenna pattern.

All of this is probably not of significance.

Paul

From: "David L, N5OIQ" <dmlo...@gmail.com>
To: Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: Tuesday, April 17, 2018 9:58 PM

Marcus D. Leech

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Apr 18, 2018, 12:22:19 AM4/18/18

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On 04/17/2018 10:24 PM, Paul Oxley wrote:

David

A agree. The coax characteristics is not constant over temperature.

Another factor that is always present is the pointing accuracy of the antennas. Wind can slightly change the direction of the antenna and result in a phase difference from the antenna pattern.

All of this is probably not of significance.

It all rather depends on what your goal is, and what the geometry of your array is, and what the primary beam geometry of your antennae is.
Phase offsets in an interferometer lead to the interferometer fringe phase center not being aligned with the geometric axis of the instrument.

IF your goal is to just produce fringes (nothing wrong with that), then you don't need to worry that much about phase matching--you'll get fringes, but
they won't necessarily give you the spatial information accuracy you're looking for.

I have been assuming that if you're talking about arrays of coherent receivers, that doing reasonably-precise sky mapping is the goal. In which case
unpredictable phase errors can really play havoc with your ability to produce quality sky maps.

If you look at the equations for phase-increment to achieve a given pointing offset for a "degenerate" two-element phased-array, you can see that
relatively small phase errors can lead to significant effective skew in the fringe geometric phase.

phase_incr = (360 * (d/L) * sin(a)

Where:

     d = antenna physical spacing--in the case of an interferometer, this would be your baseline length
     L = wavelength
     a = offset angle

I think one would agree that an effective array pointing offset larger than perhaps 1/4 of the primary beamwidth would be unpleasant.

Now, consider the previously-linked-to treatise on fractional-N synthesizer behavior.    The magnitude of the possible phase error
covers -pi to +pi radians.   For a divider modulus of (chosen arbitrarily) 13, that means that the mutual phase error between any two
identically-tuned synthesizer could be any one of 13 different values uniformly distributed between -pi and +pi.

You can then plug that in to the phase-increment formula for phased arrays and determine whether any of those 13 phase errors produces an
"acceptable" (small enough that you don't need to compensate) fringe-center offset.

Certainly, if your goal is just to produce fringes, then worrying about this is a waste of time, I agree. But when you start talking about building
arrays, some consideration should probably be given...

David L, N5OIQ

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Apr 18, 2018, 10:13:56 AM4/18/18

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My goal is very much to do actual imaging, not just produce fringes. Old fashioned interferometers relied on having a solid, known phase relationship between antennas, but modern multi-baseline interferometers use a process called self-calibration to align their many baselines using a sky model. These links describe the self-cal process and aipy interferometry package has tools in it to do the work that I have experimented with and discussed before.

https://science.nrao.edu/facilities/alma/naasc-workshops/nrao-cd-wm16/Selfcal_Madison.pdf

https://www.youtube.com/watch?v=7ghe0Hsub5s

https://casper.berkeley.edu/astrobaki/index.php/AIPY

https://www.mpifr-bonn.mpg.de/3244052/IMPRS_BB_HRK5.pdf

Errors far beyond pi/-pi can and are corrected through self-calibration for things like the ionosphere and atmosphere. The bottom line is that SDRs don't need to have their receivers locked to a known phase every time they are started up. The somewhat dissatisfying reality is that self-calibration requires prior knowledge about the sky and the complex visibility data is fitted to this known sky model. With 16 receivers and 120 baselines, a good over-determined modeling solution can be made that can generate a scientifically meaningful sky image from a SDR like the XTRX Octopack. Lots of closure phase solutions can be calculated and used to correct the beams for each baseline from such a multi-channel SDR.

Paul

From: "David L, N5OIQ" <dml...@gmail.com>

Michiel Klaassen

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Apr 18, 2018, 11:08:27 AM4/18/18

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Hi All,

Wow,; while I was asleep a lot of knowledge was exchange; interesting.

I do not have the goal yet as David has; my goal is just to make two dongles give two phase correct data streams.

So, my question is, does anybody have two short phase coherent data files from two RTL dongles, so I can use it as a reference

Made available via dropsend or other means.

regards

Michiel

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Marcus D. Leech

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Apr 18, 2018, 12:36:53 PM4/18/18

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On 04/18/2018 11:08 AM, Michiel Klaassen wrote:

Hi All,

Wow,; while I was asleep a lot of knowledge was exchange; interesting.

I do not have the goal yet as David has; my goal is just to make two dongles give two phase correct data streams.

So, my question is, does anybody have two short phase coherent data files from two RTL dongles, so I can use it as a reference

Made available via dropsend or other means.

regards

Michiel

So, here's the sad thing about the RTLSDR dongles. Even when you give them a common clock, and turn off "dither" in the PLL synthesizer, there's
*STILL* a residual phase-walk that is of the order of 10s of seconds. Or put another way, a residual frequency offset of a fraction of a Hz.
This *can* be characterized and compensated for, but it's an extra pain in the butt you have to consider with making RTLSDRs phase-coherent
for our purposes.

Michiel Klaassen

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Apr 18, 2018, 1:51:28 PM4/18/18

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Marcus,

I read your DTP_RX.pdf, so you have managed to get good results.

Do you still some bytes available.

Michiel

Marcus D. Leech

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Apr 18, 2018, 1:55:01 PM4/18/18

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On 04/18/2018 01:51 PM, Michiel Klaassen wrote:

Marcus,

I read your DTP_RX.pdf, so you have managed to get good results.

Do you still some bytes available.

Michiel

I don't have any data from those experiments, but keep in mind that the technique in that article does not require phase-coherence. You're basically
measuring the outputs of two *adding* interferometers that are 180deg out-of-phase with each other, and the phasing is implemented entirely
in hardware (either coax line lengths, or a mixer configured appropriately).

Michiel Klaassen

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Apr 18, 2018, 3:11:22 PM4/18/18

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Marcus,

I see now; yes that is thesame I have myself already.

Hopefully some else has got some coherent RTLSDR bytes laying around

Meanwhile I keep trying; next experiment is with a common inserted pilot signal

Michiel

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