Linrad scope

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Earl Shaffer

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Dec 21, 2020, 5:44:13 PM12/21/20
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Hi All..

I'm still trying to decide how much using an SDR of wider bandwidth might help the operation
of the Linrad noise blanker for EME signals at normal EME elevations. I am feeding the same pulse generator to two different SDR's. The first one is IQ+, Delta44 and Linrad at 96Khz.
The second one is SDRPlay RSP1a at 5 Mhz bandwidth 14bit.
The 96Khz SDR must run at 200 hz pulse rate to show two spikes on the scope while the 5Mhz SDR must run at 20Khz pulse rate to show two spikes on the scope. The 96Khz SDR resolves to about at 2Khz pulse rate and seems to get into trouble at higher repetition rates than that while the 5Mhz SDR seems to handle up to about an 80 Khz pulse repetition rate and gets into trouble above that. On both of these SDR's the maximum pulse rate shows to be about 10 pulses. These two SDR  scopes are obviously operating at two very different time bases. I would like to change the time base to resolve the pulses more fully. What is not obvious is how to change the timebase on Linrad. I can only change amplitude. Linrad has evolved over time and many of the manuals and descriptions are accurate at the time of the writings but since the program has evolved some of the newer features can be hard to figure out. I am using a 2 Ghz i3 Windows10 laptop computer and it is running 80 to 100% CPU at 5 Mhz SDR.  I have tried all the usual tricks for conserving the CPU. When looking at real antenna noise here it looks like the two SDR's are very similar with very similar noises pulses until one realizes that the two time bases operate at about a 100 to 1 ratio. That means the 5 Mhz SDR is resolving much finer pulses as expected. When looking at the 5 Mhz SDR some of the pulses are very close together suggesting perhaps something close to a 200 or 300 Khz pulse repetition rate. There is also a strong source of ordinary power line noise at a 120 Hz pulse repetition rate. It's peak power must be very strong because it is evident at most antenna headings. This noise is easily resolved and blanked. It's the higher pulse rate lower amplitude pulses that remain a challenge. If I can resolve the pulses better by using a faster sweep rate, I think I can optimize the calibration of Linrad to minimize the width of the pulses.
The power line pulse that shows up at many antenna headings looks a bit wider than it should but I suspect that if I am able to increase the timebase of the scope I will be able to see that each power line pulse actually has several higher frequency pulses bunched together. I would then expect my pulse generator by comparison to produce a narrower pulse after Linrad is properly calibrated.

I'm starting to suspect what might be an ideal setup for fully blanking Urban noise might be a setup as follows: A 4 yagi array of Log periodic yagis with a 20 Mhz bandwidth centered at 135 Mhz. A preamp system with a 20 Mhz bandwidth. A dual channel SDR with a 20 Mhz bandwidth at 16 or even 24 bit resolution. A VERY fast computer. And of course Linrad as a noise blanker sending to the network. Linrad as a converter to feed Map65. Then finally Map65. Looking at it this way it seems the best SDR hardware has not yet been built.

Leif Asbrink

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Dec 24, 2020, 2:11:24 PM12/24/20
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Hello Jim,

> I'm still trying to decide how much using an SDR of wider bandwidth might
> help the operation
> of the Linrad noise blanker for EME signals at normal EME elevations. I am
> feeding the same pulse generator to two different SDR's. The first one is
> IQ+, Delta44 and Linrad at 96Khz.
> The second one is SDRPlay RSP1a at 5 Mhz bandwidth 14bit.
> The 96Khz SDR must run at 200 hz pulse rate to show two spikes on the scope
> while the 5Mhz SDR must run at 20Khz pulse rate to show two spikes on the
> scope. The 96Khz SDR resolves to about at 2Khz pulse rate and seems to get
> into trouble at higher repetition rates than that while the 5Mhz SDR seems
> to handle up to about an 80 Khz pulse repetition rate and gets into trouble
> above that. On both of these SDR's the maximum pulse rate shows to be about
> 10 pulses. These two SDR scopes are obviously operating at two very
> different time bases. I would like to change the time base to resolve the
> pulses more fully.
The timf2 oscilloscope runs with the same sampling rate as the signal source.
Each point corresponds to one sample at the input when input is in the form
of I and Q. When the input is in the form of a single channel like the loudspeaker
output from a radio at i.e. 22100 Hz, the timf2 oscilloscope shows the same signal
as pairs of I and Q at 11050 Hz. It is not meaningful to change the time base.
If you want to see more details, just use a magnifying glass or the corresponding
software tool of Windows - or set a lower screen resolution to make pixels
bigger and wider separated.

If 96 kHz sampling needs a pulse repetition rate of 200 Hz for two pulses to
become visible you should need (5000/96)*200 Hz = 10 kHz. Correspondingly,
if you resolve 2 pulses at 2 kHz (48 pixels) you should resolve two pulses
separated by 48 pixels at 5 MHz also if frequency responses are similar
in flatness.



> What is not obvious is how to change the timebase on
> Linrad. I can only change amplitude. Linrad has evolved over time and many
> of the manuals and descriptions are accurate at the time of the writings
> but since the program has evolved some of the newer features can be hard to
> figure out. I am using a 2 Ghz i3 Windows10 laptop computer and it is
> running 80 to 100% CPU at 5 Mhz SDR. I have tried all the usual tricks for
> conserving the CPU. When looking at real antenna noise here it looks like
> the two SDR's are very similar with very similar noises pulses until one
> realizes that the two time bases operate at about a 100 to 1 ratio.
Hmm, 5000/96 is more like a 50 ratio;-)

> That
> means the 5 Mhz SDR is resolving much finer pulses as expected. When
> looking at the 5 Mhz SDR some of the pulses are very close together
> suggesting perhaps something close to a 200 or 300 Khz pulse repetition
> rate.
Yes. This gives only about 20 pixels per pulse. For the blanker to be
efficient you will the smart blanker with a good calibration to not
loose too much data. That could be challenging since the frequency
response of your antenna could be far from flat - and different in
different directions.
An improved smart blanker could learn how frequently occuring pulses
look, but it is a non-trivial task to implement that...

> There is also a strong source of ordinary power line noise at a 120
> Hz pulse repetition rate. It's peak power must be very strong because it is
> evident at most antenna headings. This noise is easily resolved and
> blanked. It's the higher pulse rate lower amplitude pulses that remain a
> challenge. If I can resolve the pulses better by using a faster sweep rate,
> I think I can optimize the calibration of Linrad to minimize the width of
> the pulses.
To minimize the width of the pulses you would have to ask for a desired
response that has VERY soft fall-off on both sides. The parabolic slope
with the flat region only about 50%. That would not be optimum because
pulses do not have to be fully resolved when you use the smart blanker
which would know the precise shape of each pulse and subtract the contribution
over as many pixels as required. That would of course require a flat
response of the antenna.

> The power line pulse that shows up at many antenna headings looks a bit
> wider than it should but I suspect that if I am able to increase the
> timebase of the scope I will be able to see that each power line pulse
> actually has several higher frequency pulses bunched together. I would then
> expect my pulse generator by comparison to produce a narrower pulse after
> Linrad is properly calibrated.
The calibrated pulse is symmetric and has oscillations that extend equally
in both directions in time from the peak for pulses that happen to have
the maximum on a sample. The oscillations extend roughly 1/bw on both
sides where bw is the bandwith of the fall-off region. The smart blanker
knows the shape of those oscillations and removes them very efficiently.
You should be able to see that when using the pulse generator.

> I'm starting to suspect what might be an ideal setup for fully blanking
> Urban noise might be a setup as follows: A 4 yagi array of Log periodic
> yagis with a 20 Mhz bandwidth centered at 135 Mhz. A preamp system with a
> 20 Mhz bandwidth. A dual channel SDR with a 20 Mhz bandwidth at 16 or even
> 24 bit resolution. A VERY fast computer. And of course Linrad as a noise
> blanker sending to the network. Linrad as a converter to feed Map65. Then
> finally Map65. Looking at it this way it seems the best SDR hardware has
> not yet been built.
I am sorry, the frequency response of a 4-stack has sidelobes and nulls
that vary with frequency. Pulses will be different depending on direction.

The solution would be adaptive pulse calibration. Certainly possible, but I do
not think I will go in that direction. Currently my interest is in low
noise oscillators....

73

Leif

Earl Shaffer

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Dec 25, 2020, 11:40:20 AM12/25/20
to Linrad
Hi Leif

Ok, thanks for clearing that up. I'll stop looking for a time base adjustment in Linrad.
If I reduce the sample rate the resolution goes down so I probably won't do that.
I am missing out on some possibly interesting pulses. With 5 Mhz bandwidth I see
two pulses when sending a  20 Khz pulse repetition rate. That means if I were looking at a 2 Khz
pulse repetition rate, then I am only looking at 10% of the total pulses (or area between pulses). For 200 hz
repetition pulses I see 1% of the total area around that pulse that is repeating at a 200 hz rate.
Maybe I could use a frequency converter and look at it on my 60 Mhz scope. Or maybe its
not important to see more than a small sample of the pulses. It is similar to another issue I had.
For some time, I thought I had an intermittent noise source when presented with only a 96Khz
bandwidth to look at. When I started looking at wider bandwidths between 1 and 5 Mhz I could easily
see that one or more specific noise sources were always there and simply moved around in frequency 
much of the time being outside of my frequency area of interest presenting itself as multiple spurs about 60Khz wide.

I see that you have some interesting new features on Linrad for signal to noise measurements and looking
at phase noise. I'm curious about your interest in low noise oscillators. Is it your objective now to invent superior test equipment?  We all know that receivers are  already better than most transmitters and there seems to be little interest in improving transmitters. I see that in recent years you are moving away from written text and moving more toward videos which I find very interesting and which may be an efficient use of your time.  
I hope there is some plan in place so that all the knowledge gained from your extensive work can live on beyond your years here. The rest of us have some catching up to do. I have heard stories of important resources going away with the passage of time.

I am considering using the dual channel Afedri SDR for EME. I have watched your videos that used and compared the Afedri and the only thing I don't like about the testing is that the RF filters used in a specific SDR have a significant effect on the results as well as the time of day. I suppose the videos are good for seeing the real world effects of using certain SDRs without external filters, but in my application the filters are less important because I will be using very good 2 pole helical filters ahead of the SDR at 144 Mhz.  External filters can be very cheap compared to the cost of an otherwise good SDR.  I'm still trying to figure out exactly how much difference there might be between the IQ+ Delta44 combination versus the Afedri dual channel SDR, but I suspect there is something like a 20db difference in dynamic range. I think that would be ok here.

Merry Christmas and 73, Jim. WB9UWA

--
There is an excellent Linrad User Guide by Gaetan, ON4KHG, at:
http://w3sz.com/Linrad%20Installation%20&%20Configuration%20User%20Guide%20-%20V1-0.pdf
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