Panadapters and Sample Rate

[Sybaris]

So I'm looking at a few SDR's to use as panadpaters but they vary greatly in their sample rates e.g. ELAD FDM-S2 122Mhz, RSPdx 20Mhz. Being that the SDR input would be the IF out from the receiver does it matter what the sample rate is?

 

[KA1RBI]

As long as the sample rate matches or exceeds the spectrum width coming out of the IF-out you should be OK. If it exceeds it hugely there will be excess CPU consumption and/or bus usage in the computer hosting the SDR. Of course the SDR could also be narrower if you know that you'll never need to see more than that amount of spectrum.

However another (more or less unrelated) consideration is the dynamic range of signals coming out of the IF-out could well exceed that of the SDR. In this case your only option is to add attenuation to get the largest signal in the passband into the SDR without overloading it.

A specific case is attempting to connect the IF-out of the Icom IC-R7000 to an SDR. The R-7000 also utilizes AGC which ends up working against you - the entire spectrum is reduced in strength when the receiver is tuned to a station strong enough to engage the AGC. You can defeat the AGC but with the same dynamic range penalties as already mentioned... One usually hears folks pipe up at such point saying "just use a straight SDR and dispense with the receiver and the IF-out"...

As a very rough rule of thumb you can compute the dynamic range of an SDR as 6dB per bit - so the run of the mill cheap SDR having 8 bits would work out to a dynamic range of approx. 48dB - this is quite poor by contemporary radio receiver standards...

Max

 

[Token]

So I'm looking at a few SDR's to use as panadpaters but they vary greatly in their sample rates e.g. ELAD FDM-S2 122Mhz, RSPdx 20Mhz. Being that the SDR input would be the IF out from the receiver does it matter what the sample rate is?

Nice rating badge, when were you at Corry?

It might help if you define, a little further, your desired configuration, such as what specific receiver will be the front end? What freq range is your target tuning range?

For such an application, as a panadapter, the sample rate is not a huge concern. However, you do want to have a instantaneous bandwidth that is at least as wide as the IF of the tuning receiver, and IBW will be partially driven by sample rate.

I mostly use SDRs as direct tuning receivers, but I do also use them on the IF of various off-the-shelf receivers and home brew block converters.

T!

 

[Sybaris]

Nice rating badge, when were you at Corry?

It might help if you define, a little further, your desired configuration, such as what specific receiver will be the front end? What freq range is your target tuning range?

I was at Corry in '82 then '86 for V3. Retired in '03. Maybe if I was WLR I might have retained all this better

I have a couple receivers (AR-5000 and R-8500). Don't really want to part with them and want to keep their freq range so looking to take advantage of what functions I can get out of an SDR as a panadapter and associated GUI. I may not be able to use the Icom though as I think its IF Out is powered well unless I get hold of some kind of blocking cap.

 

[Token]

I was at Corry in '82 then '86 for V3. Retired in '03. Maybe if I was WLR I might have retained all this better

I have a couple receivers (AR-5000 and R-8500). Don't really want to part with them and want to keep their freq range so looking to take advantage of what functions I can get out of an SDR as a panadapter and associated GUI. I may not be able to use the Icom though as I think its IF Out is powered well unless I get hold of some kind of blocking cap.

I was in Corry in 83. The last time I was back there was in 2019.

Yes, the R8500 has power on the IF, and yes, you can use a blocking cap. You can get low cost F type models as low as under $10, and use the appropriate adapters. Some SDRs come with built in blocking caps, like the RFSpace NetSDR and I think SDR-IQ.

The IF bandwidth of the R8500 is about 10 MHz centered on 10.7 MHz, but both of mine roll off about +/- 3.5 MHz, making the flat'ish bandwidth around 7 MHz or a tad over. It might be usable to 10 Mhz, but it can be a tad sketchy at that width.

T!

 

[KA1RBI]

Can't speak for the AOR-5000 but specifically with the IC-R8500 it is worth noting that

1) as previously mentioned the AGC in the R8500 will activate and when spinning the tuning knob the spectrum peaks will bobble up and down depending on the strength of the station tuned. This will affect the ENTIRE bandwidth shown in the panadapter, not just the station(s) immediately adjacent to the station tuned.

The SETI site has interesting commentary about this

The Icom model R-7000, R-7100, R-8500, and PRC-1000 scanning microwave receivers are gaining popularity among SETI League members. Their chief drawback is an Automatic Gain Control (AGC) circuit which cannot be turned off. AGC effectively reduces the sensitivity of your Digital Signal Processing (DSP) program by making weak signals stronger, and strong signals weaker....

2) The spectrum output is backwards. An IC-7300 tuned to 10.7 MHz with the spectrum span (width) set to 1MHz and connected (via a DC block cable) to the IF output of the R-8500 will show higher frequencies on the left and lower frequencies toward the right. To give a specific example if the R-8500 is tuned to 162.55 and the panadapter tuned to 10.7 MHz, a frequency of 162.6 would appear LEFT of center; 162.7 would be even further to the left. This is sort of counter intuitive. Depending on the SDR in use it might have an option to reverse the spectrum (or might not). For a hardware panadapter implementations, it's possible some may have made provisions for this, allowing for either high or low side LO injection.

3) need to watch out for HF shortwave activity bleeding into what should be a closed circuit. When I tried this with an R-9000 there was significant undesired reception of HF stations. The R-8500 and R-7100 seem to be better on this...

Max

 

[Token]

This is opinion, but personally I find the WinRadio G3xDDC series to be most useful in this application. They (or rather their software) allow you to zoom in and out on, and shift the visual center of, the waterfall without loosing data.

The below screen shot is the main GUI of the WinRaido G31DDC, but all of them, G31, G33, G35, G39, and G65 use a similar layout.

The "DDC Display" is the selected operational bandwidth of the SDR. I.e. if you have 1 MHz of bandwidth selected this window will show the spectrum or the waterfall (user selectable) for that bandwidth. It can be visually zoomed in to show narrower pieces of spectrum, i.e. you have it set to 1 MHz, but you are zoomed in and only show 25 kHz of spectrum.

The "Demodulator Spectrum" is a 64 kHz (or smaller, depending on user DDC bandwidth selection) display around the tuned frequency. With a 64 kHz or larger DDC selection this is 64 kHz wide, but below 64 kHz DDC selection reduces this to match the DDC selection. I.e. if you have 2, 1, 0.64, 0.2 (etc) MHz selected in the DDC window this window is 64 kHz wide, but if you have 50 kHz in the DDC window this window is 50 kHz wide, 40 kHz selected on DDC will make it 40 kHz wide, etc.

The "Wideband Display" in the bottom third shows the full bandwidth of the SDR, in this case it can be set to either 30 or 50 MHz of bandwidth. This spectrum window can also be zoomed to narrower values than the full width, i.e. you have 50 MHz of spectrum selected, but are visually zoomed in to show only 1 MHz of spectrum.

The inclusion of two different waterfalls or spectrums, one the full tuning width and one sample width, is unusual, and extremely useful. But more importantly, once written to the waterfall the data is not gone until the signal times out, rolling off the waterfall. That means I can zoom in, look closely at a signal on the waterfall, and zoom back out, without the waterfall being re-written, and the data of other signals lost.

To the best of my knowledge the WinRadio GUI is the only one that allows this combination of operations (two different waterfalls and the ability to zoom in/out without data loss).

Now, if WinRadio would understand that waterfalls fall down, not up, it would be better.

T!

 

[Sybaris]

Thanks for all the input.
I'm thinkin' now it's not possible to tick all the boxes without 3rd party software and several extra doo-dads and even then maybe not happy with the outcome.
Selling what I have for an R8600 is looking pretty good right now.