Topics

Direct Sampling frequency limit in r 1628 onwards #sdrsharp #spyserver


Martin - G8JNJ
 
Edited

Hi,


I see that in the release notes for r 11628 onward that "Set RTL max frequency to 14.4 MHz in direct sampling mode." has been added.

Is there any particular reason for this ?

I understand the desire use the tuner chip as much as possible in order to avoid Nyquist Alias signals and to have RF gain control. But I have an application where I wish to continue using direct sampling above 14.4MHz and this change has caused problems for me.

Could I suggest that it may be better if the user / admin could set this as required, as some configurations may need to use direct sampling above 14.4MHz and others may not.

It would be even better if it was possible to define frequency limits for direct / quadrature sampling in SDR Sharp and Spyserver, so that this could change automatically when the defined frequency boundary is crossed. This would save having to reconfigure things manually for example when you wish to move from HF to VHF and allow a mix of HF / VHF / UHF frequencies to be scanned.

It may also be possible to extend this proposed configuration option to automatically turn spyserver on-off (along with the frequency off-set) or turn the bias tee on / off (if folks are using this to switch antennas or pre-amps in / out).

Regards,

Martin - G8JNJ 


prog
 

On Tue, Nov 21, 2017 at 10:05 am, Martin wrote:
Hi,


I see that in the release notes for r 11628 onward that "Set RTL max frequency to 14.4 MHz in direct sampling mode." has been added.

Is there any particular reason for this ?

I understand the desire use the tuner chip as much as possible in order to avoid Nyquist Alias signals and to have RF gain control. But I have an application where I wish to continue using direct sampling above 14.4MHz and this change has caused problems for me.

Could I suggest that it may be better if the user / admin could set this as required, as some configurations may need to use direct sampling above 14.4MHz and others may not.

It would be even better if it was possible to define frequency limits for direct / quadrature sampling in SDR Sharp and Spyserver, so that this could change automatically when the defined frequency boundary is crossed. This would save having to reconfigure things manually for example when you wish to move from HF to VHF and allow a mix of HF / VHF / UHF frequencies to be scanned.

It may also be possible to extend this proposed configuration option to automatically turn spyserver on-off (along with the frequency off-set) or turn the bias tee on / off (if folks are using this to switch antennas or pre-amps in / out).

Regards,

Martin - G8JNJ 

Know your SDR. RTL's sample at 28.8MSPS, thus offering a first nyquist zone between 0 and 14.4MHz. Nope. This won't change.


Martin - G8JNJ
 

Hi,

OK, it's your call, and I understand why you may wish to do it for both technical and commercial reasons.

As I said previously, there is a Nyquist Alias wrapped around 14.4MHz, but adding this restriction alone still won't completely stop alias signals from above this frequency appearing at lower frequencies. RTL dongles used in this way will always be a compromise, so I've never considered this particular issue to ever be a major problem. Especially as there are other sampling aliases present depending upon the sampling rate chosen in SDR Sharp (and others).

RTL dongles are never going to compete with hardware like Airspy's or HF+'s. But they do provide a good introduction to SDR's and facilitate a lot of experimentation and other applications that may otherwise be too costly to implement by casual or 'hobby' users.

I often use a diplexing filter to minimise this problem when running RTL dongles in direct sampling mode with SDR Sharp (and others), so my plan was to use three dongles connected by a triplexer to cover 0-14.4 / 14.4-28.8 / 28.8 and above.

The suggestion was intended to try and maintain the same functionality of SDR Sharp in both normal and Spyserver mode  wherever possible.

Personally I'd appreciate it if you didn't put this restriction in, but as I said before it's your call.

Regards,

Martin - G8JNJ


prog
 

On Tue, Nov 21, 2017 at 02:02 pm, Martin wrote:
OK, it's your call, and I understand why you may wish to do it for both technical and commercial reasons.
Nope. Correctness first. We don't have an s-meter for the same reason. We have an SNR-Meter.


Martin - G8JNJ
 
Edited

Hi,

Unfortunately, although I'm also keen on technical correctness, it can stifle experimentation, innovation and perhaps even joy :-)

Regards,

Martin - G8JNJ


jdow
 

The guts of the dongles don't really allow tuning higher than 14.4 MHz. For that matter 14.4 MHz may be rather pushing the RTL chip, the one which performs IF filtering and A/D conversion.

{^_^}

On 2017-11-21 10:05, Martin via Groups.Io wrote:
[Edited Message Follows]
Hi,
I see that in the release notes for r 11628 onward that "Set RTL max frequency to 14.4 MHz in direct sampling mode." has been added.
Is there any particular reason for this ?
I understand the desire use the tuner chip as much as possible in order to avoid Nyquist Alias signals and to have RF gain control. But I have an application where I wish to continue using direct sampling above 14.4MHz and this change has caused problems for me.
Could I suggest that it may be better if the user / admin could set this as required, as some configurations may need to use direct sampling above 14.4MHz and others may not.
It would be even better if it was possible to define frequency limits for direct / quadrature sampling in SDR Sharp and Spyserver, so that this could change automatically when the defined frequency boundary is crossed. This would save having to reconfigure things manually for example when you wish to move from HF to VHF and allow a mix of HF / VHF / UHF frequencies to be scanned.
It may also be possible to extend this proposed configuration option to automatically turn spyserver on-off (along with the frequency off-set) or turn the bias tee on / off (if folks are using this to switch antennas or pre-amps in / out).
Regards,
Martin - G8JNJ


Martin - G8JNJ
 

Hi Joanne,

They are not as bad as you may think.

Two examples.........

Some time ago I tried running a pair of Dongles in direct sampling mode to cover two frequency ranges 0-14.4MHz and 14.4-28.8MHz. I used two instances of PC-ALE and HDSDR (with CAT to HDSDR enabled to control it). Then used it to scan the HF-Link ALE channels. It worked quite well, the only limitation being the amount of PC resources being consumed. 

The number of stations logged was greater than when I used an SDR Play RSP1 instead of the Dongles.

Today I tried an experiment logging FT8 signals on 15m.

I used and RTL SDR V3 in direct samping mode, an RTL 8020T2 dongle in normal quadrature mode and my KiWi SDR all fed from the same antenna.

There wasn't much to choose between them in terms of the number of stations logged and the RX S/N. However it wasn't particularly scientific as I had to swap between the different radios at different times, and the propagation changed over the test period.

Tomorrow I may try and get all three running concurrently but logging to PSK reporter with different callsigns. I think this would be a more realistic test.

The problem using the RTL SDR V3 with Spyserver in quadrature mode above 14.4MHz is that it has an internal diplexer feeding the Direct sampling port and Quadrature tuner port. So anything using the Quadrature tuner port between 14.4MHz and 30MHz is attenuated. This is another reason why I'd prefer to be able to select the receiver mode when using Spyserver.

I also find that if I use another dongle without the diplexer, and select the Quadrature tuner port on these frequencies, I frequently find that I have to put an attenuator in circuit as the strong broadcast stations on 15 & 19MHz cause ADC clipping and intermod even when the RF gain is set to zero. 

Regards,

Martin - G8JNJ




 


Martin - G8JNJ
 
Edited

Hi Joanne,

Yesterday I tried to setup three concurrent FT8 decode sessions on 24MHz in order to test the receive performance of an Airspy R1, generic 820T2 dongle and RTL SDR V3 in direct sampling mode.

Unfortunately the propagation gods were not favourable and I discovered a few other issues, so today I connected them up to my test kit to try and figure out what was going on.

First the maximum sensitivity I could obtain for each type. These were measured using plain carrier and USB in a 2400KHz BW. The generator output was adjusted to obtain 10dB SINAD at 27MHz. Note my generator and attenuators may not be fully in calibration, but I used the same setup for each test, so I'm comparing like with like.

Airspy R1 = -122dBm (Gain setting in free mode 10/10/15)

Airspy Mini = -117dBm (Gain setting in free mode 10/10/15)

Generic 820T2 = -119dBm (Gain setting 49.6)

RTL SDR V3 Quadrature = -118dBm (Gain setting 49.6)

RTL SDR V3 Direct Sampling = -107 (No gain adjustment)

So in theory the Quadrature mode seems to offer about 10dB better sensitivity than when used in Direct Sampling mode.

However when I connected an antenna to the various SDR's using the maximum gain settings they all showed signs of overload and IMD products, apart from the V3 in Direct Sampling mode.

In each case I adjusted the gain settings until the overload and IMD products just disappeared and then remeasured the sensitivity with the new gain settings.

Airspy R1 = -116dBm (Gain setting in free mode 10/10/6)

Airspy Mini = -115dBm (Gain setting in free mode 10/10/9)

Generic 820T2 = -112dBm (Gain setting 29.7)

RTL SDR V3 Quadrature = -111dBm (Gain setting 29.7)

RTL SDR V3 Direct Sampling = -107 (No gain adjustment)

So now the difference is only 8dB best case (Airspy Mini) and is typically only 4dB (Generic 820T2 and RTL SDR V3)

In practice these figures could be improved by placing bandpass filters ahead of the SDR's but I just wanted to test their basic out of the box performance.

Other observations:-

All the dongle sensitivities remained fairly consistent across their frequency ranges.

When using Quadrature mode none of the dongles locked consistently below 24MHz. The 820T2  would not go below 24MHz, the Airspy R1 occasionally manged it down to 19MHz and the Airspy Mini would normally only lock down to about 25MHz. I could sometimes see a signal but it was not on the correct frequency (usually 1-1.5MHz low) So if you have it connected to an antenna you may think you are seeing signals below 24MHz, but they may not be on the correct frequency.

There is no mechanism to receive signals between 14.4MHz and 24MHz using Quadrature mode. Direct Sampling does work up to about 27MHz but above this frequency the 28.8MHz clock oscillator causes very high level unwanted noise spikes.

When using the RTL SDR V3 in Direct Sampling mode the sensitivity is typically -107dBm from 2MHz to 25MHz. It falls outside these limits as the Bias tee inductor and input diplexer add additional losses. This permits good reception on most bands up to about 20MHz (with a suitable antenna) and is only limited by the natural atmospheric noise floor. Reception is still adequate on higher frequencies up to about 27MHz, especially in urban areas where the noise floor is likely to be higher.

The RTL SDR V3 in Direct Sampling mode does have Nyquist Alias signals wrapped around 14.4MHz (1/2 of 28.8MHz sample rate). This results in signals appearing twice (one the correct frequency and one unwanted on the Nyquist image). Most of these can be removed by adding a simple high / low / Diplexing filter or if only certain frequency ranges are of interest a band pass filter.

This is why like to have the option of being able to use the Direct Sampling mode between 14.4MHz and 27MHz.

Regards,

Martin - G8JNJ


David J Taylor
 

Hi Joanne,
[]
This is why like to have the option of being able to use the Direct Sampling mode between 14.4MHz and 27MHz.

Regards,
Martin - G8JNJ
=====================================

Thanks for your report, Martin. For the reasons you state, I guess you will already have an Airspy HF+ on order!

73,
David GM8ARV
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
Twitter: @gm8arv


Martin - G8JNJ
 

On Fri, Nov 24, 2017 at 08:31 am, David J Taylor wrote:
Thanks for your report, Martin. For the reasons you state, I guess you will already have an Airspy HF+ on order!
Ha maybe :-)

Although I'm quite happy with my KiWi SDR's, so perhaps not just yet, I like to let designs settle down a bit first and the early adopters to be the Beta testers for me....

The particular application I had in mind for the cheap dongles requires something to be as low cost as possible, almost a throw away item. It only needs to be good enough for the purpose.

Judging from the woeful performance of many existing WEB SDR's etc. I suspect that only a few folks will have a good enough antenna system and low enough noise floor to be able to take full advantage of the sensitivity and dynamic range of the HF+. 

In many cases these days an 'average' performance receiver is probably good enough. Although that still shouldn't stop us striving for the best performance possible to deal with 'edge cases'

Regards,

Martin - G8JNJ


Kenneth Sejkora
 

Based on the revision log, it looks as though build 1627 was the last version of SDRSharp available before the 14.4 MHz ceiling was implemented in direct sampling mode.  As Martin points out, there are situations where it might be advantageous to at least have the option to run a dongle in direct sampling mode in the 14.4 to 28.8 MHz range as opposed to being forced to use quadrature sampling in this frequency range.  I totally respect Youssef's desire to implement the 14.4 MHz ceiling for the technical reasons he stated, but for an end user the option to exceed the ceiling should also be respected and left up to the user.  The user can 'assume' the negative consequences of operating direct sampling in the 14.4 to 28.8 MHz frequency range. However, at least for now, SDRSharp is freeware and Youssef has every right to call the shots.

Although it circumvents any improvements that might be afforded by new revisions to SDRSharp, it might behoove anyone that wants the option of using direct sampling in the 14.4 to 28.8 MHz range to obtain a copy of build 1627 to at least have the option of using that range of frequencies in either direct sampling or quadrature mode.  Not the best "fix", but a workable one.  Fortunately, I still have the Zip file for build 1627, and can maintain that version as a separate installation for whenever I need to use direct sampling above 14.4 MHz.

Ken, WBØOCV Norton, MA USA
41.959546N, 71.163996W  FN41kx



From: Martin via Groups.Io <martin_ehrenfried@...>
To: main@airspy.groups.io
Sent: Friday, November 24, 2017 9:36 AM
Subject: Re: [airspy] Direct Sampling frequency limit in r 1628 onwards #sdrsharp #spyserver

Hi Joanne,

Yesterday I tried to setup three concurrent FT8 decode sessions on 24MHz in order to test the receive performance of an Airspy R1, generic 820T2 dongle and RTL SDR V3 in direct sampling mode.

Unfortunately the propagation gods were not favourable and I discovered a few other issues, so today I connected them up to my tes tkit to try and figure out what was going on.

First the maximum sensitivity I could obtain for each type. These were measured using plain carrier and USB in a 2400KHz BW. The generator output was adjusted to obtain 10dB SINAD at 27MHz. Note my generator and attenuators may not be fully in calibration, but I used the same setup for each test, so I'm comparing like with like.

Airspy R1 = -122dBm (Gain setting in free mode 10/10/15)

Airspy Mini = -117dBm (Gain setting in free mode 10/10/15)

Generic 820T2 = -119dBm (Gain setting 49.6)

RTL SDR V3 Quadrature = -118dBm (Gain setting 49.6)

RTL SDR V3 Direct Sampling = -107 (No gain adjustment)

So in theory the Quadrature mode seems to offer about 10dB better sensitivity than when used in Direct Sampling mode.

However when I connected an antenna to the various SDR's using the maximum gain settings they all showed signs of overload and IMD products, apart from the V3 in Direct Sampling mode.

In each case I adjusted the gain settings until the overload and IMD products just disappeared and then remeasured the sensitivity with the new gain settings.

Airspy R1 = -116dBm (Gain setting in free mode 10/10/6)

Airspy Mini = -115dBm (Gain setting in free mode 10/10/9)

Generic 820T2 = -112dBm (Gain setting 29.7)

RTL SDR V3 Quadrature = -111dBm (Gain setting 29.7)

RTL SDR V3 Direct Sampling = -107 (No gain adjustment)

So now the difference is only 8dB best case (Airspy Mini) and is typically only 4dB (Generic 820T2 and RTL SDR V3)

In practice these figures could be improved by placing bandpass filters ahead of the SDR's but I just wanted to test their basic out of the box performance.

Other observations:-

All the dongle sensitivities remained fairly consistent across their frequency ranges.

When using Quadrature mode non of the dongles locked consistently below 24MHz. The 820T2  would not go below 24MHz, the Airspy R1 occasionally manged it down to 19MHz and the Airspy Mini would normally only lock down to about 25MHz. I could sometimes see a signal but it was not on the correct frequency (usually 1-1.5MHz low) So if you have it connected to an antenna you may think you are seeing signals below 24MHz, but they may not be on the correct frequency.

There is no mechanism to receive signals between 14.4MHz and 24MHz using Quadrature mode. Direct Sampling does work up to about 27MHz but above this frequency the 28.8MHz clock oscillator causes very high level unwanted noise spikes.

When using the RTL SDR V3 in Direct Sampling mode the sensitivity is typically -107dBm from 2MHz to 25MHz. It falls outside these limits as the Bias tee inductor and input diplexer add additional losses. This permits good reception on most bands up to about 20MHz (with a suitable antenna) and is only limited by the natural atmospheric noise floor. Reception is still adequate on higher frequencies up to about 27MHz, especially in urban areas where the noise floor is likely to be higher.

The RTL SDR V3 in Direct Sampling mode does have Nyquist Alias signals wrapped around 14.4MHz (1/2 of 28.8MHz sample rate). This results in signals appearing twice (one the correct frequency and one unwanted on the Nyquist image). Most of these can be removed by adding a simple high / low / Diplexing filter or if only certain frequency ranges are of interest a band pass filter.

This is why like to have the option of being able to use the Direct Sampling mode between 14.4MHz and 27MHz.

Regards,

Martin - G8JNJ



Jon Fear
 
Edited

@Ken@WBØOCV

I managed to miss that final version r1627. I would like a copy please. I am not sure if we can do PM's on this board or whether you can put the file somewhere I can pick it up from. Until the HF+ arrives and without any support for the RSP1A, the dongle is the only game in town....

Best wishes

Jon in Weston super Mare, Clearly not far from Martin 


Dick
 

Look here:
 
 
Cheers,
Dick
 

From: Jon Fear
Sent: Saturday, November 25, 2017 12:23
To: main@airspy.groups.io
Subject: Re: [airspy] Direct Sampling frequency limit in r 1628 onwards #sdrsharp #spyserver
 

[Edited Message Follows]
[Reason: Add r1627 to clarify version needed...]

@Ken@WBØOCV

I managed to miss that final version r1627. I would like a copy please. I am not sure if we can do PM's on this board or whether you can put the file somewhere I can pick it up from. Until the HF+ arrives and without any support for the RSP1A, the dongle is the only game in town....

Best wishes

Jon in Weston super Mare, Clearly not far from Martin


n2msqrp
 

Where does find the previous versions of SDRSharp?


Mike N2MS

 

On November 25, 2017 at 6:33 AM Dick <hobbyd@...> wrote:

Look here:
 
 
Cheers,
Dick
 
From: Jon Fear
Sent: Saturday, November 25, 2017 12:23
To: main@airspy.groups.io
Subject: Re: [airspy] Direct Sampling frequency limit in r 1628 onwards #sdrsharp #spyserver
 

[Edited Message Follows]
[Reason: Add r1627 to clarify version needed...]

@Ken@WBØOCV

I managed to miss that final version r1627. I would like a copy please. I am not sure if we can do PM's on this board or whether you can put the file somewhere I can pick it up from. Until the HF+ arrives and without any support for the RSP1A, the dongle is the only game in town....

Best wishes

Jon in Weston super Mare, Clearly not far from Martin