SDR-4++ dual diversity SDR receiver


Chris Moulding
 

Recently we have been asked to design a dual diversity SDR receiver for a radio propagation research project.

The receiver design was based on two SDR-4+ PCBs using a single oscillator so that the two SDR receivers were phase coherent.

The receiver has now been dispatched to the customer but following the workshop tests especially the on-air tests using two antennas the performance improvement was so surprising we have decided to offer it for general sale.

There is a new web page for the receiver with several screenshots of dual HDSDR sessions using a vertical and a horizontally polarised antenna. It's interesting to see how some signals and interference appear on one or the other antenna. The difference between the two antennas can be quite dramatic especially during fading where the polarisation of the signal is rapidly changing due to the ionosphere.

Listening to the combined audio output where both receivers were fed to a set of headphones was amazing. White noise appears as a wide stereo field while wanted signals appear in the centre of the stereo image. To hear 40m stations while the ionosphere changes the polarisation is fascinating, the signal level remains the same but the noise varies in each headphone as the AGC in each HDSDR session tracks the signal. You get the sense that you can feel the propagation changes rather than just hear them.

The web page is:

http://www.crosscountrywireless.net/sdr-4_plus_plus.htm

Regards,

Chris


WA8LMF
 

On 2/18/2017 1:37 PM, Chris Moulding wrote:
 

Listening to the combined audio output where both receivers were fed to a set of headphones was amazing. White noise appears as a wide stereo field while wanted signals appear in the centre of the stereo image. To hear 40m stations while the ionosphere changes the polarisation is fascinating, the signal level remains the same but the noise varies in each headphone as the AGC in each HDSDR session tracks the signal. You get the sense that you can feel the propagation changes rather than just hear them.

The web page is:

http://www.crosscountrywireless.net/sdr-4_plus_plus.htm

Regards,

Chris


How prominent is the shifting background noise?    Would it be significantly irritating while listening to shortwave broadcasts?
 


In the early 1980's, there was a push by several competing standards to be adopted as "the standard" for AM stereo broadcasting.   One of the criticisms leveled against some of the systems was that during nighttime long-range skywave reception shifting phases, multipath and selective fading across even the 10-12KHz bandpass of a single AM standard broadcast channel would make the stereo image wander drunkenly from side to side as propagation changed.  

The (US) NAB (National Association of Broadcasters) holds an annual trade show showcasing all the new radio-TV-proaudio hardware.  At one NAB show in the early '80s, one AM stereo proponent went so far as to hand out airline barf bags imprinted with a message to the effect that the rival system was going to sicken listeners with the unstable stereo image.

I once had a chance to listen to KFI AM 640KHz 50KW in Los Angeles broadcasting Motorola C-Quam AM stereo.  KFI is a monster signal routinely heard over most of the western US at night. It enjoys the rare privilege of being a "clear channel" at night; i.e. no other station on the channel in North America after sundown.   I once had the chance to listen to stereo music from KFI in San Francisco (about 400 miles/640 KM) to the north) in the early evening hours when KFI absolutely booms into northern California on a single skywave hop.    Most of the time, the stereo was very good with with an excellent image from left to right channels. But about every 5 or 10 minutes, the sound would suddenly lurch to the left or right channel only, followed by a distorted signal that sounded like SSB received on an AM receiver (i.e. when the selective fade notched out the carrier, followed by the receiver switching back to mono.  (C-QUAM mode in receivers was triggered by a continuous low-level 20 Hz pilot tone which would be lost when selective fading took out the carrier). This was followed by the return of reasonably good mono, followed by a return to full stereo.    

On the other hand, within it's stable groundwave coverage area  (about 100 miles/160KM radius day or night)  in greater Los Angeles the AM stereo performed wonderfully.  The greater Los Angeles area is very rugged terrain that is cut through with numerous mountain ranges up to 7000 feet/2100 m, riddled with hundreds of canyons. Even the City of Los Angeles is cut in half by a 1800 ft/550m mountain range.   This kind of terrain causes FM stereo to be a nightmare of multipath-induced distortion in many parts of the city, even just a few miles from transmitters located on Mt Wilson, about 5200 ft/1600m above the city.   The AM stereo signal worked perfectly anywhere in the area, even deep into canyons.     I had an AM/FM car radio at time that was equipped with one of Motorola's more advanced C-QUAM decoder chips.  This decoder could actually vary the IF bandwidth of the radio based on strength and S/N.    [NOTE: This was long before the era of DSP. This was all done with analog technology!]    On a nice strong local signal, the AM bandwidth would actually open up to 20 KHz plus, yielding almost FM-quality stereo audio on AM!

The technology worked, but it was killed by station managers obsessed with the loudness horsepower race in broadcasting.  C-QUAM couldn't tolerate asymmetrical "super"modulation that yielded positive peaks of 120-130-140% without hideous distortion.   (Stations would pay 10's of thousands of dollars to install magic mystery audio processor boxes that would produce this asymmetric audio in an effort to be louder than the next guy.)  With C-QUAM, the max modulation had to be limited to about 90-95%.  If you drove modulation % above this, the carrier power would approach zero on each negative swing, causing the receiver quadrature demodulator to loose phase lock on the carrier.  In turn this would cause a horrible "spattery" distortion that sounded similar to FM multipath.


Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
EchoLink:  Node #  14400  [Think bottom of the 2-meter band]
Home Page:          http://wa8lmf.net

Live Off-The-Air APRS Activity Maps
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David Cutter
 


Great project, Chris, this is a frontal assault on Elecraft' s diversity receive system: good luck, it's a terrific functionality.  Before Elecraft you had to have a rack full of the most expensive kit on the planet, now you have opened the gates to a moderate cost solution.  I think you will find others jumping on this bandwagon very quickly.
 
David
G3UNA

----- Original Message -----
Sent: Saturday, February 18, 2017 6:37 PM
Subject: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

Recently we have been asked to design a dual diversity SDR receiver for a radio propagation research project.

The receiver design was based on two SDR-4+ PCBs using a single oscillator so that the two SDR receivers were phase coherent.

The receiver has now been dispatched to the customer but following the workshop tests especially the on-air tests using two antennas the performance improvement was so surprising we have decided to offer it for general sale.

There is a new web page for the receiver with several screenshots of dual HDSDR sessions using a vertical and a horizontally polarised antenna. It's interesting to see how some signals and interference appear on one or the other antenna. The difference between the two antennas can be quite dramatic especially during fading where the polarisation of the signal is rapidly changing due to the ionosphere.

Listening to the combined audio output where both receivers were fed to a set of headphones was amazing. White noise appears as a wide stereo field while wanted signals appear in the centre of the stereo image. To hear 40m stations while the ionosphere changes the polarisation is fascinating, the signal level remains the same but the noise varies in each headphone as the AGC in each HDSDR session tracks the signal. You get the sense that you can feel the propagation changes rather than just hear them.

The web page is:

http://www.crosscountrywireless.net/sdr-4_plus_plus.htm

Regards,

Chris


Chris Moulding
 

I only had an hour to "play" with the receiver after the workshop tests before we shipped it out.

The amount of background noise affecting the AGC could be controlled on SSB by adjusting the AGC threshold in each session of HDSDR.

If the AGC threshold was low so that AGC was working on the background noise then headphone listening was dominated by the changes in noise level. It wasn't too bad if the audio was fed to a stereo amplifiers and wall mounted monitor speakers. The non-expert listeners present in the workshop were impressed with the SSB audio.

If the AGC threshold was adjusted so that the AGC knee was well above background noise then the background noise effectively fell away giving a good listening experience on headphones.

The medium and short wave AM broadcasts I listened to were well above the noise floor so the stereo image was fairly constant.

One thing that should be possible in SDR software would be to compare the FFT outputs from both receiver channels. If it's a similar level in both channels it's likely to be a wanted signal. If it's high on one receiver and not on the other it's probably noise and could be attenuated. This could make a very effective noise reducing algorithm as there is a definite way to prove what is a signal and what is random noise.

Very interesting to hear your experience of AM stereo. I remember reading about the C-QUAM system but as far as I know it wasn't trialled in the UK and I wondered what happened to it. I occasionally hear test broadcasts of the DRM system on short wave from Radio Romania International and it works very well.

Regards,

Chris G4HYG


Chris Moulding
 

I hadn't realised that Elecraft had a diversity receive system.

I've just searched via Google and listened to the audio clips on N1EU's Elecraft site.  These give an identical performance to what I heard from the new receiver.

I recall thinking while I was testing it to try and make some recordings of the stereo diversity audio. I just ran out of time.

It's got me thinking that adding a second receiver in the Sentinel 5 transceiver would be very easy. Adding the extra soundcard channel into the SDR software wouldn't be so easy but I think it would be worth the effort.

Regards,

Chris G4HYG



David Cutter
 


I think you've just thrown a firecracker into the sdr world.
 
the obvious next step is for folks with 2 identical sdrs to get them to run from one oscillator and the appropriate software. 
 
David
G3UNA
 

----- Original Message -----
Sent: Sunday, February 19, 2017 3:10 PM
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

I hadn't realised that Elecraft had a diversity receive system.

I've just searched via Google and listened to the audio clips on N1EU's Elecraft site.  These give an identical performance to what I heard from the new receiver.

I recall thinking while I was testing it to try and make some recordings of the stereo diversity audio. I just ran out of time.

It's got me thinking that adding a second receiver in the Sentinel 5 transceiver would be very easy. Adding the extra soundcard channel into the SDR software wouldn't be so easy but I think it would be worth the effort.

Regards,

Chris G4HYG



g3repcomms@...
 

This is precisely what the SDR-4++ does but i will let Chris confirm this :-)
(delivery this week and one big learning curve!)

73s
Bob
G3REP
--------------------------------------------

On Sun, 19/2/17, David Cutter <d.cutter@ntlworld.com> wrote:

Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver
To: CrossCountryWireless@groups.io
Date: Sunday, 19 February, 2017, 16:34


I think you've just thrown a
firecracker into the sdr
world.
 
the obvious next step is for folks
with 2 identical sdrs
to get them to run from one oscillator and the appropriate
software. 

 
David
G3UNA
 

----- Original Message -----

From:
Chris
Moulding
To: CrossCountryWireless@groups.io


Sent: Sunday,
February 19, 2017 3:10
PM
Subject: Re:
[CrossCountryWireless]
SDR-4++ dual diversity SDR receiver


I hadn't realised that Elecraft had a diversity
receive system.
I've just searched via Google and listened to the
audio clips on N1EU's
Elecraft site.  These give an identical performance to
what I heard from
the new receiver.
I recall thinking while I was testing it to try and
make some recordings of
the stereo diversity audio. I just ran out of time.
It's got me thinking that adding a second receiver
in the Sentinel 5
transceiver would be very easy. Adding the extra soundcard
channel into the
SDR software wouldn't be so easy but I think it would
be worth the effort.
Regards,
Chris G4HYG


Chris Moulding
 

Welcome to the forum, Bob.

Yes, the SDR-4++ uses a single oscillator fed to two identical receivers.

Reading up on the Elecraft K3 diversity system ( I didn't know it existed until today) it uses a similar system where the oscillators are phase locked together. It's vital to do that or use a common oscillator with identical receivers to get the synchronisation right.

Regards,

Chris G4HYG


Chris Morrison
 

Previously I was looking at transceivers that allowed receive diversity. 

If I remember correctly the only ones that phase locked the receivers were the Elecraft K3, Flex 5000 (with 2nd RX option installed) and the Flex 6700.

The Yaesu FT1000MP (all of them), FT2000, FT DX5000 and the TenTec Orion and Orion II allowed stereo receive but their receivers weren't phase locked. I saw a mod for the TenTec Orion to phase lock the receivers but since the the 2nd receiver wasn't identical in performance to the 1st the results were not as good as it could be. Tentec released an update to the 2nd receiver so it performance matched the first but I never found any info on the web about having both modifications at the same time.

I was looking for a Flex 5000 with the 2nd RX installed, but ended up getting a Flex 6300. Receive diversity is the only feature missing that I looking for in a new transceiver. The Flex 6700 was way out my price range.    

Chris
M1PTT 



Robert Parkes <g3repcomms@...>
 

If folk want to look into some background on the idea of Diversity it has come up from time to time on the Topband Contesting Newsgroup. http://lists.contesting.com/archives//html/Topband/ Just Search "Diversity" and there are threads going back years. On the LF bands stations need a separate antenna for receiving (rather than the TX antenna - too noisy) ) which is another requirement for amateurs to consider.

73s
Bob
G3REP
--------------------------------------------

On Sun, 19/2/17, chris.mm1ptt@gmail.com <chris.mm1ptt@gmail.com> wrote:

Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver
To: CrossCountryWireless@groups.io
Date: Sunday, 19 February, 2017, 20:34

Previously I was
looking at transceivers that allowed receive
diversity. If I remember correctly the only ones
that phase locked the receivers were the Elecraft K3, Flex
5000 (with 2nd RX option installed) and the Flex
6700.The Yaesu FT1000MP (all of them), FT2000, FT
DX5000 and the TenTec Orion and Orion II allowed stereo
receive but their receivers weren't phase locked. I saw
a mod for the TenTec Orion to phase lock the receivers but
since the the 2nd receiver wasn't identical in
performance to the 1st the results were not as good as it
could be. Tentec released an update to the 2nd receiver so
it performance matched the first but I never found any info
on the web about having both modifications at the same time.
I was looking for a Flex 5000 with the 2nd RX
installed, but ended up getting a Flex 6300. Receive
diversity is the only feature missing that I looking for in
a new transceiver. The Flex 6700 was way out my price range.
   Chris
M1PTT


Robert Parkes <g3repcomms@...>
 

Chris M1PTT,
the only FT1000 with diversity that I am aware off is a modified FT1000D where a modification adds a relay and enables the source for LO on the second receiver to be switched to the first Rx.

It is bad enough having the two diversity channels moving around in ones head, the last thing you want is the phase of the second LO drifting when compared to the main channel !

cheers
Bob
G3REP



From: "chris.mm1ptt@..." <chris.mm1ptt@...>
To: CrossCountryWireless@groups.io
Sent: Sunday, 19 February 2017, 20:34
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

Previously I was looking at transceivers that allowed receive diversity. 
If I remember correctly the only ones that phase locked the receivers were the Elecraft K3, Flex 5000 (with 2nd RX option installed) and the Flex 6700.
The Yaesu FT1000MP (all of them), FT2000, FT DX5000 and the TenTec Orion and Orion II allowed stereo receive but their receivers weren't phase locked. I saw a mod for the TenTec Orion to phase lock the receivers but since the the 2nd receiver wasn't identical in performance to the 1st the results were not as good as it could be. Tentec released an update to the 2nd receiver so it performance matched the first but I never found any info on the web about having both modifications at the same time.
I was looking for a Flex 5000 with the 2nd RX installed, but ended up getting a Flex 6300. Receive diversity is the only feature missing that I looking for in a new transceiver. The Flex 6700 was way out my price range.    
Chris
M1PTT 




David Cutter
 


I'm not familiar with the problem, but I wonder if summing the 2 audio outputs would still work.  I noticed when combining music stereo sources that the noise significantly reduces.
 
David
G3UNA

----- Original Message -----
Sent: Monday, February 20, 2017 10:08 AM
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

Chris M1PTT,
the only FT1000 with diversity that I am aware off is a modified FT1000D where a modification adds a relay and enables the source for LO on the second receiver to be switched to the first Rx.

It is bad enough having the two diversity channels moving around in ones head, the last thing you want is the phase of the second LO drifting when compared to the main channel !

cheers
Bob
G3REP



From: "chris.mm1ptt@..." <chris.mm1ptt@...>
To: CrossCountryWireless@groups.io
Sent: Sunday, 19 February 2017, 20:34
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

Previously I was looking at transceivers that allowed receive diversity. 
If I remember correctly the only ones that phase locked the receivers were the Elecraft K3, Flex 5000 (with 2nd RX option installed) and the Flex 6700.
The Yaesu FT1000MP (all of them), FT2000, FT DX5000 and the TenTec Orion and Orion II allowed stereo receive but their receivers weren't phase locked. I saw a mod for the TenTec Orion to phase lock the receivers but since the the 2nd receiver wasn't identical in performance to the 1st the results were not as good as it could be. Tentec released an update to the 2nd receiver so it performance matched the first but I never found any info on the web about having both modifications at the same time.
I was looking for a Flex 5000 with the 2nd RX installed, but ended up getting a Flex 6300. Receive diversity is the only feature missing that I looking for in a new transceiver. The Flex 6700 was way out my price range.    
Chris
M1PTT 




Robin Maddison
 

As log as they are in phase!


RobinM  G0GNE

"Sure beats working!" :-)


On 20 Feb 2017, at 12:07, "David Cutter" <d.cutter@...> wrote:

I'm not familiar with the problem, but I wonder if summing the 2 audio outputs would still work.  I noticed when combining music stereo sources that the noise significantly reduces.
 
David
G3UNA
----- Original Message -----
Sent: Monday, February 20, 2017 10:08 AM
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

Chris M1PTT,
the only FT1000 with diversity that I am aware off is a modified FT1000D where a modification adds a relay and enables the source for LO on the second receiver to be switched to the first Rx.

It is bad enough having the two diversity channels moving around in ones head, the last thing you want is the phase of the second LO drifting when compared to the main channel !

cheers
Bob
G3REP



From: "chris.mm1ptt@..." <chris.mm1ptt@...>
To: CrossCountryWireless@groups.io
Sent: Sunday, 19 February 2017, 20:34
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

Previously I was looking at transceivers that allowed receive diversity. 
If I remember correctly the only ones that phase locked the receivers were the Elecraft K3, Flex 5000 (with 2nd RX option installed) and the Flex 6700.
The Yaesu FT1000MP (all of them), FT2000, FT DX5000 and the TenTec Orion and Orion II allowed stereo receive but their receivers weren't phase locked. I saw a mod for the TenTec Orion to phase lock the receivers but since the the 2nd receiver wasn't identical in performance to the 1st the results were not as good as it could be. Tentec released an update to the 2nd receiver so it performance matched the first but I never found any info on the web about having both modifications at the same time.
I was looking for a Flex 5000 with the 2nd RX installed, but ended up getting a Flex 6300. Receive diversity is the only feature missing that I looking for in a new transceiver. The Flex 6700 was way out my price range.    
Chris
M1PTT 




David Cutter
 

Hello Robin
 
I doubt if my simple stereo tape recordings of yesteryear had that benefit, but it was very evident that the best quality (listener satisfaction) was gained when combining left and right with a simple resistive adder. 
 
David
G3UNA

----- Original Message -----
Sent: Monday, February 20, 2017 12:26 PM
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

As log as they are in phase!


RobinM  G0GNE

"Sure beats working!" :-)


On 20 Feb 2017, at 12:07, "David Cutter" <d.cutter@...> wrote:

I'm not familiar with the problem, but I wonder if summing the 2 audio outputs would still work.  I noticed when combining music stereo sources that the noise significantly reduces.
 
David
G3UNA
----- Original Message -----
Sent: Monday, February 20, 2017 10:08 AM
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

Chris M1PTT,
the only FT1000 with diversity that I am aware off is a modified FT1000D where a modification adds a relay and enables the source for LO on the second receiver to be switched to the first Rx.

It is bad enough having the two diversity channels moving around in ones head, the last thing you want is the phase of the second LO drifting when compared to the main channel !

cheers
Bob
G3REP



From: "chris.mm1ptt@..." <chris.mm1ptt@...>
To: CrossCountryWireless@groups.io
Sent: Sunday, 19 February 2017, 20:34
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

Previously I was looking at transceivers that allowed receive diversity. 
If I remember correctly the only ones that phase locked the receivers were the Elecraft K3, Flex 5000 (with 2nd RX option installed) and the Flex 6700.
The Yaesu FT1000MP (all of them), FT2000, FT DX5000 and the TenTec Orion and Orion II allowed stereo receive but their receivers weren't phase locked. I saw a mod for the TenTec Orion to phase lock the receivers but since the the 2nd receiver wasn't identical in performance to the 1st the results were not as good as it could be. Tentec released an update to the 2nd receiver so it performance matched the first but I never found any info on the web about having both modifications at the same time.
I was looking for a Flex 5000 with the 2nd RX installed, but ended up getting a Flex 6300. Receive diversity is the only feature missing that I looking for in a new transceiver. The Flex 6700 was way out my price range.    
Chris
M1PTT 




Robin Maddison
 

When I was making radio commercials Phase problems we had with poor azimuth tracking on broadcast cartridges meant that in stereo most stuff sounded ok. But went all muffled in mono!


RobinM  G0GNE

"Sure beats working!" :-)


On 20 Feb 2017, at 14:20, "David Cutter" <d.cutter@...> wrote:

Hello Robin
 
I doubt if my simple stereo tape recordings of yesteryear had that benefit, but it was very evident that the best quality (listener satisfaction) was gained when combining left and right with a simple resistive adder. 
 
David
G3UNA
----- Original Message -----
Sent: Monday, February 20, 2017 12:26 PM
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

As log as they are in phase!


RobinM  G0GNE

"Sure beats working!" :-)


On 20 Feb 2017, at 12:07, "David Cutter" <d.cutter@...> wrote:

I'm not familiar with the problem, but I wonder if summing the 2 audio outputs would still work.  I noticed when combining music stereo sources that the noise significantly reduces.
 
David
G3UNA
----- Original Message -----
Sent: Monday, February 20, 2017 10:08 AM
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

Chris M1PTT,
the only FT1000 with diversity that I am aware off is a modified FT1000D where a modification adds a relay and enables the source for LO on the second receiver to be switched to the first Rx.

It is bad enough having the two diversity channels moving around in ones head, the last thing you want is the phase of the second LO drifting when compared to the main channel !

cheers
Bob
G3REP



From: "chris.mm1ptt@..." <chris.mm1ptt@...>
To: CrossCountryWireless@groups.io
Sent: Sunday, 19 February 2017, 20:34
Subject: Re: [CrossCountryWireless] SDR-4++ dual diversity SDR receiver

Previously I was looking at transceivers that allowed receive diversity. 
If I remember correctly the only ones that phase locked the receivers were the Elecraft K3, Flex 5000 (with 2nd RX option installed) and the Flex 6700.
The Yaesu FT1000MP (all of them), FT2000, FT DX5000 and the TenTec Orion and Orion II allowed stereo receive but their receivers weren't phase locked. I saw a mod for the TenTec Orion to phase lock the receivers but since the the 2nd receiver wasn't identical in performance to the 1st the results were not as good as it could be. Tentec released an update to the 2nd receiver so it performance matched the first but I never found any info on the web about having both modifications at the same time.
I was looking for a Flex 5000 with the 2nd RX installed, but ended up getting a Flex 6300. Receive diversity is the only feature missing that I looking for in a new transceiver. The Flex 6700 was way out my price range.    
Chris
M1PTT 




 

Some time ago I purchased a MFJ 1026 noise cancelling unit, and it's a really neat toy. You can use it either for noise cancelling, beam steering depending on how you place the antennas, or just a transmit/receive antenna switch.

But having this capability built in (and at that price!) is really amazing.


 

Speaking of diversity, I've noticed that some recent SDR receivers include a 10 MHz input to discipline the oscillator.

Wouldn't it be possible to place receivers at somewhat remote locations with their oscillators synchronized by GPS? It can be useful especially in the lower bands.