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Author Topic: What advantages does an SRD Transmitter provide?  (Read 3796 times)
W7SMJ
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Posts: 126




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« on: June 22, 2011, 08:48:35 PM »

I have been dabbling in SDR for a few months now with an Rfspace SDR-IQ and I have really enjoyed the experience.  It’s amazing to me the versatility and utility this little SDR provides!

I currently own a TS-2000X and it seems to me it would be a pretty straight forward mod to utilize the internal T/R switching of the TS-2000X for antenna switching with any SDR.  There are a few SDR’s that appear to offer outstanding receiver performance, Perseus, Excalibur and QS1R.  My thoughts are by pairing any of these SDR’s with my TS-2000X I would have a pretty good station!
  
The above idea of course precludes utilizing an SDR transmitter.  My question is while there are several clear advantages to an SDR receiver, what are the advantages, if any, to an SDR transmitter?

73,
Scott

Edit: <sigh>... The above subject should of course read "What advantages does an SDR Transmitter provide?
« Last Edit: June 22, 2011, 08:52:45 PM by W7SMJ » Logged
N3OX
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« Reply #1 on: June 22, 2011, 09:09:33 PM »

In principle you can make the transmitter have much lower distortion by PRE-distorting the drive signal to compensate for the distortion in the final amplifier.  You can reduce the intermodulation distortion that causes splatter outside of a SSB passband.

You could have impeccable CW keying, completely click free but very readable.

In practice I don't think enough people actually care about the quantitative details of their transmitter as long as no one yells at them on the air for it. 
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W7SMJ
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Posts: 126




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« Reply #2 on: June 23, 2011, 11:42:11 AM »

In principle you can make the transmitter have much lower distortion by PRE-distorting the drive signal to compensate for the distortion in the final amplifier.  You can reduce the intermodulation distortion that causes splatter outside of a SSB passband.

You could have impeccable CW keying, completely click free but very readable.

In practice I don't think enough people actually care about the quantitative details of their transmitter as long as no one yells at them on the air for it. 

Thanks for the reply, I had heard about predistortion, but I never actually took the time to educate myself on it.  I can see the value and practicality!

However, I wonder if modern day transcievers are lacking enough in spectrual purity to warrant an SDR approach to transmission?  Another opportunity for more research!

73,
Scott
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ZENKI
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Posts: 980




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« Reply #3 on: June 24, 2011, 03:00:11 AM »

Everybody assumes that on SSB  they can use all their radios potential dynamic range. This assumption is false since its the other stations transmitter spectral purity that is defining how well our  receivers potential that can be exploited. You probably never  ever hear these so called top end radios block simply because SSB splatter would cover  up the results of blocking.  Its the same story with the ARRL lab measurements  of dynamic that dig out IMD products out of the phase noise and report that as a valid measurement! Who can hear through receiver phase noise or bad splatter or QRM? So if we ever intend to exploit our radios best ability on SSB, we do require legislated SSB IMD performance like most commercial HF users. Bragging about 100db IMD dynamic range radios on SSB is futile since probably all we need during a SSB contest is 70 db  based on the amount and level of splatter that we hear! Why waste money on high performance receivers when the transmitters block the receivers potential with crud? Ham radio manufacturers are becoming receiver number extremist and dont realize that more effort should be place on transmitter IMD performance. The ADAT is the only transceiver that has a  almost perfect transmitter using pre-distortion techniques.

In principle you can make the transmitter have much lower distortion by PRE-distorting the drive signal to compensate for the distortion in the final amplifier.  You can reduce the intermodulation distortion that causes splatter outside of a SSB passband.

You could have impeccable CW keying, completely click free but very readable.

In practice I don't think enough people actually care about the quantitative details of their transmitter as long as no one yells at them on the air for it. 

Thanks for the reply, I had heard about predistortion, but I never actually took the time to educate myself on it.  I can see the value and practicality!

However, I wonder if modern day transcievers are lacking enough in spectrual purity to warrant an SDR approach to transmission?  Another opportunity for more research!

73,
Scott
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KE5JPP
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« Reply #4 on: June 24, 2011, 03:30:52 AM »

The ADAT is the only transceiver that has a  almost perfect transmitter using pre-distortion techniques.


That is nice and all, but good luck getting an ADAT here in the States.  If you could, the price is pretty high for what it is.  All the talk about the ADAT using pre-distortion is interesting, but from what I have heard from the few that actually have an ADAT, the pre-distortion kind of works sometimes.  In theory it is interesting and fun to talk about.  In reality it is very hard to put into practice other than on a transmitter that always transmits on the same frequency.  For a transmitter that covers the HF bands from 160 to 10 meters, it is very difficult to do.  The same problem exists for the high efficiency class E amplifiers.  They work well on a certain band but it is very hard to design a class E amp that works on all Ham bands.

Gene
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WB6RQN
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« Reply #5 on: June 24, 2011, 09:17:51 AM »

In reality it is very hard to put into practice other than on a transmitter that always transmits on the same frequency.  For a transmitter that covers the HF bands from 160 to 10 meters, it is very difficult to do.

No doubt. OTOH, if you have a full-duplex radio that can listen to its own output, it can be adaptive and autocorrect the errors. That strikes me as a fruitful line of investigation.

73 de Brian, WB6RQN/J79BPL
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KE5JPP
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« Reply #6 on: June 24, 2011, 09:39:06 AM »

In reality it is very hard to put into practice other than on a transmitter that always transmits on the same frequency.  For a transmitter that covers the HF bands from 160 to 10 meters, it is very difficult to do.

No doubt. OTOH, if you have a full-duplex radio that can listen to its own output, it can be adaptive and autocorrect the errors. That strikes me as a fruitful line of investigation.

73 de Brian, WB6RQN/J79BPL


Uh-mm, it has been investigated and is used quite successfully for transmitters that are on a fixed frequency.   There a lot of interesting reading on adaptive pre-distortion application to transmitters.  It works great at a fixed frequency and has its applications in those uses.

Gene
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M0HCN
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Posts: 473




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« Reply #7 on: July 01, 2011, 01:09:25 PM »

You probably do not want to use a conventional receive path for the error term as it will almost certainly have too much group delay or too little bandwidth and loop stability will become seriously problematic.   

Cartesian feedback works and can now (with the advent of low cost DDS parts) be made to work for a multi band set without major pain, but it is far more applicable to a integrated transmitter then it is to something designed to work with an add on power amplifier.
The issue is one of loop bandwidth as the group delay through the PA can (as long as it is small compared to the highest modulating frequency), be compensated by tweaking the phase of the local oscillators for the forward and reverse paths.

I have a design I am working on that I may try to get into Radcom or QEX at some point in the future.

There is a paper from IIRC Marconi dated sometime late 70's or early 80's that discusses an approach to correcting the phase errors in the feedback term. 

They quote 2 tone IMD levels of better then -60dBc (As compared to maybe -30 or so for an uncorrected PA).

The place where this becomes really useful is for folks experimenting with advanced digital modes where things like OFDM impose really nasty linearity requirements on the amplifiers.

As for class E, I suspect that the way to skin that cat is to switch the output network as well as the lowpass filter on a per band basis. You are already switching the lowpass network so moving the relay to the drain side of the tank should not be that hard (for all that the voltages are nasty).
A polar loop would probably be required to linearize such a rig, but it is worth thinking on.
It should be possible to design a switchmode modulation supply that could run directly off the mains and would provide direct conversion to the required envelope drive, while the carrier is produced with appropriate phase modulation at low power. All the switching fets and modulation amplifier circuitry would be common to all bands with the output tank and LPF switched per band.

None of this is unique to a SDT or course, but some DSP will make bits of it easier.

73's Dan M6ATV.
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ZENKI
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Posts: 980




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« Reply #8 on: July 07, 2011, 03:34:02 AM »

Hi Dan

I am looking forward to   reading more about your designs. I would also like to know how the ADAT principle of incorporating an external PA into the adaptive pre-distortion loop works. This would be a great leap forward when the transceiver can help improve the IMD performance of any external PA.

It would be great day for amateur radio when you can buy a perfect transmitter that has no ALC overshoot, no ALC induced splatter and perfect power control thats impossible too over-drive.  Splatter on the ham bands could well and truly be a thing of the past.

ADAT seems to be the only company at the moment that offers us a glimpse of such an ideal transmitter. No other HF transmitter manufacturer in the world offers an off the shelf pre-distortion transmitter, not Selex nor even R&S. Mobat  has done extensive work in this area.

You probably do not want to use a conventional receive path for the error term as it will almost certainly have too much group delay or too little bandwidth and loop stability will become seriously problematic.   

Cartesian feedback works and can now (with the advent of low cost DDS parts) be made to work for a multi band set without major pain, but it is far more applicable to a integrated transmitter then it is to something designed to work with an add on power amplifier.
The issue is one of loop bandwidth as the group delay through the PA can (as long as it is small compared to the highest modulating frequency), be compensated by tweaking the phase of the local oscillators for the forward and reverse paths.

I have a design I am working on that I may try to get into Radcom or QEX at some point in the future.

There is a paper from IIRC Marconi dated sometime late 70's or early 80's that discusses an approach to correcting the phase errors in the feedback term. 

They quote 2 tone IMD levels of better then -60dBc (As compared to maybe -30 or so for an uncorrected PA).

The place where this becomes really useful is for folks experimenting with advanced digital modes where things like OFDM impose really nasty linearity requirements on the amplifiers.

As for class E, I suspect that the way to skin that cat is to switch the output network as well as the lowpass filter on a per band basis. You are already switching the lowpass network so moving the relay to the drain side of the tank should not be that hard (for all that the voltages are nasty).
A polar loop would probably be required to linearize such a rig, but it is worth thinking on.
It should be possible to design a switchmode modulation supply that could run directly off the mains and would provide direct conversion to the required envelope drive, while the carrier is produced with appropriate phase modulation at low power. All the switching fets and modulation amplifier circuitry would be common to all bands with the output tank and LPF switched per band.

None of this is unique to a SDT or course, but some DSP will make bits of it easier.

73's Dan M6ATV.

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KE5JPP
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Posts: 0




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« Reply #9 on: July 07, 2011, 05:08:50 AM »

It would be great day for amateur radio when you can buy a perfect transmitter that has no ALC overshoot, no ALC induced splatter and perfect power control thats impossible too over-drive.  Splatter on the ham bands could well and truly be a thing of the past.

Yeah, good luck banning all of the transceivers that are currently on the air to get to your splatter free utopia.   Roll Eyes

Gene
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M0HCN
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Posts: 473




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« Reply #10 on: July 07, 2011, 08:10:02 AM »

External PAs  are tricky things!

A semiconductor external amp can usually be brought into the loop without too much trouble (My experiments are using an old military amp brick (BLW50F *4 and BLW96 *4 for ~500W, a blast from the past!), but that is because its bandwidth is much greater then the bandwidth of the correction loop. The same cannot unfortunately be said for a typical tube PA with its high Q tank circuit.

Due to the fact that until I take the next level exam (a few days) I am restricted on the air to commercial kit all my experiments have been into a dummy load (Hate the UK licensing setup - why can I not just take the advanced exam? Been designing commercial transmission gear for years).

Basically my loop looks like this:
There are two DDS chips fed by a common clock, one driving the forward direction switching I/Q modulator and the other the mixer for the reverse path from a power sampler at the output of the PA. To compensate the group delay through the PA and so bring the forward and reverse path baseband I/Q pairs into approximate phase so they can be subtracted, the reverse path DDS is programmed to a hundred Hz below the frequency of the forward path, so that the phase of the received IQ pair slips relative to the transmitted pair, the loop is only closed once the point corresponding to minimum error term is found.

A small micro (ARM LPC17xx) controls the whole mess.
There is actually a third DDS to take the IF up and down from the transmit frequency, but that is a detail.

So far results are promising.

I dislike RF AGC in PAs, the approach is just wrong!
You know what the PA is rated for and measure its gain on each band,  so you know the required drive power on each band..... The rest is just a case of setting the drive power limit appropriately. The loop bandwidth inherent in a typical PA AGC loop is too small and as you never really know the transfer function of either the rig or the PA, so how is that loop supposed to work correctly?

If you sold the things, some 'screwdriver expert' would find a way to screw it up, probably by monkeying with the power detector head. See enough of that even on broadcast sites.

Given that the gear manufacturers mostly don't even do the basics when it comes to lowering two tone IMD (Diplexer filters are not rocket science, and they help), instead preferring to boast about how inefficient the rigs are "Class A to 75 watts!", as if that was a selling point, I don't see  commercial predistortion or Cartesian loop (not quite the same thing!) appearing in the mainstream any time soon.

Regards, Dan. 
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