Receiving and demodulation
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In playing with PSK31 recently using a *very* suboptimal receiver (my
old Icom IC-706mkII) I realized how important it is to couple the demodulation system with the the rest of the receiver. For example, I can watch the AGC respond to unwanted signals and make the desired signal uncopyable. So it seems to me that, in the process of demodulating a signal the demodulator has information about the energy in the desired signal that should be used to drive the AGC. That implies to me that you really cannot effectively separate the two. The "sound card plugged into the radio" approach is interesting as a way to start but it is not the final solution. One of the interesting features of the K3 is that PSK31 and RTTY operation are part of the radio. Does that mean that the PSK31 and RTTY demodulators in the K3 control the AGC so that the desired signal and only the desired signal controls AGC? As we come up with more and more interesting digital modulation schemes for HF I can see the desire to closely couple the demodulator with the rest of the receiving system. The K3 has some real possibility here. Will it be incorporated? How about an API to let other programmers add demodulators and couple them to the rest of the receiver, controlling roofing filter selection, AGC, etc.? 73 de Brian, WB6RQN Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Brian,
The 1st IF (hardware) AGC is highly dependent on the width of the 'roofing filter'. We do not have the relevant figures for the relative dynamic range of the 8.215 kHz IF vs. the 15 kHz DSP portion of the receiver. When those specifics are available, your question can be answered. For now, I only assume that a narrow filter will be the best solution for AGC 'pumping' when operating digital modes. The capability to demodulate PSK31 and RTTY signals and the AGC 'pumping' situation are entirely two different animals. One thing is for certain, the AGC cannot be totally controlled by the DSP section, so the real answer depends on the actual dynamic range that the A-D conversion can handle - we don't have that answer yet. 73, Don W3FPR Brian Lloyd wrote: > In playing with PSK31 recently using a *very* suboptimal receiver (my > old Icom IC-706mkII) I realized how important it is to couple the > demodulation system with the the rest of the receiver. For example, I > can watch the AGC respond to unwanted signals and make the desired > signal uncopyable. So it seems to me that, in the process of > demodulating a signal the demodulator has information about the energy > in the desired signal that should be used to drive the AGC. That implies > to me that you really cannot effectively separate the two. The "sound > card plugged into the radio" approach is interesting as a way to start > but it is not the final solution. > > One of the interesting features of the K3 is that PSK31 and RTTY > operation are part of the radio. Does that mean that the PSK31 and RTTY > demodulators in the K3 control the AGC so that the desired signal and > only the desired signal controls AGC? > > As we come up with more and more interesting digital modulation schemes > for HF I can see the desire to closely couple the demodulator with the > rest of the receiving system. The K3 has some real possibility here. > Will it be incorporated? > > How about an API to let other programmers add demodulators and couple > them to the rest of the receiver, controlling roofing filter selection, > AGC, etc.? > > 73 de Brian, WB6RQN > Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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On May 13, 2007, at 11:03 PM, Don Wilhelm wrote:
> Brian, > > The 1st IF (hardware) AGC is highly dependent on the width of the > 'roofing filter'. We do not have the relevant figures for the > relative dynamic range of the 8.215 kHz IF vs. the 15 kHz DSP > portion of the receiver. > > When those specifics are available, your question can be answered. > For now, I only assume that a narrow filter will be the best > solution for AGC 'pumping' when operating digital modes. > > The capability to demodulate PSK31 and RTTY signals and the AGC > 'pumping' situation are entirely two different animals. In what we have come to see as a traditional receiving system I would completely agree with you. As we migrate to newer and more efficient modulation schemes I think we are going to find a need to couple them together in order to extract maximum performance. With CW and SSB we have very simple modulation and demodulation schemes. We can use a relatively simple envelope detector to generate AGC. It gets a bit more difficult with more complex modulation schemes. OTOH, all we are trying to do is to ensure that the peak signal level does not exceed the range of our A:D. Perhaps a simple envelope peak detector will work for that too. > One thing is for certain, the AGC cannot be totally controlled by > the DSP section, Oh? That seems odd to me. It strikes me that the desired signal and only the desired signal should have an effect on AGC. Since the final bandwidth determination and demodulation take place in the DSP stage, the AGC needs to be derived at that point. That implies that the DSP stage needs to generate the AGC. Hmm, no, we need the AGC to be derived from the signal arriving at the A:D and not post bandwidth limiting in order to reduce gain and prevent clipping at the A:D. So it still needs to be derived in the DSP after the A:D but before we do the final bandwidth limiting. OK, here is the scenario I have in mind. I am going to use made-up numbers for convenience and these are not to imply that they are or should be specific numbers for the K3. I am going to copy a relatively weak PSK31 signal. He is going to be about 100Hz wide and high enough out of the noise for good copy but no higher. I have a 500Hz roofing filter. My desired signal is toward the edge of the roofing filter's passband. A QSO between two very strong PSK31 stations is going on 300Hz away but still inside the passband of my roofing filter. I have used the DSP to limit the final bandwidth to include only my desired signal. I cannot "hear" the undesired QSO but the receiver still has to deal with it as it is arriving at the A:D. If the AGC is entirely derived at analog before the A:D the undesired signals are going to modulate my AGC and hence the gain for the desired signal thus changing the S:N or Eb/No. If the AGC is derived after my final filtering in the DSP, there is a possibility that the strong signals will drive the A:D into clipping. Back in the golden days of tube-type radios (my first receiver was a Hammarlund HQ-129x and my first transmitters were an ARC-5 and a 10w Central Electronics Multi-Phase exciter -- I was a poor kid in junior high and I couldn't afford anything better) I would solve the problem by turning off AGC and riding the RF gain myself to keep both signals in the "sweet spot" (dynamic range) of the receiver. It would be low enough that the strong signal would not overload the receiver and high enough that the desired signal was far enough out of the noise to be useful copy. I was, in effect, an intelligent AGC. (Well, in my case "intelligent" might be pushing it a bit. :-) So getting back to the point, as I understand it, the reason for a very high IP3 and BDR is to prevent strong signals that do not affect the AGC from inducing compression in earlier stages or from exceeding the range of the A:D. I understand that, in order to keep the A:D from clipping and causing the whole receiver to go to hell, we have to provide some AGC on everything in the passband of the roofing filter. OTOH, we also need some awareness of the state of our desired signal in order to prevent undesired changes of AGC from making our desired signal uncopyable. In a perfect world my demodulator would look at my desired signal and adjust the AGC to the highest point that would give me an acceptable Eb/No for a low bit error rate. Increasing the gain to get a higher Eb/No doesn't get me anything but it means that undesired signals will have more chance to clip in the A:D. So I see a new, smart receiver looking at what is in the analog passband, looking at my desired signal, and then "ride the gain" to do the right thing. If it is not possible to meet both requirements, i.e. sufficient Eb/No without A:D clipping, it could even give me a light on the front panel that says something like, "I'm sorry Dave but I don't think I can do that." (Apologies to Arthur C. Clarke.) I then know that I have exceeded the limitations of my receiver. > so the real answer depends on the actual dynamic range that the A-D > conversion can handle - we don't have that answer yet. I understand. Still, I think a systems approach and understanding the possibilities is reasonable. I look at the topology of the K3 receiver and I know that what I am talking about is possible given the existing hardware. It just depends on how much more processing you want to do in the DSP to achieve it. As more demodulators get moved to the DSP I think this might become mandatory. > > 73, > Don W3FPR 73 de Brian, WB6RQN Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Brian Lloyd-6
> One of the interesting features of the K3 is that PSK31 and RTTY operation
>are part of the radio. Does that mean that the PSK31 and RTTY demodulators >in the K3 control the AGC so that the desired signal and only the desired >signal controls AGC? The use of an SSB bandwidth "waterfall" display for PSK31 is a great example of using the wrong filter for the job, at least after you've established the QSO. The Rx AGC gets pumped by strong signals inside the "roofing filter" bandwidth, as you've observed. On the K3, when I am copying a PSK31 signal, I spin the selectivity right down to 50 Hz. Then, the PSK31 signjal I am copuying is the only one inside the fitler bandwidth, and is the one driving the AGC. 73, Lyle KK7P _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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On May 23, 2007, at 9:33 AM, lyle johnson wrote: >> One of the interesting features of the K3 is that PSK31 and RTTY >> operation are part of the radio. Does that mean that the PSK31 >> and RTTY demodulators in the K3 control the AGC so that the >> desired signal and only the desired signal controls AGC? > > The use of an SSB bandwidth "waterfall" display for PSK31 is a > great example of using the wrong filter for the job, at least after > you've established the QSO. The Rx AGC gets pumped by strong > signals inside the "roofing filter" bandwidth, as you've observed. > > On the K3, when I am copying a PSK31 signal, I spin the selectivity > right down to 50 Hz. Then, the PSK31 signjal I am copuying is the > only one inside the fitler bandwidth, and is the one driving the AGC. Thanks Lyle. That helps. Sounds like AGC is derived from the envelope energy in the passband. I was thinking of actually looking at the energy contained in the desired signal, in this case the PSK- modulated carrier and the associated sidebands, and using that to control the AGC even tho' there are other signals in the passband. This could result in the AGC being set such that the A:D saturates but that's life. Thought: set the Eb/No of the desired signal to the desired 12dB or so using the AGC and then track it. That would guarantee the maximum headroom regardless of filter selection. And narrowing the roofing filter as much as possible is also nice ... unless you are trying to copy more than one signal in the passband simultaneously. 73 de Brian, WB6RQN Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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