Re: [Flexradio] Zero IF SDR
On 5/23/06, Peter Martinez [EMAIL PROTECTED] wrote: Sami: Please could you try an experiment with this crud I have performed all kinds of tests with SDR-1000, but unfortunately haven't really documented the results. Now, in response to your request, I looked at my old files and quickly put up this page: http://kotisivu.dnainternet.net/ahti/sdr-1000/spec-d44-sdr1k.html Noise around 0 Hz is always going to be an issue with *every* direct-conversion receiver and not just SDR-1000. With extremely careful design of receiver circuit, grounding setup, etc. you can probably lower the near-DC noise to a negligible level. Or alternatively it may prove to be infinitely difficult. So I'm wondering: why insist on zero-IF? I think Gerald's idea of using a frequency offset in SDR-1000 software is an improvement over the original zero-IF idea. A simple software operation solves a difficult hardware problem. 73, Sami ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
From G3PLX: From what Ahti says, there is not a 1/f problem with the current SDR1000, which is what I found with the earlier version, but the oscillator radiation effect may or may not still be there. In the last topic I raised, concerning 24 bit soundcards, I speculated that the 'ideal' ADC would have the size of the least-significant bit the same as the input thermal noise. Present-day cards seem to have about 48dB (8 bits) more noise than this. This means, conceptually at least, we need 48dB gain before them in order to hear the noise floor. If we put all this gain after the QSD (i.e. at the zero I.F), that would certainly make any 1/f problem show, and I think Ahti was saying that it does show with the Softrock hardware. The answer is to move some of the gain in front of the QSD. Personally I am not even convinced that 1/f noise is real - whenever I encountered noise like this in a circuit I was always able to find a cause and fix it, like the oscillator radiation and power-supply effects I mentioned before. The triple op-amp instrumentation amplifier, used in the SDR1000 for the post-QSD stages, may seem to some people as overkill for what seems to be just an audio pre-amp but it does inherently null the power-supply noise. Notice that this part of the circuit has NO coupling capacitors. I have not seen the softrock circuit diagram. Jim's point about poor (or missing) DC coupling probably crossed with my answer to Frank. It really is not a problem. Jim mentioned clock noise again, and I have to say I haven't studied this at all, but my understanding is that it is a strong-signal effect, not a noise level one. I don't think I have seen any such effects, but I haven't looked for them either. I have not yet heard any convincing arguments to say that zero-IF is a bad idea. If the present SDR1000 software can do it, I would be very interested to hear from anyone who would try some tests with it on the latest hardware. If there is noise around the zero-point, does it change if you insert/remove the pre-amp? Does it vanish if you use the SDR1000 to demodulate a sniff of I.F. signal from another receiver? Are there any nasty strong-signal effects? Can you hear the central notch and does it bother you? How far off can you put the image balance before the wanted signal sounds bad? Do some experiments. Let us know the results. 73 Peter ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
Sami Aintila wrote: Peter, I am seeing all kinds of nasty crud around DC, and so are many other SDR-1000 users. On some systems - maybe with some sound cards - maybe with better grounding - the problem is not very prominent. But for optimal (receiver) performance it's always safer to operate at some frequency offset instead of DC. (I haven't checked the source code lately so I'm not sure, but I think the current PowerSDR software actually uses 0 Hz IF for TX.) This corresponds to my experiences with three different sound cards and my SDR. Keep in mind that whatever the defects of my particular receiver, it has an extensive practical lineage. In addition to all my well-documented adventures with it on HF, recall that I originally purchased it from Terry, W0VB, who performed the first moonbounce QSO with it (and in circumstances that Terry considered very challenging in terms of beforehand preparation of that famous event). So, whether mine is the best receiver that came off the line or not is kind of beside the point. Even if it is a sample size of one (three if you count sound card _types_), it has certainly been well-proven in all respects. In particular, much weak signal work has been done with it under a wide variety of conditions, geographies, and bands. Yet, I see, visually, all kinds of noise on the left of the panadapter display under a variety of conditions. Whatever the theory may say, my practical experience suggests that an offset from DC is a good idea. I basically ignore whatever the last kilohertz or so of the panadapter says. If I understand the display at all, it is very suggestive that Zero IF wouldn't be very successful compared to the actual implementation. Larry WO0Z ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
Peter Martinez wrote: ...poor (or missing) DC coupling...really is not a problem... As a generalization, I believe this is untrue. 73 Frank AB2KT ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
An HTML attachment was scrubbed... URL: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/attachments/20060522/55b14537/attachment.html -- next part -- A non-text attachment was scrubbed... Name: zeropeak.gif Type: image/gif Size: 6096 bytes Desc: not available Url : http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/attachments/20060522/55b14537/zeropeak.gif ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
At 02:05 AM 5/22/2006, Peter Martinez wrote: From G3PLX: Jim's point about poor (or missing) DC coupling probably crossed with my answer to Frank. It really is not a problem. Jim mentioned clock noise again, and I have to say I haven't studied this at all, but my understanding is that it is a strong-signal effect, not a noise level one. I don't think I have seen any such effects, but I haven't looked for them either. Clock noise comes from a variety of sources, but imperfect power supply rejection is one case (that is, the A/D clock generator puts some amount of ripple on the power supply, which then couples into the signal path). In any case, noise at the sampling rate aliases right to DC. You see it with no input signal. The tricky thing is if the noise isn't perfectly narrow band, because then it spreads out a bit. Otherwise, you could just take it out like any other DC bias. Depending on how the circuit that makes the A/D clock frequency works, you can also get other noise components harmonically related to the sample rate, most of which will alias to either DC or to fs/2. I would imagine that there's other sampling/mixing artifacts (i.e. leakage of the signals going to the QSD) that might manifest themselves at DC, but in a frequency dependent way (i.e. the I/Q imbalance or the skew between the mux control signals isn't symmetric). Jim ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
Alberto I2PHD wrote: An HTML attachment was scrubbed... Well, the reflector didn't like my HTML formatted message with an embedded picture. This is the price we must pay for the existence of a bunch of idiots that send out spam and viruses...so I repeat here my message in plain text, with a link to the picture. Peter Martinez wrote: Are there any nasty strong-signal effects? Can you hear the central notch and does it bother you? How far off can you put the image balance before the wanted signal sounds bad? Do some experiments. Let us know the results. Well, not always the 0 peak is very pronounced. The picture at the address below shows what I see with the Delta 44, sampling at 96 kHz, using the WMME drivers (but ASIO would not make any difference here), with the inputs left unconnected. The horizontal frequency span is 96 kHz. The program used is Winrad http://sundry.i2phd.com/zeropeak.gif Listening to the 40m band using my own QSD with a couple of ADG704, I can listen to an SSB signal than spans the 0 Hz spot without any adverse artefacts. 73 Alberto I2PHD . . ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
From G3PLX: Alberto: Thanks for your input! The peak in the centre of the spectrum plot you showed at http://sundry.i2phd.com/zeropeak.gif is certainly the small DC offset of the ADC's in the Delta 44. The Zero-IF software would remove this of course. It can be done automatically, it doesn't need a calibration process. Without this null, the DC offset might show as a faint tone, typically at 1700Hz, in the receiver audio. But you didn't hear it anyway on 40m, so there was enough band noise even to drown it. This is a promising result. But it still doesn't show if the new SDR1000 hardware has a low-enough oscillator radiation to be able to do this successfully. Sami's report of 'crud around DC' may say that it will not. Sami: Please could you try an experiment with this crud: First make sure that it is not generated inside the soundcard (see if it stays there with the audio input removed) or in the audio input cable groundloops (unplug the jack from the SDR1000 but touch the jack ground to the SDR100 chassis). If these tests are clear but there is still crud coming through the SDR1000, then see what happens when the antenna is removed and replaced with a 50 ohm termination. If the crud vanishes then it was surely caused by local oscillator radiation intermodulating with LF noise outside the receiver. Or maybe some others could try it. I would really like to know the answer. If it's not going to work with the new SDR1000 hardware then I will take my Zero-IF idea elsewhere. 73 Peter ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
[Flexradio] Zero IF SDR
From G3PLX: The software that comes with the SDR1000 uses an 11.025kHz intermediate frequency. I understand the reasons for doing it this way, but even before the SDR1000 appeared I was doing software radio with an I.F. of zero. By this I mean that the sine and cosine RF oscillators were set right in the middle of the wanted signal, not offset by 11kHz. This may sound impossible to those who were brought up with analogue RF, but that's because it could never be done with analogue circuitry. With DSP it's actually easier to have the 'IF' frequency down in the audio band than to push it up where you can't hear it. The big advantage of zero IF is that the 22kHz image response problem vanishes. Any slight amplitude or phasing unbalance in the Tayloe sampler just results in an equally-slight amount of in-band distortion. The strongest image-frequency signal you ever need to reject is the wanted signal itself. You don't need to worry about a much stronger unwanted signal 22kHz up the band. When I got the SDR1000 kit (I got a very early one), I used it with this technique, and the results were excellent, except for one thing. It took me a while to trace the problem, but I found it in the end and the cause was a surprise. The problem showed as noise around the centre-frequency of all received signals, but it varied across the bands, and was absent when I unplugged the antenna. It was so bad that it made the receiver unusable on some bands with some antennas. But if I used the SDR1000 to tap-off and demodulate the intermediate-frequency of another receiver, it worked perfectly. The cause was oscillator radiation. The DDS oscillator (right in the middle of the wanted signal) was radiating, intermodulating with all kinds of low-frequency noise sources external to the receiver, and the resulting unwanted products (either side of the oscillator frequency) were re-radiated into the antenna. The effect is well-known to anyone who has ever experimented with home-brew direct-conversion receivers, where it usually shows as a raw power-line buzz in the speaker. It's possible that this effect may well have shown in the early work on SDR and it may have been one reason for offsetting the passband by 11kHz in the present software. The fix is to stop the local oscillator radiation. Screening helps a lot but another way is to add an RF stage, or configure the receiver as a superhet with the Tayloe sampler at the I.F. frequency. My early SDR1000 kit didn't have a pre-amp and I understand the current kits do. The local oscillator radiation is probably considerably lower on the present kits, so the zero-IF technique would probably work a lot better than it does on mine. Has anyone here who is writing his own SDR software tried this on the latest hardware? I can provide more details of the zero-IF technique if required. All the well-known modes can be implemented this way, both for receive and transmit. Maybe the present SDR software could be patched to implement zero-IF, or my own zero-IF software could be run in parallel on another soundcard. Would anyone like to have a go? 73 Peter ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
The DSP software is already capable of 0 Hz IF, and has been since its earliest version. IIRC the 11025 Hz IF is primarily a consequence of the frequency response of typical soundcards, which start rolling off somewhere in the vicinity of 200-300 Hz. 73 Frank AB2KT Peter Martinez wrote: From G3PLX: The software that comes with the SDR1000 uses an 11.025kHz intermediate frequency. I understand the reasons for doing it this way, but even before the SDR1000 appeared I was doing software radio with an I.F. of zero. By this I mean that the sine and cosine RF oscillators were set right in the middle of the wanted signal, not offset by 11kHz. This may sound impossible to those who were brought up with analogue RF, but that's because it could never be done with analogue circuitry. With DSP it's actually easier to have the 'IF' frequency down in the audio band than to push it up where you can't hear it. The big advantage of zero IF is that the 22kHz image response problem vanishes. Any slight amplitude or phasing unbalance in the Tayloe sampler just results in an equally-slight amount of in-band distortion. The strongest image-frequency signal you ever need to reject is the wanted signal itself. You don't need to worry about a much stronger unwanted signal 22kHz up the band. When I got the SDR1000 kit (I got a very early one), I used it with this technique, and the results were excellent, except for one thing. It took me a while to trace the problem, but I found it in the end and the cause was a surprise. The problem showed as noise around the centre-frequency of all received signals, but it varied across the bands, and was absent when I unplugged the antenna. It was so bad that it made the receiver unusable on some bands with some antennas. But if I used the SDR1000 to tap-off and demodulate the intermediate-frequency of another receiver, it worked perfectly. The cause was oscillator radiation. The DDS oscillator (right in the middle of the wanted signal) was radiating, intermodulating with all kinds of low-frequency noise sources external to the receiver, and the resulting unwanted products (either side of the oscillator frequency) were re-radiated into the antenna. The effect is well-known to anyone who has ever experimented with home-brew direct-conversion receivers, where it usually shows as a raw power-line buzz in the speaker. It's possible that this effect may well have shown in the early work on SDR and it may have been one reason for offsetting the passband by 11kHz in the present software. The fix is to stop the local oscillator radiation. Screening helps a lot but another way is to add an RF stage, or configure the receiver as a superhet with the Tayloe sampler at the I.F. frequency. My early SDR1000 kit didn't have a pre-amp and I understand the current kits do. The local oscillator radiation is probably considerably lower on the present kits, so the zero-IF technique would probably work a lot better than it does on mine. Has anyone here who is writing his own SDR software tried this on the latest hardware? I can provide more details of the zero-IF technique if required. All the well-known modes can be implemented this way, both for receive and transmit. Maybe the present SDR software could be patched to implement zero-IF, or my own zero-IF software could be run in parallel on another soundcard. Would anyone like to have a go? 73 Peter ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
Another reason for using IF higher than 0 Hz is the high inherent noise level of typical transistors and opamps at low frequencies (so called 1/f-noise). This and the leakage of the VFO signals made me to move away from the zero-IF in my early switching (and Tayloe) mixer experiments. Fortunately, before spending too much time for re-inventing the wheel came Gerald's famous first article in QSX - and here I am! Now is the time to modify the wheel! 73, Ahti OH2RZ On 21/05/06, Frank Brickle [EMAIL PROTECTED] wrote: The DSP software is already capable of 0 Hz IF, and has been since its earliest version. IIRC the 11025 Hz IF is primarily a consequence of the frequency response of typical soundcards, which start rolling off somewhere in the vicinity of 200-300 Hz. 73 Frank AB2KT Peter Martinez wrote: From G3PLX: The software that comes with the SDR1000 uses an 11.025kHz intermediate frequency. I understand the reasons for doing it this way, but even before the SDR1000 appeared I was doing software radio with an I.F. of zero. By this I mean that the sine and cosine RF oscillators were set right in the middle of the wanted signal, not offset by 11kHz. This may sound impossible to those who were brought up with analogue RF, but that's because it could never be done with analogue circuitry. With DSP it's actually easier to have the 'IF' frequency down in the audio band than to push it up where you can't hear it. The big advantage of zero IF is that the 22kHz image response problem vanishes. Any slight amplitude or phasing unbalance in the Tayloe sampler just results in an equally-slight amount of in-band distortion. The strongest image-frequency signal you ever need to reject is the wanted signal itself. You don't need to worry about a much stronger unwanted signal 22kHz up the band. When I got the SDR1000 kit (I got a very early one), I used it with this technique, and the results were excellent, except for one thing. It took me a while to trace the problem, but I found it in the end and the cause was a surprise. The problem showed as noise around the centre-frequency of all received signals, but it varied across the bands, and was absent when I unplugged the antenna. It was so bad that it made the receiver unusable on some bands with some antennas. But if I used the SDR1000 to tap-off and demodulate the intermediate-frequency of another receiver, it worked perfectly. The cause was oscillator radiation. The DDS oscillator (right in the middle of the wanted signal) was radiating, intermodulating with all kinds of low-frequency noise sources external to the receiver, and the resulting unwanted products (either side of the oscillator frequency) were re-radiated into the antenna. The effect is well-known to anyone who has ever experimented with home-brew direct-conversion receivers, where it usually shows as a raw power-line buzz in the speaker. It's possible that this effect may well have shown in the early work on SDR and it may have been one reason for offsetting the passband by 11kHz in the present software. The fix is to stop the local oscillator radiation. Screening helps a lot but another way is to add an RF stage, or configure the receiver as a superhet with the Tayloe sampler at the I.F. frequency. My early SDR1000 kit didn't have a pre-amp and I understand the current kits do. The local oscillator radiation is probably considerably lower on the present kits, so the zero-IF technique would probably work a lot better than it does on mine. Has anyone here who is writing his own SDR software tried this on the latest hardware? I can provide more details of the zero-IF technique if required. All the well-known modes can be implemented this way, both for receive and transmit. Maybe the present SDR software could be patched to implement zero-IF, or my own zero-IF software could be run in parallel on another soundcard. Would anyone like to have a go? 73 Peter ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
Typo correction: ... article in QEX... Ahti OH2RZ On 21/05/06, Ahti Aintila [EMAIL PROTECTED] wrote: Another reason for using IF higher than 0 Hz is the high inherent noise level of typical transistors and opamps at low frequencies (so called 1/f-noise). This and the leakage of the VFO signals made me to move away from the zero-IF in my early switching (and Tayloe) mixer experiments. Fortunately, before spending too much time for re-inventing the wheel came Gerald's famous first article in QSX - and here I am! Now is the time to modify the wheel! 73, Ahti OH2RZ On 21/05/06, Frank Brickle [EMAIL PROTECTED] wrote: The DSP software is already capable of 0 Hz IF, and has been since its earliest version. IIRC the 11025 Hz IF is primarily a consequence of the frequency response of typical soundcards, which start rolling off somewhere in the vicinity of 200-300 Hz. 73 Frank AB2KT Peter Martinez wrote: From G3PLX: The software that comes with the SDR1000 uses an 11.025kHz intermediate frequency. I understand the reasons for doing it this way, but even before the SDR1000 appeared I was doing software radio with an I.F. of zero. By this I mean that the sine and cosine RF oscillators were set right in the middle of the wanted signal, not offset by 11kHz. This may sound impossible to those who were brought up with analogue RF, but that's because it could never be done with analogue circuitry. With DSP it's actually easier to have the 'IF' frequency down in the audio band than to push it up where you can't hear it. The big advantage of zero IF is that the 22kHz image response problem vanishes. Any slight amplitude or phasing unbalance in the Tayloe sampler just results in an equally-slight amount of in-band distortion. The strongest image-frequency signal you ever need to reject is the wanted signal itself. You don't need to worry about a much stronger unwanted signal 22kHz up the band. When I got the SDR1000 kit (I got a very early one), I used it with this technique, and the results were excellent, except for one thing. It took me a while to trace the problem, but I found it in the end and the cause was a surprise. The problem showed as noise around the centre-frequency of all received signals, but it varied across the bands, and was absent when I unplugged the antenna. It was so bad that it made the receiver unusable on some bands with some antennas. But if I used the SDR1000 to tap-off and demodulate the intermediate-frequency of another receiver, it worked perfectly. The cause was oscillator radiation. The DDS oscillator (right in the middle of the wanted signal) was radiating, intermodulating with all kinds of low-frequency noise sources external to the receiver, and the resulting unwanted products (either side of the oscillator frequency) were re-radiated into the antenna. The effect is well-known to anyone who has ever experimented with home-brew direct-conversion receivers, where it usually shows as a raw power-line buzz in the speaker. It's possible that this effect may well have shown in the early work on SDR and it may have been one reason for offsetting the passband by 11kHz in the present software. The fix is to stop the local oscillator radiation. Screening helps a lot but another way is to add an RF stage, or configure the receiver as a superhet with the Tayloe sampler at the I.F. frequency. My early SDR1000 kit didn't have a pre-amp and I understand the current kits do. The local oscillator radiation is probably considerably lower on the present kits, so the zero-IF technique would probably work a lot better than it does on mine. Has anyone here who is writing his own SDR software tried this on the latest hardware? I can provide more details of the zero-IF technique if required. All the well-known modes can be implemented this way, both for receive and transmit. Maybe the present SDR software could be patched to implement zero-IF, or my own zero-IF software could be run in parallel on another soundcard. Would anyone like to have a go? 73 Peter ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com ___ FlexRadio mailing list
Re: [Flexradio] Zero IF SDR
From G3PLX: I just checked my two soundcards for the low-frequency roll-off. My new Firebox is 2.4dB down at 1.8Hz and the MP3+ is 1.5dB down at 1.2Hz. And that was done quickly by linking line-out to line-in, so it includes the LF roll-off of the transmit side too. I am quite certain the music business wouldn't touch a soundcard that rolled-off at 200Hz. The LF roll-off is really not a problem for zero-IF anyway. Even if you put the oscillator right in the centre, which theoretically puts a deep narrow null in the passband, I defy anyone to notice it's there on an SSB signal. There are ways to eliminate this null completely, but I really don't think we need to do it. To Ahti: I have never seen 1/f noise in my zero-IF work (I designed such a receiver before I retired, for HF GMDSS working). The local oscillator radiation problem looks just like 1/f noise, but that can be fixed once it is recognised. It's also possible that poor post-mixer design could result in supply-line noise being a problem (this has a 1/f spectrum), but the post-mixer amplifier design of the SDR1000 kit is excellent in this respect. If 1/f noise was present, it would show as a noise peak at the centre of the output spectrum. There is no such peak. If, as Frank says, the SDR1000 software can do zero-IF already, has anyone done any tests with it? What were the results? Were there any problems? Has the local oscillator radiation problem gone now that the RF amplifier is in place? I think it's worth looking at this area again. The 22kHz image problem will be tolerated by SDR1000 fans but this is surely not a proper solution. My GMDSS receiver would not have gained it's approval certificate if the operator had to balance the image rejection each time he changed bands! 73 Peter ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
Peter, You are right about the 22 kHz image in transmission. That is why I am reluctant to transmit without checking (and adjusting) the attenuation on the used frequency. With the preamplifier board the leakage of the sampling signal still can be detected by my other receivers. In practice it is no problem on the usual noisy bands. That is true also with the 1/f-noise, if you are running with the gains now used in the SDR-1000. In my earlier experiments with zero-IF I tried to maximize the dynamic range without any preamplifier. Then the 1/f-noise determines your weak signal performance. The maximum signal will be about 4 Vpp at the 200 ohm level that the QSD sees and can handle. This makes about 0 dBm at the antenna connector. Just for an explanation, this experiment was made for a commercial instrumentation project handling about 20 kHz bandwidth. If you have a SoftRock receiver available, you may tune across the 0 Hz IF. With the present high gain opamp you hardly can see anything special. Try to set the gain to 0 dB, then you possibly will find a difference. Measure the signals before the sound card. 73, Ahti OH2RZ On 21/05/06, Peter Martinez [EMAIL PROTECTED] wrote: From G3PLX: I just checked my two soundcards for the low-frequency roll-off. My new Firebox is 2.4dB down at 1.8Hz and the MP3+ is 1.5dB down at 1.2Hz. And that was done quickly by linking line-out to line-in, so it includes the LF roll-off of the transmit side too. I am quite certain the music business wouldn't touch a soundcard that rolled-off at 200Hz. The LF roll-off is really not a problem for zero-IF anyway. Even if you put the oscillator right in the centre, which theoretically puts a deep narrow null in the passband, I defy anyone to notice it's there on an SSB signal. There are ways to eliminate this null completely, but I really don't think we need to do it. To Ahti: I have never seen 1/f noise in my zero-IF work (I designed such a receiver before I retired, for HF GMDSS working). The local oscillator radiation problem looks just like 1/f noise, but that can be fixed once it is recognised. It's also possible that poor post-mixer design could result in supply-line noise being a problem (this has a 1/f spectrum), but the post-mixer amplifier design of the SDR1000 kit is excellent in this respect. If 1/f noise was present, it would show as a noise peak at the centre of the output spectrum. There is no such peak. If, as Frank says, the SDR1000 software can do zero-IF already, has anyone done any tests with it? What were the results? Were there any problems? Has the local oscillator radiation problem gone now that the RF amplifier is in place? I think it's worth looking at this area again. The 22kHz image problem will be tolerated by SDR1000 fans but this is surely not a proper solution. My GMDSS receiver would not have gained it's approval certificate if the operator had to balance the image rejection each time he changed bands! 73 Peter ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
Re: [Flexradio] Zero IF SDR
At 09:41 AM 5/21/2006, Peter Martinez wrote: From G3PLX: The software that comes with the SDR1000 uses an 11.025kHz intermediate frequency. I understand the reasons for doing it this way, but even before the SDR1000 appeared I was doing software radio with an I.F. of zero. By this I mean that the sine and cosine RF oscillators were set right in the middle of the wanted signal, not offset by 11kHz. This may sound impossible to those who were brought up with analogue RF, but that's because it could never be done with analogue circuitry. With DSP it's actually easier to have the 'IF' frequency down in the audio band than to push it up where you can't hear it. snip There's actually several things pushing towards choosing an IF near fs/4.. whether doing I/Q or single channel sampling: 1) most sampling systems (particularly sound cards with AC coupling) have poor response at DC. 2) most sampling systems have significant clock noise at the sample rate (which aliases to DC) and at half the clock rate. Putting the signal at fs/4 puts it squarely between the noise contributions at zero and fs/2. Doing I/Q (with any IF) helps with effectively doubling the sample rate (so that the signal of interest is a smaller fraction of the sampling bandwidth, which helps with filtering.. the filters can be less steep) However I/Q sampling comes at a cost of having to deal with balance. When talking about I/Q balance we need to realize that it's not just two numbers (phase and amplitude) although that's a reasonable approximation in the middle of the passband. But, as you get closer to the analog passband edges (i.e. buffer amp upper frequency rolloff, and low frequency AC coupling) then the odds of the slopes exactly matching are less (mostly because the analog filter components, coupling capacitors and resistors and the like) tend not to be exactly the same (particularly over temperature). So, to do a good job of image cancellation, etc., you need to measure the imbalance at every frequency and apply the inverse filter. Or, equivalently, measure the impulse response and deconvolve it with the incoming signal. (the single step balance fix in the current PowerSDR is basically a zero order time domain filter) Has anyone here who is writing his own SDR software tried this on the latest hardware? I can provide more details of the zero-IF technique if required. All the well-known modes can be implemented this way, both for receive and transmit. Maybe the present SDR software could be patched to implement zero-IF, or my own zero-IF software could be run in parallel on another soundcard. Would anyone like to have a go? Another advantage of running at an IF instead zero-IF is that once you've dealt with the frequency conversion in the signal processing domain, you can move the IF around a bit without too much trouble. That way, your LO can go in discrete steps, even if your tuned frequency is continuous. The fine tuning is in the signal processing. That way you can adjust the DDS frequency to put spurs where the signal isn't. 73 Peter ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com James Lux, P.E. Spacecraft Radio Frequency Subsystems Group Flight Communications Systems Section Jet Propulsion Laboratory, Mail Stop 161-213 4800 Oak Grove Drive Pasadena CA 91109 tel: (818)354-2075 fax: (818)393-6875 ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
