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Direct conversion spectrum analyser
here is a spectrum analyser design that i would like the group to
comment upon. 1) we take the input via a low pass filter, up convert it to an IF of 100Mhz or so, and follow it up with a direct conversion receiver at 100 Mhz with 20 khz bandwidth. 2) the upconverting local oscillator is a VCO that is controlled by a sweep generator. the sweep is controlled by a PWM modulated signal in the audio range. 3) the sweep generator input is connected to the output of a PC sound card. the output of the direct conversion receiver is connected to the input of the PC sound card. Now, by clever programming of the sound card on the PC, we can make the VCO sweep our passband of interest. The sound is often digitsed at 16 bit levels (in the better systems at 32-bit level). This will effectively give us 90db range. the lograithmic scale can be implemented in software. DSP can be used to set the bandwidth to any particular size. the most important benefit of this design will be that even hams without expensive oscilloscopes will be able to easily make a PC based analyser that is easy to assemble and use. if there isn't any glaring problem with this design, i would like to pull out my soldering iron and take a go. is anyone here with spectrum analyser experience willing to share knowledge? |
"Ashhar Farhan" wrote in message om... here is a spectrum analyser design that i would like the group to comment upon. 1) we take the input via a low pass filter, up convert it to an IF of 100Mhz or so, and follow it up with a direct conversion receiver at 100 Mhz with 20 khz bandwidth. 2) the upconverting local oscillator is a VCO that is controlled by a sweep generator. the sweep is controlled by a PWM modulated signal in the audio range. 3) the sweep generator input is connected to the output of a PC sound card. the output of the direct conversion receiver is connected to the input of the PC sound card. Now, by clever programming of the sound card on the PC, we can make the VCO sweep our passband of interest. The sound is often digitsed at 16 bit levels (in the better systems at 32-bit level). This will effectively give us 90db range. the lograithmic scale can be implemented in software. DSP can be used to set the bandwidth to any particular size. the most important benefit of this design will be that even hams without expensive oscilloscopes will be able to easily make a PC based analyser that is easy to assemble and use. if there isn't any glaring problem with this design, i would like to pull out my soldering iron and take a go. is anyone here with spectrum analyser experience willing to share knowledge? I have recently completed a spectrum analyser, see http://www.hanssummers.com/electroni...yser/index.htm. It is awaiting possible magasine publication so there are not yet any circuit or construction details on the page above. If you want the full details, email me privately and I'll show them to you. I also tried a direct conversion receiver initially. It doesn't work on in analogue (i.e. non-PC) analyser, because there are all sorts of heterodynes of the sweep frequency against the directly converted incoming signal. Of course I kicked myself afterwards for not thinking of it in advance to save myself the time of the experiment. I think broadly speaking the final IF should be substantially higher than the frequency of the sweep waveform, so that the final filtering works faster than the sweep. There are lots of people in this forum far more advanced than me who will probably be able to explain it better in terms of filter response times or group delays or something. Though there might be a way of untangling everything in software so it may work. Being direct conversion you'll also have both sidebands present, which will create further complications. Again, clever software might untangle it but I think it's far from straightforward. Another problem is the narrow bandwidth. 20KHz is a nice bandwidth to have but I think in a spectrum analyser you also want wider bandwidths available. In particular, if you are digitally generating your sweep voltage, and trying to cover the whole 100MHz, you need of the order of 100,000 / 20 = 5,000 discrete measurement intervals. You can't display that many horizontal pixels on screen. You could average them in software, but at the low 20KHz bandwidth, you're going to need quite a slow sweep rate. 5,000 measurements are a lot and will take a long time. It's a nice idea but I don't think it will work as it stands. My recommendation would be to add a 2nd IF to your design, 2nd IF amp and logarithmic detector. In my design I used a 145MHz 1st IF, so the VCO sweeps 145 - 290MHz. The 2nd local oscillator is at 153MHz for an 8MHz 2nd IF, amplified then passed into an AD8307 logarithmic amplifier. Anything similar would work well. I used an SA602 front end for simplicity, but a diode ring mixer would give potentially better performance than the 65-70dB dynamic range I achieved. You can still use the PC for a nice display, rather than an oscilloscope. Just feed the log output into your PC sound card, and have the PC sound card control the sweep as you suggest. I think you'll solve a lot of problems by adding these few extra modules to the analogue front end before introducing the PC. Incidentally, this is exactly what I'm doing with my Mk2 analyser, see http://www.hanssummers.com/electroni...ser2/index.htm. Now I'm hoping for someone to put it all more clearly and professionally than I have here ;-) 73 Hans G0UPL http://www.HansSummers.com |
"Ashhar Farhan" wrote in message om... here is a spectrum analyser design that i would like the group to comment upon. 1) we take the input via a low pass filter, up convert it to an IF of 100Mhz or so, and follow it up with a direct conversion receiver at 100 Mhz with 20 khz bandwidth. 2) the upconverting local oscillator is a VCO that is controlled by a sweep generator. the sweep is controlled by a PWM modulated signal in the audio range. 3) the sweep generator input is connected to the output of a PC sound card. the output of the direct conversion receiver is connected to the input of the PC sound card. Now, by clever programming of the sound card on the PC, we can make the VCO sweep our passband of interest. The sound is often digitsed at 16 bit levels (in the better systems at 32-bit level). This will effectively give us 90db range. the lograithmic scale can be implemented in software. DSP can be used to set the bandwidth to any particular size. the most important benefit of this design will be that even hams without expensive oscilloscopes will be able to easily make a PC based analyser that is easy to assemble and use. if there isn't any glaring problem with this design, i would like to pull out my soldering iron and take a go. is anyone here with spectrum analyser experience willing to share knowledge? I have recently completed a spectrum analyser, see http://www.hanssummers.com/electroni...yser/index.htm. It is awaiting possible magasine publication so there are not yet any circuit or construction details on the page above. If you want the full details, email me privately and I'll show them to you. I also tried a direct conversion receiver initially. It doesn't work on in analogue (i.e. non-PC) analyser, because there are all sorts of heterodynes of the sweep frequency against the directly converted incoming signal. Of course I kicked myself afterwards for not thinking of it in advance to save myself the time of the experiment. I think broadly speaking the final IF should be substantially higher than the frequency of the sweep waveform, so that the final filtering works faster than the sweep. There are lots of people in this forum far more advanced than me who will probably be able to explain it better in terms of filter response times or group delays or something. Though there might be a way of untangling everything in software so it may work. Being direct conversion you'll also have both sidebands present, which will create further complications. Again, clever software might untangle it but I think it's far from straightforward. Another problem is the narrow bandwidth. 20KHz is a nice bandwidth to have but I think in a spectrum analyser you also want wider bandwidths available. In particular, if you are digitally generating your sweep voltage, and trying to cover the whole 100MHz, you need of the order of 100,000 / 20 = 5,000 discrete measurement intervals. You can't display that many horizontal pixels on screen. You could average them in software, but at the low 20KHz bandwidth, you're going to need quite a slow sweep rate. 5,000 measurements are a lot and will take a long time. It's a nice idea but I don't think it will work as it stands. My recommendation would be to add a 2nd IF to your design, 2nd IF amp and logarithmic detector. In my design I used a 145MHz 1st IF, so the VCO sweeps 145 - 290MHz. The 2nd local oscillator is at 153MHz for an 8MHz 2nd IF, amplified then passed into an AD8307 logarithmic amplifier. Anything similar would work well. I used an SA602 front end for simplicity, but a diode ring mixer would give potentially better performance than the 65-70dB dynamic range I achieved. You can still use the PC for a nice display, rather than an oscilloscope. Just feed the log output into your PC sound card, and have the PC sound card control the sweep as you suggest. I think you'll solve a lot of problems by adding these few extra modules to the analogue front end before introducing the PC. Incidentally, this is exactly what I'm doing with my Mk2 analyser, see http://www.hanssummers.com/electroni...ser2/index.htm. Now I'm hoping for someone to put it all more clearly and professionally than I have here ;-) 73 Hans G0UPL http://www.HansSummers.com |
"Hans Summers" wrote in message news:bmgam6
I also tried a direct conversion receiver initially. It doesn't work on in analogue (i.e. non-PC) analyser, because there are all sorts of heterodynes of the sweep frequency against the directly converted incoming signal. Of course I kicked myself afterwards for not thinking of it in advance to save myself the time of the experiment. a) i am not proposing a direct direct conversion. i am proposing an up conversion exactly like han's MK1. for the second conversion, i am suggesting a direct conversion to base-band from the high IF. as you rightly point out harmonic mixing is a problem with broad-band direct conversion receivers, therefore, you need to have a low-pass or a band-pass ahead of a diode mixer working as a product detector. b) my second suggestion is to do the logarithmic calculations digitally on the PC. that will simplify the design. c) as for the granuality of the sweep, for finer resolution the sweep range will be decreased. that is one way to get higher resolution, the other is to slow down the sweep. what kind of a VCO are you using? and how are you ensuring that the vco output remains constant throughout the sweep? i expect that the VCO's amplitude will also effect the first mixer gain. - farhan |
"Hans Summers" wrote in message news:bmgam6
I also tried a direct conversion receiver initially. It doesn't work on in analogue (i.e. non-PC) analyser, because there are all sorts of heterodynes of the sweep frequency against the directly converted incoming signal. Of course I kicked myself afterwards for not thinking of it in advance to save myself the time of the experiment. a) i am not proposing a direct direct conversion. i am proposing an up conversion exactly like han's MK1. for the second conversion, i am suggesting a direct conversion to base-band from the high IF. as you rightly point out harmonic mixing is a problem with broad-band direct conversion receivers, therefore, you need to have a low-pass or a band-pass ahead of a diode mixer working as a product detector. b) my second suggestion is to do the logarithmic calculations digitally on the PC. that will simplify the design. c) as for the granuality of the sweep, for finer resolution the sweep range will be decreased. that is one way to get higher resolution, the other is to slow down the sweep. what kind of a VCO are you using? and how are you ensuring that the vco output remains constant throughout the sweep? i expect that the VCO's amplitude will also effect the first mixer gain. - farhan |
"Ashhar Farhan" wrote in message om... "Hans Summers" wrote in message news:bmgam6 I also tried a direct conversion receiver initially. It doesn't work on in analogue (i.e. non-PC) analyser, because there are all sorts of heterodynes of the sweep frequency against the directly converted incoming signal. Of course I kicked myself afterwards for not thinking of it in advance to save myself the time of the experiment. a) i am not proposing a direct direct conversion. i am proposing an up conversion exactly like han's MK1. for the second conversion, i am suggesting a direct conversion to base-band from the high IF. as you rightly point out harmonic mixing is a problem with broad-band direct conversion receivers, therefore, you need to have a low-pass or a band-pass ahead of a diode mixer working as a product detector. That wasn't the effect I was worrying about... after all your up mixing and direct conversion of the VHF IF down to baseband, and low pass filtered it, you'll end up with say 20KHz of audio baseband to feed into your soundcard. Unfortunately your sweep rate is also down in the same range. Which is a problem because whatever method you use for envelope detection of the baseband will have to react faster than the sweep rate. Though as I say, if you do it all in software you might be able to untangle it. b) my second suggestion is to do the logarithmic calculations digitally on the PC. that will simplify the design. It simplifies the analogue design and complicates the software. 16 bits of resolution should give adequate dynamic range. Depends what you want to spend time on... The AD8307 chip is very easy to use, if a little on the expensive side. If you have to use the PC to unwrap the envelope detection mess as mentioned above then you have to do the logarithmic bit in PC anyway. c) as for the granuality of the sweep, for finer resolution the sweep range will be decreased. that is one way to get higher resolution, the other is to slow down the sweep. Agreed, but if the widest resolution is only 20KHz, then you have to do 5,000 samples to cover 0-100MHz input bandwidth. This fine resolution is imposed by the relatively narrow bandwidth, otherwise you will suffer significant loss of accuracy on your displayed result. The way I think of it is in terms of frequency spikes falling into the holes between samples. In the extreme imagine taking a VCO going in 100 steps, so making an amplitude measurement at each 1MHz of the range 1-100MHz. Your bandwidth 20KHz. Signals exactly on the MHz will be no problem. But what about a signal at say 12.5MHz... what does it look like on the analyser? It's 500KHz away from the centre of the 20KHz passband for both adjacent measurement points 12MHz and 13MHz. It's fallen into the hole. What you'd see on your display depends on the skirt selectivity of your filters. If you're using a 20KHz soundcard as the filter, the skirts will be quite sharp, so it's likely you'd see almost nothing at 500KHz baseband. This would mean the majority of frequencies in your input spectrum would be absent from your display or at entirely the wrong amplitude. what kind of a VCO are you using? and how are you ensuring that the vco output remains constant throughout the sweep? i expect that the VCO's amplitude will also effect the first mixer gain. I'm just using the internal oscillator of the SA602 mixer/oscillator chip. As you suggest, probably not at all constant ;-) Hans http://www.HansSummers.com |
"Ashhar Farhan" wrote in message om... "Hans Summers" wrote in message news:bmgam6 I also tried a direct conversion receiver initially. It doesn't work on in analogue (i.e. non-PC) analyser, because there are all sorts of heterodynes of the sweep frequency against the directly converted incoming signal. Of course I kicked myself afterwards for not thinking of it in advance to save myself the time of the experiment. a) i am not proposing a direct direct conversion. i am proposing an up conversion exactly like han's MK1. for the second conversion, i am suggesting a direct conversion to base-band from the high IF. as you rightly point out harmonic mixing is a problem with broad-band direct conversion receivers, therefore, you need to have a low-pass or a band-pass ahead of a diode mixer working as a product detector. That wasn't the effect I was worrying about... after all your up mixing and direct conversion of the VHF IF down to baseband, and low pass filtered it, you'll end up with say 20KHz of audio baseband to feed into your soundcard. Unfortunately your sweep rate is also down in the same range. Which is a problem because whatever method you use for envelope detection of the baseband will have to react faster than the sweep rate. Though as I say, if you do it all in software you might be able to untangle it. b) my second suggestion is to do the logarithmic calculations digitally on the PC. that will simplify the design. It simplifies the analogue design and complicates the software. 16 bits of resolution should give adequate dynamic range. Depends what you want to spend time on... The AD8307 chip is very easy to use, if a little on the expensive side. If you have to use the PC to unwrap the envelope detection mess as mentioned above then you have to do the logarithmic bit in PC anyway. c) as for the granuality of the sweep, for finer resolution the sweep range will be decreased. that is one way to get higher resolution, the other is to slow down the sweep. Agreed, but if the widest resolution is only 20KHz, then you have to do 5,000 samples to cover 0-100MHz input bandwidth. This fine resolution is imposed by the relatively narrow bandwidth, otherwise you will suffer significant loss of accuracy on your displayed result. The way I think of it is in terms of frequency spikes falling into the holes between samples. In the extreme imagine taking a VCO going in 100 steps, so making an amplitude measurement at each 1MHz of the range 1-100MHz. Your bandwidth 20KHz. Signals exactly on the MHz will be no problem. But what about a signal at say 12.5MHz... what does it look like on the analyser? It's 500KHz away from the centre of the 20KHz passband for both adjacent measurement points 12MHz and 13MHz. It's fallen into the hole. What you'd see on your display depends on the skirt selectivity of your filters. If you're using a 20KHz soundcard as the filter, the skirts will be quite sharp, so it's likely you'd see almost nothing at 500KHz baseband. This would mean the majority of frequencies in your input spectrum would be absent from your display or at entirely the wrong amplitude. what kind of a VCO are you using? and how are you ensuring that the vco output remains constant throughout the sweep? i expect that the VCO's amplitude will also effect the first mixer gain. I'm just using the internal oscillator of the SA602 mixer/oscillator chip. As you suggest, probably not at all constant ;-) Hans http://www.HansSummers.com |
I say go for it ! Ignore the doubters and those that don't like the idea. just go for it I say. It's a good idea and a fully workable one, I've been a hardware/software designer for years and theirs nothing that's so difficult in the idea. Though what I'd do is for the unit to have its own fast ADC - at least 18-bit. and it's own little cpu - ATmega's are nice and easy to use. Then send the current freq and log level down the RS232 port to a PC. Happy experimenting ! Clive |
I say go for it ! Ignore the doubters and those that don't like the idea. just go for it I say. It's a good idea and a fully workable one, I've been a hardware/software designer for years and theirs nothing that's so difficult in the idea. Though what I'd do is for the unit to have its own fast ADC - at least 18-bit. and it's own little cpu - ATmega's are nice and easy to use. Then send the current freq and log level down the RS232 port to a PC. Happy experimenting ! Clive |
If you have log before ADC I think you need a superhet conversion rather than direct conversion to baseband, for the reasons I mentioned earlier. If you just ADC'ed the lot there'd be some chance of sorting the confusion out in software, though it'd take a brave heart to try it. So use 2 18-bit ADC's (sound card maybe 16-bit but you won't get 90dB range from it, you'll find a fair bit of noise from your average sound card - well the first 3 bits will be noisy), one with the I and the other with the Q (from the DC output) and do it that way ? Clive |
If you have log before ADC I think you need a superhet conversion rather than direct conversion to baseband, for the reasons I mentioned earlier. If you just ADC'ed the lot there'd be some chance of sorting the confusion out in software, though it'd take a brave heart to try it. So use 2 18-bit ADC's (sound card maybe 16-bit but you won't get 90dB range from it, you'll find a fair bit of noise from your average sound card - well the first 3 bits will be noisy), one with the I and the other with the Q (from the DC output) and do it that way ? Clive |
"Hans Summers" wrote in message news:bmj291
Interesting ideas Len. I guess the idea of an all-digital spectrum analyser is similar to that of an all-digital HF amateur radio tranceiver. It can be done but at the current state of the art, it's a difficult proposal for the hobbyist and certainly difficult to obtain the same level of performance as the equivalent analogue device for the same amount of cost and/or effort. I have a PC-based oscilloscope that does something pretty close. It digitizes at the rate of 4ns per sample (taking 8bit samples unfortunately) and generates an FFT display of magnitude/power spectrum/power density). I think it is feasible to use an analogue RF front end under computer control, the a PC controlling the VCO and sampling the logarithmic output. In essence just replacing the oscilloscope as the display system. I guess, it is not necessary to PC control the VCO. Sweep generators are easy to come by. probably, if there is a way to feed the sweep into the PC to generate the X-axis, then that might be a better alternative. - farhan |
"Hans Summers" wrote in message news:bmj291
Interesting ideas Len. I guess the idea of an all-digital spectrum analyser is similar to that of an all-digital HF amateur radio tranceiver. It can be done but at the current state of the art, it's a difficult proposal for the hobbyist and certainly difficult to obtain the same level of performance as the equivalent analogue device for the same amount of cost and/or effort. I have a PC-based oscilloscope that does something pretty close. It digitizes at the rate of 4ns per sample (taking 8bit samples unfortunately) and generates an FFT display of magnitude/power spectrum/power density). I think it is feasible to use an analogue RF front end under computer control, the a PC controlling the VCO and sampling the logarithmic output. In essence just replacing the oscilloscope as the display system. I guess, it is not necessary to PC control the VCO. Sweep generators are easy to come by. probably, if there is a way to feed the sweep into the PC to generate the X-axis, then that might be a better alternative. - farhan |
i just ran a check with the sound card:
a) the sound card contributes noise. i shoved in a 50 ohms resistor into the mic input, cranked up the mic level, save a 'silent' wave file and saw it through a hex editor. There is a random noise of about 2-3 bits. That means, the effective range is not 16 bits but 12 bits. 12 bits can represent 4096-1 as the highest number. That means, the dynamic range is 4096 squared: about 16 million or 72db. A little low really for interesting work. there isnt enough 'roof' to resolve a spur about 60db down without touching the grass b)the sampling is at 44khz. some of the posts got confused between 20khz b/w and 20khz sampling rate. hans is right about the PC being a kludge when compared to an oscilloscope. I live in India and second-hand oscilloscopes are a rarity. The new ones cost an engineer's whole year's salary. I have purchased a tektronix 454 last month on ebay for $300, it is still on its way to India. So, while i can imagine that in a number of more developed countries oscilloscopes are not really a problem, they do remain a problem in many parts of the world. More often than not, an amateur already has a PC. Adding a simple hardware to be able to do quick and dirty spectrum analysis might be an interesting option. As Wes writes in SSD and EMRFD, the purpose of test instruments is to help with the projects on hand, rather than be projects themselves (then he went on to homebrewing a spectrum analyser, hehe). - farhan |
i just ran a check with the sound card:
a) the sound card contributes noise. i shoved in a 50 ohms resistor into the mic input, cranked up the mic level, save a 'silent' wave file and saw it through a hex editor. There is a random noise of about 2-3 bits. That means, the effective range is not 16 bits but 12 bits. 12 bits can represent 4096-1 as the highest number. That means, the dynamic range is 4096 squared: about 16 million or 72db. A little low really for interesting work. there isnt enough 'roof' to resolve a spur about 60db down without touching the grass b)the sampling is at 44khz. some of the posts got confused between 20khz b/w and 20khz sampling rate. hans is right about the PC being a kludge when compared to an oscilloscope. I live in India and second-hand oscilloscopes are a rarity. The new ones cost an engineer's whole year's salary. I have purchased a tektronix 454 last month on ebay for $300, it is still on its way to India. So, while i can imagine that in a number of more developed countries oscilloscopes are not really a problem, they do remain a problem in many parts of the world. More often than not, an amateur already has a PC. Adding a simple hardware to be able to do quick and dirty spectrum analysis might be an interesting option. As Wes writes in SSD and EMRFD, the purpose of test instruments is to help with the projects on hand, rather than be projects themselves (then he went on to homebrewing a spectrum analyser, hehe). - farhan |
Actually, I'd be tempted to rig a phasing type conversion to baseband from
the 300 MHz or whatever the first IF is.... At best it might get 40-50 dB of suppression of the other sideband, but it would remove most of the undesired spurs and something like an AD8307 log detector could be used to convert to a DC level. After that, feed it into an A/D converter and into a PC using the printer port. Given a dual channel A/D converter, I'd also sample the tuning voltage for the VCO and feed that into the PC as well. All of this would move the programming from a possibly complex DSP problem to a relatively simple Parallel port input and then number crunching to a display. There would be spur responses 40-50 dB down, but generally speaking, it should give useful information. =========== Granted, one problem is how fast the data transfer is through the parallel port, that would limit how many samples/second the system could process. There is the second issue, about maximum sweep speed Vs the IF bandwidth, and the maximum useful bandwidth would be a function of the Phase shift detector system. Still, I think this approach has some possibilities. Almost forgot... WIN XP and WIN2000 make getting to the parallel port for something like this MUCH more difficult then it was under WIN 98. You could use RS232 and perhaps two COM ports to allow getting the AD8307 and the VCO sweep voltage in parallel, so to speak. Once again, it would be a limit on how many samples per second the systme could provide. If you want to be more complex, you could feed OUT commands to a synthesizer to tune the VCO and this would allow really slow sweeps and very precise accuracy, but it complicates the design. I have to admit, I've been looking at the synthesizer design John Miles, KE5FX, did which tunes from 1 GHz to 2 GHz with very fine tuning steps and all osrts of ideas for various test gear built around it keep dancing in my mind. I suppose I should see if I can build one and get some idea as to the cost and so on. But it'd make a wonderful starting point for a digitally tuned spectrum analyzer !!! Admitted, the close in phase noise could limit dynamic range 'way below the range pssible with an AD8307 detector, but even so the frequency accuracy of an analyzer would be fantastic. ========= Overall, Avery is probably correct that the result may be a lot of work an less than perfect performance, but as a homebrewer it does seem like a fun way to get something fairly useful and learn a lot as the project goes on. Jim Pennell N6BIU |
Actually, I'd be tempted to rig a phasing type conversion to baseband from
the 300 MHz or whatever the first IF is.... At best it might get 40-50 dB of suppression of the other sideband, but it would remove most of the undesired spurs and something like an AD8307 log detector could be used to convert to a DC level. After that, feed it into an A/D converter and into a PC using the printer port. Given a dual channel A/D converter, I'd also sample the tuning voltage for the VCO and feed that into the PC as well. All of this would move the programming from a possibly complex DSP problem to a relatively simple Parallel port input and then number crunching to a display. There would be spur responses 40-50 dB down, but generally speaking, it should give useful information. =========== Granted, one problem is how fast the data transfer is through the parallel port, that would limit how many samples/second the system could process. There is the second issue, about maximum sweep speed Vs the IF bandwidth, and the maximum useful bandwidth would be a function of the Phase shift detector system. Still, I think this approach has some possibilities. Almost forgot... WIN XP and WIN2000 make getting to the parallel port for something like this MUCH more difficult then it was under WIN 98. You could use RS232 and perhaps two COM ports to allow getting the AD8307 and the VCO sweep voltage in parallel, so to speak. Once again, it would be a limit on how many samples per second the systme could provide. If you want to be more complex, you could feed OUT commands to a synthesizer to tune the VCO and this would allow really slow sweeps and very precise accuracy, but it complicates the design. I have to admit, I've been looking at the synthesizer design John Miles, KE5FX, did which tunes from 1 GHz to 2 GHz with very fine tuning steps and all osrts of ideas for various test gear built around it keep dancing in my mind. I suppose I should see if I can build one and get some idea as to the cost and so on. But it'd make a wonderful starting point for a digitally tuned spectrum analyzer !!! Admitted, the close in phase noise could limit dynamic range 'way below the range pssible with an AD8307 detector, but even so the frequency accuracy of an analyzer would be fantastic. ========= Overall, Avery is probably correct that the result may be a lot of work an less than perfect performance, but as a homebrewer it does seem like a fun way to get something fairly useful and learn a lot as the project goes on. Jim Pennell N6BIU |
I think it is feasible to use an analogue RF front end under computer control, the a PC controlling the VCO and sampling the logarithmic output. In essence just replacing the oscilloscope as the display system. I guess, it is not necessary to PC control the VCO. Sweep generators are easy to come by. probably, if there is a way to feed the sweep into the PC to generate the X-axis, then that might be a better alternative. Having the PC control the VCO via a digital-to-analogue converter gives the PC a lot of control over the sweep rate etc. You could then control the sweep rate, and zoom in on areas of your choice, via the PC rather than analogue controls. But either way would work fine. You can feed the sweep into the PC with a simple analogue-to-digital converter no problem. Hans |
I think it is feasible to use an analogue RF front end under computer control, the a PC controlling the VCO and sampling the logarithmic output. In essence just replacing the oscilloscope as the display system. I guess, it is not necessary to PC control the VCO. Sweep generators are easy to come by. probably, if there is a way to feed the sweep into the PC to generate the X-axis, then that might be a better alternative. Having the PC control the VCO via a digital-to-analogue converter gives the PC a lot of control over the sweep rate etc. You could then control the sweep rate, and zoom in on areas of your choice, via the PC rather than analogue controls. But either way would work fine. You can feed the sweep into the PC with a simple analogue-to-digital converter no problem. Hans |
wrote in message ... If you have log before ADC I think you need a superhet conversion rather than direct conversion to baseband, for the reasons I mentioned earlier. If you just ADC'ed the lot there'd be some chance of sorting the confusion out in software, though it'd take a brave heart to try it. So use 2 18-bit ADC's (sound card maybe 16-bit but you won't get 90dB range from it, you'll find a fair bit of noise from your average sound card - well the first 3 bits will be noisy), one with the I and the other with the Q (from the DC output) and do it that way ? I still think you stand a fair chance of problems, when essentially you're doing your signal strength measurement in baseband and sweeping at rates in the same frequency range. The aquisition time for the signal strength measurement needs to be much shorter than the sweep, or the signal strength will change while you're attmpting to measure it. I guess it's a bit like the problem of designing audio-derived AGC in direct conversion receivers. It takes too many cycles to measure the audio volume at low frequencies to get a correction signal to control gain. Similar problem. There might be a way of disentangling it in software, I don't know the mathematics but I can intuitively imagine that it could work. But I'm no expert on this, just a clown with soldering iron... Hans G0UPL http://www.hanssummers.com |
wrote in message ... If you have log before ADC I think you need a superhet conversion rather than direct conversion to baseband, for the reasons I mentioned earlier. If you just ADC'ed the lot there'd be some chance of sorting the confusion out in software, though it'd take a brave heart to try it. So use 2 18-bit ADC's (sound card maybe 16-bit but you won't get 90dB range from it, you'll find a fair bit of noise from your average sound card - well the first 3 bits will be noisy), one with the I and the other with the Q (from the DC output) and do it that way ? I still think you stand a fair chance of problems, when essentially you're doing your signal strength measurement in baseband and sweeping at rates in the same frequency range. The aquisition time for the signal strength measurement needs to be much shorter than the sweep, or the signal strength will change while you're attmpting to measure it. I guess it's a bit like the problem of designing audio-derived AGC in direct conversion receivers. It takes too many cycles to measure the audio volume at low frequencies to get a correction signal to control gain. Similar problem. There might be a way of disentangling it in software, I don't know the mathematics but I can intuitively imagine that it could work. But I'm no expert on this, just a clown with soldering iron... Hans G0UPL http://www.hanssummers.com |
hans is right about the PC being a kludge when compared to an oscilloscope. I live in India and second-hand oscilloscopes are a rarity. The new ones cost an engineer's whole year's salary. I have purchased a tektronix 454 last month on ebay for $300, it is still on its way to India. So, while i can imagine that in a number of more developed countries oscilloscopes are not really a problem, they do remain a problem in many parts of the world. That puts another angle on it. In that case I'd say you could produce a nice instrument by using a complete analogue analyser such as my design http://www.hanssummers.com/electroni...yser/index.htm or similar, and feeding the sweep (X-axis) and logarithmic output (Y-axis) into the PC via ADC's on the parallel port. 8-bit ADC's would probably be marginally good enough, 10 bit for sure. More often than not, an amateur already has a PC. Adding a simple hardware to be able to do quick and dirty spectrum analysis might be an interesting option. As Wes writes in SSD and EMRFD, the purpose of test instruments is to help with the projects on hand, rather than be projects themselves (then he went on to homebrewing a spectrum analyser, hehe). It became a project in its own right for me. So much that it spawned a second project http://www.hanssummers.com/electroni...ser2/index.htm. But not wasted time - I learnt so much making it and got my first experience at VHF work etc, so it was time well spent. Hans G0UPL http://www.hanssummers.com |
hans is right about the PC being a kludge when compared to an oscilloscope. I live in India and second-hand oscilloscopes are a rarity. The new ones cost an engineer's whole year's salary. I have purchased a tektronix 454 last month on ebay for $300, it is still on its way to India. So, while i can imagine that in a number of more developed countries oscilloscopes are not really a problem, they do remain a problem in many parts of the world. That puts another angle on it. In that case I'd say you could produce a nice instrument by using a complete analogue analyser such as my design http://www.hanssummers.com/electroni...yser/index.htm or similar, and feeding the sweep (X-axis) and logarithmic output (Y-axis) into the PC via ADC's on the parallel port. 8-bit ADC's would probably be marginally good enough, 10 bit for sure. More often than not, an amateur already has a PC. Adding a simple hardware to be able to do quick and dirty spectrum analysis might be an interesting option. As Wes writes in SSD and EMRFD, the purpose of test instruments is to help with the projects on hand, rather than be projects themselves (then he went on to homebrewing a spectrum analyser, hehe). It became a project in its own right for me. So much that it spawned a second project http://www.hanssummers.com/electroni...ser2/index.htm. But not wasted time - I learnt so much making it and got my first experience at VHF work etc, so it was time well spent. Hans G0UPL http://www.hanssummers.com |
Granted, one problem is how fast the data transfer is through the parallel port, that would limit how many samples/second the system could process. I think a parallel port is easily fast enough for this application Almost forgot... WIN XP and WIN2000 make getting to the parallel port for something like this MUCH more difficult then it was under WIN 98. True enough. I had this problem with a different project. There's a shareware driver available which helps access the parallel port under XP etc. See http://www.hanssummers.com/electroni...trix/index.htm for details of how it works. Hans G0UPL http://www.hanssummers.com |
Granted, one problem is how fast the data transfer is through the parallel port, that would limit how many samples/second the system could process. I think a parallel port is easily fast enough for this application Almost forgot... WIN XP and WIN2000 make getting to the parallel port for something like this MUCH more difficult then it was under WIN 98. True enough. I had this problem with a different project. There's a shareware driver available which helps access the parallel port under XP etc. See http://www.hanssummers.com/electroni...trix/index.htm for details of how it works. Hans G0UPL http://www.hanssummers.com |
I still think you stand a fair chance of problems, when essentially you're doing your signal strength measurement in baseband and sweeping at rates in the same frequency range. The aquisition time for the signal strength measurement needs to be much shorter than the sweep, or the signal strength will change while you're attmpting to measure it. Yes Sweep rate can be a problem, but it's all down the baseband bandwidth and what kind of update rate you want/need. If you want to sweep across a 100MHz within say a second, assuming your using 44100 sampling rate, that's 44100 samples you'll get across the 100MHz sweep in that one second - 2.26Khz wide freq segments (non-overlapping), but you'd need a baseband bandwidth of 1KHz to overcome the anti-alias problem. Doesn't sound to bad really. A one second scan across the entire 100MHz is fine really (depending on what your doing) - the PC is a perfect storage scope. To convert a linear I/Q baseband sample to a log scale is no problem at all in software. The software cud easily deal with any variation in VCO/Mixer level differences across the whole band. Best to have the PC control the vco though, then as you say, you can zoom in on a desired freq range etc. Clive |
I still think you stand a fair chance of problems, when essentially you're doing your signal strength measurement in baseband and sweeping at rates in the same frequency range. The aquisition time for the signal strength measurement needs to be much shorter than the sweep, or the signal strength will change while you're attmpting to measure it. Yes Sweep rate can be a problem, but it's all down the baseband bandwidth and what kind of update rate you want/need. If you want to sweep across a 100MHz within say a second, assuming your using 44100 sampling rate, that's 44100 samples you'll get across the 100MHz sweep in that one second - 2.26Khz wide freq segments (non-overlapping), but you'd need a baseband bandwidth of 1KHz to overcome the anti-alias problem. Doesn't sound to bad really. A one second scan across the entire 100MHz is fine really (depending on what your doing) - the PC is a perfect storage scope. To convert a linear I/Q baseband sample to a log scale is no problem at all in software. The software cud easily deal with any variation in VCO/Mixer level differences across the whole band. Best to have the PC control the vco though, then as you say, you can zoom in on a desired freq range etc. Clive |
wrote in message ... I still think you stand a fair chance of problems, when essentially you're doing your signal strength measurement in baseband and sweeping at rates in the same frequency range. The aquisition time for the signal strength measurement needs to be much shorter than the sweep, or the signal strength will change while you're attmpting to measure it. Yes Sweep rate can be a problem, but it's all down the baseband bandwidth and what kind of update rate you want/need. If you want to sweep across a 100MHz within say a second, assuming your using 44100 sampling rate, that's 44100 samples you'll get across the 100MHz sweep in that one second - 2.26Khz wide freq segments (non-overlapping), but you'd need a baseband bandwidth of 1KHz to overcome the anti-alias problem. The problem I'm having seeing how it can work is this. If you take 44100 measurements per second, that's one measurement every 27uS. But you've low pass filtered your baseband at 1KHz. It would take at least one of those cycles to measure the envelope amplitude with any degree of accuracy but you're allowing just 2.7% of a single 1KHz sample, how does that work? Or am I looking at it too simplistically? I certainly agree that the PC can make a nice storage and display device, specially if 'scopes aren't so easily available. Seems to me a question of how much of the signal chain is implemented in analogue and how much in digital. I Just think it saves an awful lot of hassle to add that little extra analogue stage before you go digital, i.e. 2nd IF and detector. Log could be done on the digital side no problem if desired provided enough ADC resolution was available. VCO/Mixer level differences could still be compensated in software, and the PC control the VCO. Hans G0UPL http://www.hanssummers.com |
wrote in message ... I still think you stand a fair chance of problems, when essentially you're doing your signal strength measurement in baseband and sweeping at rates in the same frequency range. The aquisition time for the signal strength measurement needs to be much shorter than the sweep, or the signal strength will change while you're attmpting to measure it. Yes Sweep rate can be a problem, but it's all down the baseband bandwidth and what kind of update rate you want/need. If you want to sweep across a 100MHz within say a second, assuming your using 44100 sampling rate, that's 44100 samples you'll get across the 100MHz sweep in that one second - 2.26Khz wide freq segments (non-overlapping), but you'd need a baseband bandwidth of 1KHz to overcome the anti-alias problem. The problem I'm having seeing how it can work is this. If you take 44100 measurements per second, that's one measurement every 27uS. But you've low pass filtered your baseband at 1KHz. It would take at least one of those cycles to measure the envelope amplitude with any degree of accuracy but you're allowing just 2.7% of a single 1KHz sample, how does that work? Or am I looking at it too simplistically? I certainly agree that the PC can make a nice storage and display device, specially if 'scopes aren't so easily available. Seems to me a question of how much of the signal chain is implemented in analogue and how much in digital. I Just think it saves an awful lot of hassle to add that little extra analogue stage before you go digital, i.e. 2nd IF and detector. Log could be done on the digital side no problem if desired provided enough ADC resolution was available. VCO/Mixer level differences could still be compensated in software, and the PC control the VCO. Hans G0UPL http://www.hanssummers.com |
This is a very interesting discussion
I have thought about doing this in the past, but have never been brave enough! What about a scheme like: Input - attenuator - LO/mixer-LPF/if amp -Direct coversionLO/Mixer - switchable LPF to say 150KHz - Log det (broadband)- ADC - software /PC For a 100MHz sweep you would probably need 300KHz bandwidth max which would be achieved by a 150KHz DSB receiver. You could go higher than 100MHz as the first IF - use ring diode mixer to a helical filter as the roofing filter I would be interested in a cooperative project Richard Ashhar Farhan wrote: "Hans Summers" wrote in message news:bmj291 Interesting ideas Len. I guess the idea of an all-digital spectrum analyser is similar to that of an all-digital HF amateur radio tranceiver. It can be done but at the current state of the art, it's a difficult proposal for the hobbyist and certainly difficult to obtain the same level of performance as the equivalent analogue device for the same amount of cost and/or effort. I have a PC-based oscilloscope that does something pretty close. It digitizes at the rate of 4ns per sample (taking 8bit samples unfortunately) and generates an FFT display of magnitude/power spectrum/power density). I think it is feasible to use an analogue RF front end under computer control, the a PC controlling the VCO and sampling the logarithmic output. In essence just replacing the oscilloscope as the display system. I guess, it is not necessary to PC control the VCO. Sweep generators are easy to come by. probably, if there is a way to feed the sweep into the PC to generate the X-axis, then that might be a better alternative. - farhan |
This is a very interesting discussion
I have thought about doing this in the past, but have never been brave enough! What about a scheme like: Input - attenuator - LO/mixer-LPF/if amp -Direct coversionLO/Mixer - switchable LPF to say 150KHz - Log det (broadband)- ADC - software /PC For a 100MHz sweep you would probably need 300KHz bandwidth max which would be achieved by a 150KHz DSB receiver. You could go higher than 100MHz as the first IF - use ring diode mixer to a helical filter as the roofing filter I would be interested in a cooperative project Richard Ashhar Farhan wrote: "Hans Summers" wrote in message news:bmj291 Interesting ideas Len. I guess the idea of an all-digital spectrum analyser is similar to that of an all-digital HF amateur radio tranceiver. It can be done but at the current state of the art, it's a difficult proposal for the hobbyist and certainly difficult to obtain the same level of performance as the equivalent analogue device for the same amount of cost and/or effort. I have a PC-based oscilloscope that does something pretty close. It digitizes at the rate of 4ns per sample (taking 8bit samples unfortunately) and generates an FFT display of magnitude/power spectrum/power density). I think it is feasible to use an analogue RF front end under computer control, the a PC controlling the VCO and sampling the logarithmic output. In essence just replacing the oscilloscope as the display system. I guess, it is not necessary to PC control the VCO. Sweep generators are easy to come by. probably, if there is a way to feed the sweep into the PC to generate the X-axis, then that might be a better alternative. - farhan |
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