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All Digital Receiver (or nearly all digital)
Do digital receivers get discussed here much? I am working on a
receiver for the WWVB signal at 60 kHz and am looking at a very low power all digital design in an FPGA. Of course some aspects are still analog such as the antenna. I have been reading about loop antennas for low frequency work. I don't plan on having an analog amplifier unless it is needed. I may be able to sample the RF signal directly and use processing to boost the signal out of the noise. Has anyone done anything like this? Right now I am looking at how to synchronize the sample rate with the carrier so that I can accumulate the signal in a coherent manner. Any pointers on where I could find more info? Rick PS I posted this to rec.radio.amateur.equipment by mistake. I don't that is the right group for home made equipment. |
All Digital Receiver (or nearly all digital)
rickman wrote:
Do digital receivers get discussed here much? I am working on a receiver for the WWVB signal at 60 kHz and am looking at a very low power all digital design in an FPGA. Of course some aspects are still analog such as the antenna. I have been reading about loop antennas for low frequency work. I don't plan on having an analog amplifier unless it is needed. I may be able to sample the RF signal directly and use processing to boost the signal out of the noise. Has anyone done anything like this? Right now I am looking at how to synchronize the sample rate with the carrier so that I can accumulate the signal in a coherent manner. Any pointers on where I could find more info? Search for "SDR". Software defined radio. It is hot these days. You will need the mathematical background to understand and create designs like this. Of course there are many articles explaining the digital signal processing, and how to build a program that does what you want. At 60 kHz it should be easy. People do this at 30 MHz. |
All Digital Receiver (or nearly all digital)
On 11/21/2012 6:27 PM, rickman wrote:
Do digital receivers get discussed here much? I am working on a receiver for the WWVB signal at 60 kHz and am looking at a very low power all digital design in an FPGA. Of course some aspects are still analog such as the antenna. I have been reading about loop antennas for low frequency work. I don't plan on having an analog amplifier unless it is needed. I may be able to sample the RF signal directly and use processing to boost the signal out of the noise. Has anyone done anything like this? Right now I am looking at how to synchronize the sample rate with the carrier so that I can accumulate the signal in a coherent manner. Any pointers on where I could find more info? Rick PS I posted this to rec.radio.amateur.equipment by mistake. I don't that is the right group for home made equipment. It looks like I got a reply to this post, but something goofed up in my reader or the server and instead of Rob's message I see the twice monthly announcement. Rick |
All Digital Receiver (or nearly all digital)
On 11/22/2012 4:07 AM, Rob wrote:
wrote: Do digital receivers get discussed here much? I am working on a receiver for the WWVB signal at 60 kHz and am looking at a very low power all digital design in an FPGA. Of course some aspects are still analog such as the antenna. I have been reading about loop antennas for low frequency work. I don't plan on having an analog amplifier unless it is needed. I may be able to sample the RF signal directly and use processing to boost the signal out of the noise. Has anyone done anything like this? Right now I am looking at how to synchronize the sample rate with the carrier so that I can accumulate the signal in a coherent manner. Any pointers on where I could find more info? Search for "SDR". Software defined radio. It is hot these days. You will need the mathematical background to understand and create designs like this. Of course there are many articles explaining the digital signal processing, and how to build a program that does what you want. At 60 kHz it should be easy. People do this at 30 MHz. There it is! I had to restart my newsreader to see this message. Yes, the term SDR is very broad. I was thinking there might be some folks here doing work in this area. Yes, the math is rather intense. While the math is always essential, there are often times when a good seat of the pants feel for the job is important too. DSP is just the digital version of analog signal processing. It is the "signal processing" part that is important. Rick |
All Digital Receiver (or nearly all digital)
rickman wrote:
On 11/22/2012 4:07 AM, Rob wrote: wrote: Do digital receivers get discussed here much? I am working on a receiver for the WWVB signal at 60 kHz and am looking at a very low power all digital design in an FPGA. Of course some aspects are still analog such as the antenna. I have been reading about loop antennas for low frequency work. I don't plan on having an analog amplifier unless it is needed. I may be able to sample the RF signal directly and use processing to boost the signal out of the noise. Has anyone done anything like this? Right now I am looking at how to synchronize the sample rate with the carrier so that I can accumulate the signal in a coherent manner. Any pointers on where I could find more info? Search for "SDR". Software defined radio. It is hot these days. You will need the mathematical background to understand and create designs like this. Of course there are many articles explaining the digital signal processing, and how to build a program that does what you want. At 60 kHz it should be easy. People do this at 30 MHz. There it is! I had to restart my newsreader to see this message. Yes, the term SDR is very broad. I was thinking there might be some folks here doing work in this area. Yes, the math is rather intense. While the math is always essential, there are often times when a good seat of the pants feel for the job is important too. DSP is just the digital version of analog signal processing. It is the "signal processing" part that is important. True. I thought I should just mention that because many people are able to construct such a receiver using analog techniques and discrete parts, using standard "building blocks" they are familiar with like an LC bandfilter, an amplifier, an AM detector etc, but would find themselves in unknown charters when they would have to write down the mathematical equivalent of what they constructed in hardware. When you can do that, and are familiar with the usual conversions between time domain and frequency domain, it should be possible to write the code (and/or construct it from existing modules). I have been able to write some DSP code that way, even on a dedicated DSP in asm (which has the extra complication that you need to learn the instruction set, that looks quite different from a normal CPU), and it was fun. However, before that DSP experiment I sometimes wrote code that looked for zerocrossings and timed the duration between them, and tries to decode data from that, and that is the wrong approach when you want good performance. |
All Digital Receiver (or nearly all digital)
On 11/22/2012 1:07 PM, Rob wrote:
I have been able to write some DSP code that way, even on a dedicated DSP in asm (which has the extra complication that you need to learn the instruction set, that looks quite different from a normal CPU), and it was fun. DSP doesn't have to be code. In this case it will be hardware in an FPGA. So the calculations will all be done in dedicated hardware. I don't plan on doing multiplies, but that might happen. These chips don't have dedicated multiplier. I've done multiplier designs with the shift and add algorithm, not so hard. However, before that DSP experiment I sometimes wrote code that looked for zerocrossings and timed the duration between them, and tries to decode data from that, and that is the wrong approach when you want good performance. Actually I did a commercial design that way for demodulating a time code signal. It only needed to measure amplitude to get the data, but had to sync to the carrier for "precise" timing. Looking for the zero crossings was a very effective method with an 8 kHz sample rate on a 1 kHz carrier that had a very high SNR. Rick |
All Digital Receiver (or nearly all digital)
"rickman" wrote in message ... Do digital receivers get discussed here much? I am working on a receiver for the WWVB signal at 60 kHz and am looking at a very low power all digital design in an FPGA. Of course some aspects are still analog such as the antenna. I have been reading about loop antennas for low frequency work. I don't plan on having an analog amplifier unless it is needed. I may be able to sample the RF signal directly and use processing to boost the signal out of the noise. Has anyone done anything like this? Right now I am looking at how to synchronize the sample rate with the carrier so that I can accumulate the signal in a coherent manner. Any pointers on where I could find more info? Rick PS I posted this to rec.radio.amateur.equipment by mistake. I don't that is the right group for home made equipment. Yikes! Here you over here too! Loop antennas sure have more "capture area" than ferrite loopsticks, using "seat of the pants" engineering, I always wondered just how the marked directivity of the loop antenna could be put to use in a "dual Diversity" scheme for noise cancelling. It was used (and perhaps still is) for "selective fading" and interference rejection on large, HF antenna farms. Marine and aircraft beacon band receivers used both rotatable loops and loopsticks in conjunction with a "sense" antenna for enhanced directional reception. I'm over my head, so I'm bailing. Please keep us informed of progress! Old Chief Lynn |
All Digital Receiver (or nearly all digital)
On 11/22/2012 2:18 PM, coffelt2 wrote:
Yikes! Here you over here too! Yeah, I think this is where I should be. The other group sounds like it is more about bought equipment than homemade. Loop antennas sure have more "capture area" than ferrite loopsticks, using "seat of the pants" engineering, I always wondered just how the marked directivity of the loop antenna could be put to use in a "dual Diversity" scheme for noise cancelling. It was used (and perhaps still is) for "selective fading" and interference rejection on large, HF antenna farms. Marine and aircraft beacon band receivers used both rotatable loops and loopsticks in conjunction with a "sense" antenna for enhanced directional reception. I'm over my head, so I'm bailing. Please keep us informed of progress! Old Chief Lynn It seems to be the null that is highly directional. I expect when they used loops for DF they turned the antenna until the signal faded and used the null as the directional part. I ordered some RG-6 with a solid copper center conductor. The shield is aluminum so I'm hoping that doesn't hurt. A number of sources talk about the importance of keeping the conductor resistance low to maximize the Q of the tuning. I need to do my calculations and select the loop size and number of turns. I don't think I want to make it very large, so I'm hoping more turns will compensate for a modest size. I'll let you know. This group is a little quiet, but that's better than all the noise in some other groups. As I'm sure you know, what is important is not the signal level, but the SNR! Rick |
All Digital Receiver (or nearly all digital)
"rickman" wrote in message ... On 11/22/2012 2:18 PM, coffelt2 wrote: Yikes! Here you over here too! Yeah, I think this is where I should be. The other group sounds like it is more about bought equipment than homemade. Loop antennas sure have more "capture area" than ferrite loopsticks, using "seat of the pants" engineering, I always wondered just how the marked directivity of the loop antenna could be put to use in a "dual Diversity" scheme for noise cancelling. It was used (and perhaps still is) for "selective fading" and interference rejection on large, HF antenna farms. Marine and aircraft beacon band receivers used both rotatable loops and loopsticks in conjunction with a "sense" antenna for enhanced directional reception. I'm over my head, so I'm bailing. Please keep us informed of progress! Old Chief Lynn It seems to be the null that is highly directional. I expect when they used loops for DF they turned the antenna until the signal faded and used the null as the directional part. I ordered some RG-6 with a solid copper center conductor. The shield is aluminum so I'm hoping that doesn't hurt. A number of sources talk about the importance of keeping the conductor resistance low to maximize the Q of the tuning. I need to do my calculations and select the loop size and number of turns. I don't think I want to make it very large, so I'm hoping more turns will compensate for a modest size. I'll let you know. This group is a little quiet, but that's better than all the noise in some other groups. As I'm sure you know, what is important is not the signal level, but the SNR! Rick "Understanding Digital Signal Processing" by Richard G. Lyons is a good source of information. If you plan to do the FPGA code yourself check out: www.myhdl.org For the antenna consider: www.febo.com/time-freq/wwvb/antenna At 60 KHz RG-58 would be as good as RG-6 for a loop antenna and much easier to work with. PCV pipe is a good material for the frame. email me directly and I'll send you a photo of a 1m VLF antenna I made. Unless you are located in Boulder, CO you will probably need an analog amplifier. |
All Digital Receiver (or nearly all digital)
On 12/17/2012 10:18 PM, garyr wrote:
"Understanding Digital Signal Processing" by Richard G. Lyons is a good source of information. If you plan to do the FPGA code yourself check out: www.myhdl.org For the antenna consider: www.febo.com/time-freq/wwvb/antenna At 60 KHz RG-58 would be as good as RG-6 for a loop antenna and much easier to work with. PCV pipe is a good material for the frame. email me directly and I'll send you a photo of a 1m VLF antenna I made. Unless you are located in Boulder, CO you will probably need an analog amplifier. Hi Gary, Thanks for the info. I am familiar with Richard's book. He seems like a good guy and will provide errata for anyone who has bought his book. I am familiar with DSP in general and digital receivers specifically. I have most of that part of the design analyzed enough to begin coding. It is the ADC that I have had some trouble analyzing. I have been planning to construct a Sigma-Delta converter in the FPGA. Turns out this might be a little power hungry and I won't be able to use the special dithering that shapes the noise. My primary goal is to do this entire project with very, very little power consumption. If I can get a strong enough signal I can just use an LVDS input without the integration of a Sigma-Delta converter, but I won't know until I get some of this built and tested. I am very experienced with HDL so I should be ok there. I have found any number of sites that talk about loop antenna construction, but most don't really explain how to analyze them. I have finally cobbled together a good picture of the technical aspects from a number of web sites and have an antenna plan. I am looking at using 50 feet of RG-6 with copper inner conductor to optimize the Q. My initial pass is a compromise between optimizing the signal strength and making the final unit easy to construct and support. It will be 8 turns on 2 foot diameter wooden spokes. I expect the stiffness of the RG-6 to help support the cable. This would likely be a decent design up to four foot diameter. In the end I may find I can use a ferrite loop. But the signal strengths I have seen from commercial ferrite antennas do not seem to be good enough, around 8 uV for 100uV/m field strength which is about what I expect to see here on the east coast. As to the amp, we will see. I have an equation to predict signal strength at the antenna output and I am expecting a decent signal level if I have a good Q and transformer couple the output. Total gain (Q and transformer) over an untuned antenna will approach 10,000. But there is many a slip between cup and lip. I hope to make some progress on this over the holidays and have some test results. Can you explain why you think RG-58 will be as good as the RG-6? The inner conductor of RG-6 has only a small impact from skin effect, I think it is around 21%. At 32 mil the inner conductor of RG-58 (compared to 40 mil for RG-6) will have even less impact from skin effect, but will have a higher resistance and so a lower Q. The capacitance per foot is nearly twice that of RG-6 as well although I'm still not clear on the specific details of this effect. I believe higher capacitance will lower the self resonant frequency although I don't expect this to be a problem in my application. Rick |
All Digital Receiver (or nearly all digital)
"rickman" wrote in message ... On 12/17/2012 10:18 PM, garyr wrote: "Understanding Digital Signal Processing" by Richard G. Lyons is a good source of information. If you plan to do the FPGA code yourself check out: www.myhdl.org For the antenna consider: www.febo.com/time-freq/wwvb/antenna At 60 KHz RG-58 would be as good as RG-6 for a loop antenna and much easier to work with. PCV pipe is a good material for the frame. email me directly and I'll send you a photo of a 1m VLF antenna I made. Unless you are located in Boulder, CO you will probably need an analog amplifier. Hi Gary, Thanks for the info. I am familiar with Richard's book. He seems like a good guy and will provide errata for anyone who has bought his book. I am familiar with DSP in general and digital receivers specifically. I have most of that part of the design analyzed enough to begin coding. It is the ADC that I have had some trouble analyzing. I have been planning to construct a Sigma-Delta converter in the FPGA. Turns out this might be a little power hungry and I won't be able to use the special dithering that shapes the noise. My primary goal is to do this entire project with very, very little power consumption. If I can get a strong enough signal I can just use an LVDS input without the integration of a Sigma-Delta converter, but I won't know until I get some of this built and tested. I am very experienced with HDL so I should be ok there. I have found any number of sites that talk about loop antenna construction, but most don't really explain how to analyze them. I have finally cobbled together a good picture of the technical aspects from a number of web sites and have an antenna plan. I am looking at using 50 feet of RG-6 with copper inner conductor to optimize the Q. My initial pass is a compromise between optimizing the signal strength and making the final unit easy to construct and support. It will be 8 turns on 2 foot diameter wooden spokes. I expect the stiffness of the RG-6 to help support the cable. This would likely be a decent design up to four foot diameter. In the end I may find I can use a ferrite loop. But the signal strengths I have seen from commercial ferrite antennas do not seem to be good enough, around 8 uV for 100uV/m field strength which is about what I expect to see here on the east coast. As to the amp, we will see. I have an equation to predict signal strength at the antenna output and I am expecting a decent signal level if I have a good Q and transformer couple the output. Total gain (Q and transformer) over an untuned antenna will approach 10,000. But there is many a slip between cup and lip. I hope to make some progress on this over the holidays and have some test results. Can you explain why you think RG-58 will be as good as the RG-6? The inner conductor of RG-6 has only a small impact from skin effect, I think it is around 21%. At 32 mil the inner conductor of RG-58 (compared to 40 mil for RG-6) will have even less impact from skin effect, but will have a higher resistance and so a lower Q. The capacitance per foot is nearly twice that of RG-6 as well although I'm still not clear on the specific details of this effect. I believe higher capacitance will lower the self resonant frequency although I don't expect this to be a problem in my application. Rick I don't understand how you intend to use an LVDS input for the WWVB signal? Could you explain. Have you considered using a WWVB receiver IC? I don't know what kind of power they require but they generate the 1pps PWM signal which could be processed by a low-power microprocessor very easily. http://www.c-max-time.com/products/showProduct.php?id=2 http://www.pvelectronics.co.uk/index...&products_id=7 http://www.ntp-time-server.com/wwvb-...b-receiver.htm I've constructed loop antennas using both RG-58 (60 KHz) and RG-6, or something similar to RG-6 (~20 KHz) and had good luck with both. I haven't compared the outputs of both tuned to 60 KHz so I can't say much about how their electrical performance compare. The RG-6 with the aluminum shield is a PITA because you can't make a solder connection to it. Self-resonance will not be a problem. 100' of RG-58 on a 1 meter dia. frame required about 25 nF to resonate at 60 KHz. One advantage of the febo-type antenna with the 1-turn pickup loop is that its output impedance is very low which means that the antenna can be located away from the receiver. |
All Digital Receiver (or nearly all digital)
On 12/19/2012 10:26 AM, garyr wrote:
I don't understand how you intend to use an LVDS input for the WWVB signal? Could you explain. I guess you didn't read my initial post. My intent is to create an all-digital receiver at very low power. The receiver ICs around use standard analog technology to detect the AM signal. WWVB has added phase modulation which should be receivable at a lower SNR. I also want to detect both modulation schemes and compare the results. Have you considered using a WWVB receiver IC? I don't know what kind of power they require but they generate the 1pps PWM signal which could be processed by a low-power microprocessor very easily. They are fairly low power as can be CPU chips when cycled on and off as needed. But I want to do this in an FPGA so I can see just how low power it can be. http://www.c-max-time.com/products/showProduct.php?id=2 http://www.pvelectronics.co.uk/index...&products_id=7 http://www.ntp-time-server.com/wwvb-...b-receiver.htm I've constructed loop antennas using both RG-58 (60 KHz) and RG-6, or something similar to RG-6 (~20 KHz) and had good luck with both. I haven't compared the outputs of both tuned to 60 KHz so I can't say much about how their electrical performance compare. The RG-6 with the aluminum shield is a PITA because you can't make a solder connection to it. Self-resonance will not be a problem. 100' of RG-58 on a 1 meter dia. frame required about 25 nF to resonate at 60 KHz. One advantage of the febo-type antenna with the 1-turn pickup loop is that its output impedance is very low which means that the antenna can be located away from the receiver. The cable I have already has F type connectors on the ends, so I don't care about the aluminum shield. I did have to shop around a bit to find one with an all copper inner conductor. Seems they use copper coated steel for CATV. I think my numbers are for a bit less than 80 nF of capacitance so I bought a bunch of 15 nF silver mica parts and some other small values to tune it in closely, then I have a triple bank, 0-500 pF tuner cap from an old receiver for final tuning. Once I have the rest of the system working well, I'll likely use fixed values of caps to tune the antenna without the variable cap. I saw one antenna that used a bank of roughly power of 2 caps with switches to allow the antenna to be tuned easily, cute. I am thinking of using a ferrite core as a coupling transformer as it will give me a lot of voltage gain. I am looking to get towards 10,000 gain from combined Q and transformer. The LVDS input on the FPGA will act as a 1 bit ADC. I was planning to use this in a Sigma-Delta converter and produce a 4 bit value at 240 kHz (4x carrier rate). I'm not sure I need the Sigma-Delta converter and might end up just using the 1 bit from the LVDS directly. By downconverting to DC and downsampling to 30 Hz I will get a *lot* of processing gain with an SNR improvement of some 33 dB (assuming uncorrelated noise). Another way to look at the digital design is as a very long FIR filter resulting in a filter with a very narrow bandpass or one bin of a DFT. Turns out they are all the same math in this case (multiply by 1, 0, -1, 0 sequence). There are a few wrinkles I haven't figured out yet. I need a sample clock that is both accurate and low power. I have yet to find that. XO oscillators I have found over 1 MHz are all in the mA range. I'm thinking of using the very poor internal oscillator in the FPGA and calibrating it to a 32.768 kHz reference which can be both accurate and low power. But I don't yet know if the internal oscillator is low power enough. I need to fire up the eval board (gratis from NuHorizons) and measure the power consumption with and without the internal oscillator enabled. If the power consumption is low enough I can frequency lock it to the clock oscillator and come up with a calibration factor. Then the sample clock can be generated internally using a DCO updated with each calibration. Thanks for discussing this with me. I like to bounce ideas off of other people. Rick |
All Digital Receiver (or nearly all digital)
I thought I would cross post this to rec.radio.amateur.moderated. They
have made a number of announcements about this and seem to be encouraging the practice. Rick On 12/19/2012 3:51 PM, rickman wrote: On 12/19/2012 10:26 AM, garyr wrote: I don't understand how you intend to use an LVDS input for the WWVB signal? Could you explain. I guess you didn't read my initial post. My intent is to create an all-digital receiver at very low power. The receiver ICs around use standard analog technology to detect the AM signal. WWVB has added phase modulation which should be receivable at a lower SNR. I also want to detect both modulation schemes and compare the results. Have you considered using a WWVB receiver IC? I don't know what kind of power they require but they generate the 1pps PWM signal which could be processed by a low-power microprocessor very easily. They are fairly low power as can be CPU chips when cycled on and off as needed. But I want to do this in an FPGA so I can see just how low power it can be. http://www.c-max-time.com/products/showProduct.php?id=2 http://www.pvelectronics.co.uk/index...&products_id=7 http://www.ntp-time-server.com/wwvb-...b-receiver.htm I've constructed loop antennas using both RG-58 (60 KHz) and RG-6, or something similar to RG-6 (~20 KHz) and had good luck with both. I haven't compared the outputs of both tuned to 60 KHz so I can't say much about how their electrical performance compare. The RG-6 with the aluminum shield is a PITA because you can't make a solder connection to it. Self-resonance will not be a problem. 100' of RG-58 on a 1 meter dia. frame required about 25 nF to resonate at 60 KHz. One advantage of the febo-type antenna with the 1-turn pickup loop is that its output impedance is very low which means that the antenna can be located away from the receiver. The cable I have already has F type connectors on the ends, so I don't care about the aluminum shield. I did have to shop around a bit to find one with an all copper inner conductor. Seems they use copper coated steel for CATV. I think my numbers are for a bit less than 80 nF of capacitance so I bought a bunch of 15 nF silver mica parts and some other small values to tune it in closely, then I have a triple bank, 0-500 pF tuner cap from an old receiver for final tuning. Once I have the rest of the system working well, I'll likely use fixed values of caps to tune the antenna without the variable cap. I saw one antenna that used a bank of roughly power of 2 caps with switches to allow the antenna to be tuned easily, cute. I am thinking of using a ferrite core as a coupling transformer as it will give me a lot of voltage gain. I am looking to get towards 10,000 gain from combined Q and transformer. The LVDS input on the FPGA will act as a 1 bit ADC. I was planning to use this in a Sigma-Delta converter and produce a 4 bit value at 240 kHz (4x carrier rate). I'm not sure I need the Sigma-Delta converter and might end up just using the 1 bit from the LVDS directly. By downconverting to DC and downsampling to 30 Hz I will get a *lot* of processing gain with an SNR improvement of some 33 dB (assuming uncorrelated noise). Another way to look at the digital design is as a very long FIR filter resulting in a filter with a very narrow bandpass or one bin of a DFT. Turns out they are all the same math in this case (multiply by 1, 0, -1, 0 sequence). There are a few wrinkles I haven't figured out yet. I need a sample clock that is both accurate and low power. I have yet to find that. XO oscillators I have found over 1 MHz are all in the mA range. I'm thinking of using the very poor internal oscillator in the FPGA and calibrating it to a 32.768 kHz reference which can be both accurate and low power. But I don't yet know if the internal oscillator is low power enough. I need to fire up the eval board (gratis from NuHorizons) and measure the power consumption with and without the internal oscillator enabled. If the power consumption is low enough I can frequency lock it to the clock oscillator and come up with a calibration factor. Then the sample clock can be generated internally using a DCO updated with each calibration. Thanks for discussing this with me. I like to bounce ideas off of other people. Rick |
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