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