On 8/5/15 8:03 PM, Donald wrote:
On 8/5/2015 6:44 PM, Jim Lux wrote:
I'm not sure it would buy you much.. you'd have something running at
240kHz switching the inputs to the detector?
It's MUCH easier to just digitize the 60kHz with a high resolution
converter. And have a nice BPF in front of the digitizer.
The tayloe/quadrature sampling detector (a key part of the Flexradio
original design) is more convenient if you're making a direct
conversion receiver that needs to tune up to 10s of MHz, since it
allows you to use a slow ADC with lots of bits. It's basically a I/Q
mixer and will take a lot more parts in total than just getting a
192ksps 24 bit converter.
Thanks for the numbers.
I was uncertain how to scale the sample rate for the I/Q.
if you want to turn a regular single channel data stream and turn it
into IQ, an easy way is to do the ++-- approach.
complex sample 1 = real sample 1 + j real sample 2
complex sample 2 = -real sample 3 - j real sample 4
complex sample 3 = real sample 5 + j real sample 6
complex sample 4 = -real sample 7 - j real sample 8
and so forth..
This will give you an I/Q sample stream centered at samplingrate/4. So
for 192 ksps (real), fs/4 = 48 kHz, so your 60 kHz signal will be at an
apparent frequency of +12 kHz (sampled at 96k i/q samples/sec)
As mentioned in a different post, I'd look at a filter/decimate, which
might be computationally faster. Say you need 1kHz bandwidth (which I
think is pretty wide for WWVB)
http://www.nist.gov/pml/div688/grp40/upload/Bin-2719.pdf
so you're looking at 192:1 decimation. with 192 taps, I'll bet you can
make a fairly decent BPF (and if you're clever, probably bring 60kHz
down to close to DC at the same time, by combining a synthetic LO with
the taps)
So you'd be looking at 192 multiplies and adds per 1kHz output point,
which is a pretty "loafing along" CPU load (e.g. a whole lot of
microcontrollers could do it)
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