[time-nuts] New WWVB modulation format receivers (NOT)

Brian Lloyd brian at lloyd.com
Sat Feb 22 16:51:24 UTC 2014



On 2/20/14, 8:51 PM, Bob Camp wrote:
> Hi
> 
> You can get parts in the 18 bit and up range for not a whole lot of money with rational sample rates for a WWVB receiver. Analog Devices and Linear Tech both make some interesting looking parts. They get you into the >=100 db  dynamic range area. 

Yes. 192ksps is considered audio and is now available as a common part
for consumer devices. Or go with a 1-bit part and then decimate the hell
out of the result.

> Even with a lower bit count part, you pick up some bits in the downsampling process. As long as you have enough noise to keep things moving, you can track pretty far down into the crud. GPS receivers do that sort of thing all the time. 
> 
> Since this is slow audio after the CIC decimator, things like ARM chips probably have enough DSP horsepower to do what you need to do. The decimator it’s self is not terribly taxing if you don’t go too crazy with the rate change. 

This all makes more sense to me than hacking a bunch of op amps and
filter hardware. Use a low bit-depth but fast part then decimate. If it
is fast enough you can get by with very simple anti-alias filtering.
Like you said, if you have enough noise to randomize the LSB you are
good to go. I bet that the AMBCB provides more than enough randomization
power. And if it doesn't, just inject enough broadband noise to
randomize the LSB. The rest is just SMOP.

Just for giggles I took a look at the output of the ADC on my Hermes
board. The input is my Pixelsat loop which is broadband from 50kHz to
30MHz. (It has significant output down to below 20kHz.) The ADC of my
Hermes HPSDR board is being fed with the raw output of the antenna with
no filtering so the ADC is seeing everything from DC to 60MHz that comes
out of the antenna. Peak ADC level at my location is -40dBFS. I live
about 35km N of San Antonio so my location is neither particularly RF
quiet nor noisy. Seems like the 16-bit ADC has plenty of headroom. I
hear WWVB very handily on this setup and I am seeing -83dBm out of my
antenna for WWVB on peaks. (Actually the S:N is pretty good at over 16dB
in a 300Hz bandwidth.) This is near noon local time.

-- 
Brian Lloyd, WB6RQN/J79BPL
706 Flightline Drive
Spring Branch, TX 78070
brian at lloyd.com
+1.916.877.5067



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