[time-nuts] WWVB d-psk-r down conversion method
kb8tq at n1k.org
Mon Sep 22 19:42:43 EDT 2014
Is it really that much harder to process a 1 KHz note out of a direct conversion receiver than it is to process a 100 Hz note?
Put another way - you already have a crystal filter in the front end of the “radio” right? It will do a better job knocking down an “adjacent channel” 2 KHz away than one 200 Hz away. Even without the crystal filter, the only likely thing you are trying to reject is noise. Effectively the “no filter” approach degrades your signal to noise by 3 to 6 db depending on the type of noise you are dealing with.
I’m asking because I may have missed something that makes 100 Hz “magic” ….
On Sep 22, 2014, at 6:15 PM, paul swed <paulswedb at gmail.com> wrote:
> One of the methods suggested for working with the wwvb psk is down
> converting to a lower frequency. When experimenting with NAA I had built a
> 24 Khz receiver that down converted that signal to a 100 Hz base band.
> It was a minor effort to shift it to 60 Khz by changing the LO. I liked
> what I saw. Essentially a simple direct conversion receiver with about 42
> DB of gain.
> The PSK flips at every 1 second depending on the data and is 1/100Hz
> delayed from the wwvb flip due to the down conversion process. This makes
> the further processing very reasonable in just about any micro.
> But the LO is a mess. Not bad if you have an HP3335a as I did till last
> week. (Another project to repair) for 100 Hz the LO must be at 60.1 or 59.9
> Khz. Or synthesize it etc. Starting to get seriously ugly.
> Or jump to a 1 Khz with 59 or 61 KHz. But I hated the idea of mixers and
> Any Time-Nuts seen those crystals or oscillators around lately? Seriously
> missing from the general haunts.
> But turns out and heavens knows why there are plentiful 5.9904 MHz xtals
> around. I ordered 5 from Mouser $ .46 ea.
> 5.9904/100 = 59.904 as a LO and suspect it can be moved to the correct
> frequency not that it has to be.
> Not sure there is a real need to lock the LO at all.
> So an exciting bit of news.
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