[time-nuts] WWVB d-psk-r down conversion method
kb8tq at n1k.org
Mon Sep 22 21:06:46 EDT 2014
Ok, well then, here’s a thought - 10 MHz divided by 164 is 60.975.xxxx Hz. 165, 166, 167 and 168 are also “close enough” to give you a reasonable tone out of the converter. That assumes you are after a 10 MHz standard at the end of all this. The fact that the tone frequency is something really weird isn’t as big a deal once you convert it to digital bits.
More or less - make the LO something easy, lock it up, and do the rest with software.
On Sep 22, 2014, at 7:59 PM, paul swed <paulswedb at gmail.com> wrote:
> You missed nothing really. 100 Hz because thats what the NAA radio used
> with the tracor doubler. So when I moved it to wwvb that was the baseband
> Now that said I liked the 1 second phase flips at 100 Hz delay. Dog slow
> and perfectly OK to resolve the phase flip and I can use my favored uproc.
> Whats funny is that any crystal close to anything existed.
> Digikey says they have a 6.1Mhz xtal but when selected goes to 6.144 MHz.
> I was amazed that both mouser and digikey had the 5.9904 xtals and both
> stocked a variety of cases. Must be some story about whatever that
> frequency is.
> Did try lots of frequencies and divider math to come up with a simple LO
> scheme for 61 or 59 KHz. Messy.
> On Mon, Sep 22, 2014 at 7:42 PM, Bob Camp <kb8tq at n1k.org> wrote:
>> 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
>>> 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
>>> 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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