[time-nuts] Phase modulation detection/NIST plan

[Tofurk Ei]

>"Ei
>Sorry if I have your name reversed. By taking this approach it
>eliminates the ability to use wwvb as a frequency reference because it
>destroys that traceability.
>Thats what we are trying to preserve. Or at least re-establish for the
>older phase measuring receivers.
>Regards
>Paul"

Are you getting at something along the lines of everything about the old
system is a
known quantity, and any error is just going to creep in at one point..the
accuracy is easy to maintain using that system?

The accuracy of the crystals on these cheap SDRs is terrible because
temperature variations are severe in those little dongles as they warm up.
Once their temps stabilize, most people use their drivers to enter in a ppm
value that cancels the error out somewhat. Uncorrected the inaccuracy
typically ranges from a few khz at 60 MHz to 40 or 50 KHz or more at 2200
Mhz. Also the dongles vary in the range of frequencies they can tune. Some
of them will go, as I said, as high as 2207-2210 MHz.

They don't cover the HF bands out of the box, but a few months back some
adventurous people discovered that by feeding a HF signal directly into the
RTL2832, bypassing the tuner chip, they can be made to tune the HF bands -
at what is described as enough sensitivity to listen to shortwave, hams
using SSB and CW. They can also be made to tune below the AM band, somehow.
Its possible they could be modified with off the shelf parts to have quite
respectable sensitivity and selectivity, even though they were not made to
tune those bands..

Nobody with any decent test equipment has characterized this direct
sampling mode's performance because most of the people who are playing with
them simply don't have the equipment to aspire to such things.. but the
fact seems striking to me that here is a device which until quite recently
could be found for under $20 in retail environments that with quite minimal
modifications could quite possibly function as both a multimode
communications receiver over a very large chunk of the spectrum, sucking
down up approximately a 2.6 to 3.2 MHz slice of spectrum at a time, or
alternatively could function as a sort of poor mans spectrum analyzer..

Throw in the ability to use gnuradio which is a very sophisticated set of
tools for communications engineering, and you have a situation where, since
the cost of entry is so low, its not so unreasonable to devote some time to
trying to grok some radio scheme and work with it to see what can be done.
Digital radios are no less capable than analog radios, nor are the results
achieved with them any less capable of being accurate. All the sources of
error are quantifiable and probably those dongles are a situation where a
small investment in replacing the crystal with a TCXO, air cooling or basic
thermal management, calibration, etc, might pay off big in results very
quickly.

Already people are doing the kind of things that people do with expensive
equipment with them, not $20 toys. So, there are just a load of
possibilities with them.

A way to see if this NIST format could be worked with would be to save a
capture file of the broadcast signal to disk and then it would be possible
to work with that offline later in gnuradio even when the transmitter was
not broadcasting that kind of modulation.

Its surprising that this broadcasting format has not been published as an
open spec. Thats a whole other (important) issue right there.

Anyway, I saw the discussion about this changeover and I thought the idea
might seem like a crazy one but I think it could potentially work and keep
the cost low. If these other issues could be dealt with. But I think they
might be straightforward to deal with for you guys as its your area of
expertise.

When you have one of these finger-sized little things in your hand and you
are fooling around with it, its pretty amazing. I say that as somebody who
has been into radio ever since I was a very little kid. I am hard to
impress with technology.. and this was pretty awesome.

The misgivings were expressed in one of the earlier posts that this
changeover was proceeding too rapidly and fears that technically it could
create a captive market - Just like with the breakup of Ma Bell,
competition is good.. and having some other options close at hand would
keep everything more honest.

(Unrelated, Just curious, did any of you guys ever live in Tarrytown NY?)

Tofurk