[aprssig] TNC-2 tuning and temperature stability

[scott at opentrac.org]

As some of you already know, I've got my next-generation OpenTracker in beta
testing at the moment.  To keep it cheap, and avoid difficult-to-find
components, I went with a TNC-2 style XR2211-based demodulator on the
receive side.  Actually, it's more like the MFJ-1270 series, in that it has
a passive filter network instead of an MF-10 filter.

Anyway, it needs to keep working across a wide temperature range, to
accommodate things like high altitude balloons and dashboards in Phoenix.  I
finished my first round of low-temperature testing today, and found that it
did stop demodulating properly beyond a certain temperature.  It works fine
sitting on a pile of dry ice, but once it's buried in dry ice it goes
downhill fast.

I'd already selected temperature-stable metal film capacitors for the parts
I thought would be most critical, but now I'm taking a closer look at the
rest of the components in that section.  Finding C0G-rated caps, and
possibly 1% metal film resistors, shouldn't be too difficult.  Might raise
the price by a buck or so, but if it works, I think it'll be worth it.

The TNC-2 and clones have been out there for many years.  Anyone care to
share their experiences with these things at extreme temperatures?

I also discovered that the tuning comes out almost perfect with the
adjustment pot set dead center, like it comes from the factory.  I'm
wondering if there's really any need to include it at all, or if it can be
replaced with a couple of fixed-value resistors.

One other thing I'm meaning to explore is exactly how useful the passive
filter network really is.  I've got a CD with both raw and de-emphasized
audio from the LA area at rush hour, and I'll be running some tests with
that to see how the receive performance compares with and without the filter
network.

And on a semi-related subject, if anyone knows where I can find a 42-pin
shrink DIP ZIF socket (0.070" pitch) for under $100, please let me know!  I
know 3M makes one, but no one seems to stock them.  I'm going to need one if
I'm going to produce these trackers in any volume.  I hadn't really planned
on making the through-hole board a production item, but there's been so much
demand for it I think it might be a good idea.

http://n1vg.net/opentracker/images/ot2m.jpg

73,

Scott
N1VG