[NLRS] DB6NT PTC crystal heater

[Dr. Gerald N. Johnson, electrical engineer]

Depending on the angle of the crystal rotation, the frequency versus
temperature for a crystal tends to be an horizontal S curve. Crystals
cut for room temperature can be cut to work at flattening the horizontal
S curve. Some fancier cuts can get the crystal curve nearly flat at room
temperatures. Crystals intended for 50 or 60� C operation are cut so
that the roll of the S curve is centered on the oven temperature. That
way minor temperature variations have minimal frequency variations. 

Crystal ovens do vary in precision and range of temperature variation.

I've used PTC thermistors for stabilizing crystal oscillators for
decades. Generally I've used 40� C rating presuming that Iowa weather
during operating periods would be cooler than that. That may not apply
to the Gulf coast. In Mouser I see a few PTC thermistors rated 110 or
120�F. I take a 1 inch or so cube of foam insulation like Dow blue board
and carve a hole to just accept the crystal and thermistor. I've never
bothered to solder the thermistor to the crystal can. It would improve
heat transfer, but since there's only a couple thin steel wires and some
gas to transfer heat to the quartz inside the can, a little more
impedance outside isn't that much of a deal, and the enclosing foam gets
the heat where its needed. I don't like the idea of heating a crystal up
to soldering temperature. I don't like cutting leads of a crystal with
ordinary diagonal cutters because I don't want to feed the crystal that
shock. Its best to hold the lead between the crystal and the wire cutter
with sturdy pliers to absorb most of the shock of cutting. Or I use
flush cutting dikes with the flush side towards the crystal so the shock
all goes to the free end of the wire.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.