Pete wrote:
"I'm planning to hook it up as an external reference for an HP59309A
clock. Now my thought is to put a Schottky diode in the ground leg of
the oscillator (to raise the ground reference slightly), then use a
trim-pot on the CV input to fine-tune it."
I'd recommend against offsetting the device ground - it's OK for
experimenting, but may cause all sorts of troubles in actual
application. Does this part have mechanical access to the coarse tuning
inside? I'm assuming probably not, so you're kind of stuck with the
existing tune range, but at least it passes through the 10 MHz ideal
value. In actual application, you should go ahead and provide the +/-
tune voltage range. It's a little more complicated since you'll need a
negative supply or offset, but will be much cleaner than moving the ground.
If it's for a dedicated, non-critical application, and you do the ground
offset, it would be much more stable with a shunt regulator IC rather
than a rectifier junction. The lowest commercial types are around 1.22
V. I can't recall all the ones available, but I always remember the
classic ICL8069, as a starting point to think about. There are newer
ones that can take lots more current. Also of course, if you move the
case/DC ground, you need make sure it's solidly bypassed to earth/system
ground, and the 10 MHz output and its ground (if necessary) are AC
coupled to the outside world.
It's fairly easy to make quiet, low current negative supplies from the
+12 V, if no other ready source is available from the system. The
simplest is to use the 10 MHz output signal to drive a small amplifier
or 74HC04 (or HC24X/54X for more power), which can supply a charge pump
circuit to make other voltages. Running it at the 10 MHz avoids making
other frequencies that could interfere (but there will be harmonics). At
this speed, use small signal Schottky diodes as rectifiers. The
associated capacitors will be small, around the nF range, so it can be
quite compact overall. There are also tricks you can do with
opto-couplers to make DC offset voltages at small current.
Ed
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
