[time-nuts] Oscillatek 2352

[ed breya]

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