Ultra low frequency sawtooth oscillators

[Martin Czech]

:::Hi All,
:::
:::Does anyone know places where I can find schematics for ultra low frequency
:::saw tooth oscillators.
:::I want to make such an oscillator for a sweep generator in a low frequency
:::gain-frequency analyser.
:::The sweep time should at least be able to be 10 minutes (1.66milliHz)
:::
:::Also which components should be carefully selected ? For instance if an
:::opamp integrator is used
:::the timing capacitor should be a low leakage type to avoid timing
:::non-linearities.

If you have that kind of application in mind, where the CRT beam
is controlled by sweep voltage waveform is not very important,
nor frequency error. Anyway:

I don't think that a very special circuit is needed. Just any old
relaxation concept will do. 
Hmmm, 10 minutes are 600s, the largest good cap is about 1uF,
say 1V amplitude, C*U=Q=I*t => I=C*U/t = > I = 1.67nA

So you are down to 1nA (just to have an idea of the order of magnitude).
It is obvious that leakage is your main enemy then.
For diy: use air insulation. i.e. bend the high impedance legs up,
out of the sockets, solder them with a good, ESD proof iron, so that
everything that could be harmfully leaky is up in the air.
At least under the climatic conditions where I live, air insulation
is very very good. (read Bob Pease What's all the femtoamper stuff, anyhow?).

Now you have solved PCB leakage and socket leakage.
Next order of magnitude error comes from junction leakage.

You need a good voltage follower op amp. Try CA3140. (protection diodes
will lead to leakage at higher temperature though).  The national LMC
series is also a good choice. Look at Bob Pease's  leakometer in Analog
Trouble shooting, I can't remember the device number now.  I hope I can
dig that out, I have just put some books away, so I have no easy access
to them.

Or a simple source follower MOSFET.

The discharge switch is the next problem.  I've listet some MOSFETs which
I think could be good enough in earlier postings.  Couldn't verify so far,
since all devices I got were damaged (!).

BS170, the other I just can't remember now.

Try to not touch semiconductor packages with your fingers, because
of contamination. Clean sensitive parts (isoprop).

I once had to fight a "serious leakage problem" which finaly 
came up as grease from my finger tips on plastic packages.


m.c.