AW: VCO hacks, tweaks

[Rene Schmitz]

Terry Michaels wrote:

>I think using a variable gain stage with a tempco opposite that of the expo
>transistor is an excellent idea, but there may be a better way to implement
>it.  Two possible alternatives come to mind:
>
>Using a differential transistor pair with a large amount of resistive
>emitter degeneration.  An example of this is a monolithic analog
>multiplier, one input can be a voltage reference which varies with absolute
>temperature.

One could use a diode connected trannie inside the array, then 
convert to the reciprocal to multiply. Or use a divider-loop like in my
circuit.
I think this could be an alternative approach for a solid-state compensation.
It would be not much more than my existing circuit, add two resistors in
the emitter paths, the diode gives a forward-drop proportional to T. Only
the second input of the multiplier would have to be added.
Another idea is to add linearizing diodes at the diffamp. (like in the
LM13700)

>Using a log/antilog conversion in tandem, to create a two quadrant
>multiplier, one input of which is made to vary with absolute temperature.

Now this sounds like too much overhead!

>I will look into both of these ideas sometime soon.  However, the
>simplicity of a tempco resistor in the signal path is beginning to have
>more appeal.
>
>Terry


At 20:24 05.11.98 -0700, Ian Fritz wrote:

>I studied the design quite carefully to be sure I understood it
>completely. I agree with what others have said about it, it is certainly
>an elegant design. It is also clearly a very carefully thought out
>design. But the most impressive thing about it is that it seems to be
>*new*, at least as a VCO feature.

Many thanks! 

>My first shot at implementing the circuit was just a bit different from
>Rene's. For the diff amp I used a 3080 OTA followed by an opamp I-V
>converter. At first I had a lot of trouble with spurious oscillation in
>the servo loop, but this was easily tamed by replacing the 3140 opamps I
>was using with the BB OPA132's. Feedback caps of 30pF around each opamp
>give rock-solid stability at all signal levels.

I had many problems with the stability too, my first attempt did latch up 
all the time. I had thought about using an OTA for the diff-amp, too, but the 
drawback would be the temperatures would not be the same for convertor and
compensator. 
(i.e. the self-heating of the OTA would play a significant role)

>As a second shot, I tried reducing the input to the diff amp by a factor
>of two. With the consequent improvement in linearity, I got a tuning
>curve that was not too bad over about 100Hz to 4kHz. I'm sure that by
>playing with the dead time setting and with different HF tracking
>schemes the tracking over this range could be improved even more.

I think a reasonable tuning curve can be archeived just by using the HFT,
because deviations from the 1V/oct curve appear only towards the low
frequency end, 
where they are less noticeable!
But another idea would be to add linearizing diodes (or even breakpoint
shapers).

>My next mod will probably be to try to make a very low noise diff amp
>and to attenuate its input even more.
>
>Rene -- do you have any numerical data on your tracking (V/oct) behavior
>that you would be willing to share with us?

Unfortunately not. I don't have any "real-world" data, but
I posted two images of the simulations I did. (The scales are pretty much
messed up!)
http://titan.cs.uni-bonn.de/~schmitz/simul1.gif and simul2.gif
Comparing the two images shows what I said above. The difference in the two
curves is 
mainly towards the low frequency end. On the high end, the curves drop the
same in both images (this is usually compensated by the HFT). Also one can
see the improvement in temperature stability, the curves for different
temperatures are closer together.
But one can also clearly see, that the error is quite big at the lower end.
Although I never noticed this in my VCOs I'll try to find a solution.
Perhaps some diode shaper which "bends" the curve more towards a straight
line.
(Ok, my next VCO will have a LFT and a HFT;-)


Bye
 Rene



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