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Old 26th February 2012, 08:55 AM   #11
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by Frex View Post
Hi,

Interesting, but when i had speak about Bode plots, it was real world measurements
I may do them, if I find some time
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Old 5th March 2012, 12:21 PM   #12
Elvee is online now Elvee  Belgium
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I made some more detailed measurements.
I didn't make complete plots, since it is of little interest: the general form is that of any other second order system.
Two details have an importance, mainly the fundamental leakage and accessorily the attenuation of harmonics.

As I suspected, the actual performance is better than predicted by the sim, at least for mid-range frequencies: at 1KHz, I measured 0.0065% leakage.
This degraded to 0.14% at 300Hz and 0.36% at 3KHz (I used 2.2nF tuning capacitors to center the geometric mean on 1KHz).

These figures are somewhat surprising: one would expect a degradation at the frequency extremes, but not of that magnitude.

In fact, all has to do with the parasitic capacitances: at 1KHz, the wiring to the frequency pots creates a slight capacitive overcompensation, which is about right to compensate the phase shift of the TLO84.
But at the frequency extremes, it works in the wrong direction: at the maximum frequency, there is no compenstion at all when it would be most needed, and at the minimum, the compensation is maximal, ie too large, and in addition, the available loop gain is decreased by the potentimetric arrangement of the frequency controls.

Better performances at low frequency would be achievable by connecting the pots as variable resistors, but I opted for the potentiometric configuration to improve the stability and reduce the effect of wiper resistance.
With high quality wirewound types, the resistive method would be preferable.

It should be possible to properly compensate the filter for the whole frequency range by adding small capacitors (several pF) across R4 and R6, and between the wiper and cold terminals of the pots.

I'll test it one of these days.

One (important) thing I forgot to mention last time: the tuning capacitors must be of a low loss type to be able to achieve the advertised null depths.

Polystyrene is best, that's what I used, second best is polypropylene (foil).
Mica or good COG ceramics are a third option.
No other commonly available dielectric is suitable: even polycarbonate would degrade performances by an order of magnitude.
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