Build -- Active Twin-T notch filter for distortion analysis

[RNMarsh]

Notching out a source's dominant harmonic.
And, for fft use with ADC. Its just a lot more convenient as most of the thing is built already and I have an excess (for parts) 339a. What is needed to add to the osc to make the Q higher for use as a notch filter with the 339a?

Thx-RNMarsh

[davada]

Originally Posted by RNMarsh

Notching out a source's dominant harmonic.
And, for fft use with ADC. Its just a lot more convenient as most of the thing is built already and I have an excess (for parts) 339a. What is needed to add to the osc to make the Q higher for use as a notch filter with the 339a?

Thx-RNMarsh

Copy the analyzer's notch filter.

[davada]

Rick the bridged T in the oscillator is operating as a band pass filter because it's in the negative feedback of the op amp. I don't think it would be an easy undertaking to convert it
to a notch configuration. You'd be better off working with the notch filter in the analyzer section.

[RNMarsh]

Right. Got it.

Thx -- doing 60 things at once over here.

[RNMarsh]

been referring to the 339a as an oscillator... meant analyzer. The notch is on the A3 pcb...Fundemental Rejection Circuit. aka Notch Filter. OK?
Using this portion and/or also modifying the variable passive twin-t to run with the twin-T active circuit are projects for me to think about. Thx-RNMarsh

[RNMarsh]

I was looking at the K-H4400A circuit which uses a bridge-T oscillator which is a notch according to Circuit Description in its manual. That gave me the idea in the first place of using the notch in the spare 339a with the QA400 or other ADC.

But I'm not going to use the K-H4400A for that. I am going to see if it will go lower in THD than the modified 339a because of the trouble getting lowest thd at all frequencies. As you said, you need another wafer to trim thd levels at all freq. The K-H might be better in this respect. I got one a few days ago off eBay for $200. Waiting for it now.

Thx-RNMarsh

[davada]

Originally Posted by RNMarsh

been referring to the 339a as an oscillator... meant analyzer. The notch is on the A3 pcb...Fundemental Rejection Circuit. aka Notch Filter. OK?
Using this portion and/or also modifying the variable passive twin-t to run with the twin-T active circuit are projects for me to think about. Thx-RNMarsh

Hi Rick,

The mod to the passive twin t would be an easier task. It would be just a matter of lifting the T grounds and inserting the output of the positive feedback amplifier and adding an output buffer.

[davada]

Originally Posted by RNMarsh

I was looking at the K-H4400A circuit which uses a bridge-T oscillator which is a notch according to Circuit Description in its manual. That gave me the idea in the first place of using the notch in the spare 339a with the QA400 or other ADC.

But I'm not going to use the K-H4400A for that. I am going to see if it will go lower in THD than the modified 339a because of the trouble getting lowest thd at all frequencies. As you said, you need another wafer to trim thd levels at all freq. The K-H might be better in this respect. I got one a few days ago off eBay for $200. Waiting for it now.

Thx-RNMarsh

It's been a while since I looked at the 4400A. If they use a Jfet for control element then you will have the same trouble.

[richiem]

Yep, the 4400 uses a JFET.

[RNMarsh]

I havent found the limit to be the jFET yet. I started with reaching the limit with an A-Prec model One; then I am now at the low end of the the ShibaSoku. Never changed the jFET... did try but went back to original now. I can get the 339a oscillator to -125-ish. Not sure due to noise there.... need the ADC to look at it now and seperate the noise from the harmonics.

I think the Passive filter I have already (B&K 1607) will be the best approach, also. The box is plenty big to house active circuit AND it is continuously variable over a wide range. Anyone else have such a pasive filter or going to get one asap? :-)

Thx- Richard