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

[richiem]

@Ken -- it's hard to see the spectra with your photos, in part due to limited pixels, but also because of the frequency span. Can you make the span less than 100kHz when looking at lower frequency fundamentals? And can you normalize the amplitude to 10dB/div for better clarity?

@AndrewT -- the Cordell DM does not work with oscillators because in action, it simply sums/nulls the input to the device under test (DUT) with the output of the DUT, and displays what's left over. This is great because it doesn't much matter about exactly how low the distortion of the source is, what you see is only what's added by the DUT -- you can even use music for a test signal once you get eveything nulled out with test signals. But with an oscillator, there's no "input" signal to do the summing of the output with. The Active Twin-T filter just nulls out the fundamental a lot -- say 60dB or more -- without adding any significant distortion of its own, and without attenuating any of the harmonics of the oscillator or other device under test, so that whatever spectrum analyzer you use can have it's best dynamic range for looking at the residuals.

Ken's explanation of what the Twin-T does is good -- for more detail than you probably want, see my webpage on this filter:
Active Twin-T filter

[klewis]

Dick,

Attached are the following:
Image:
1. 10kHz 0.5v 0db
2. 10kHz 0.5v +20db (for those following along, subtract 20db from the harmonic values to get the true value)
3. 10kHz 0.1v +20db

1k to follow in next post

Ken

[klewis]

Part two:

( I forgot to mention I cooked up this program in agilent VEE to drive the signal analyzer and the 8903A, the vrms and thd display boxes are not turned on, as the cable is not connected currently to 8903A input)

Image:
1. 1kHz 0.5v 0db (couldn't get quite as deep of a null in the middle setting range -40 db so the -20db is worth showing)
2. 1kHz 0.1v +20db

Ken

[klewis]

Also forgot to mention that the fundamental is purposely off the screen, seems to help the resolution of the analyzer.

[richiem]

Ken, what is the maximum output of the 8903 at 1kHz? 3.16VRMS into 600 ohms -- 6.3VRMS into hi-Z (like the Twin-T)? I was wondering why you are using such low output voltages?

[klewis]

Originally Posted by richiem

Ken, what is the maximum output of the 8903 at 1kHz? 3.16VRMS into 600 ohms -- 6.3VRMS into hi-Z (like the Twin-T)? I was wondering why you are using such low output voltages?

Dick,

Max output looks like 6vrms. I am used to testing power amps, so, I run in the .1v to 1v range. I'll give it a go at higher power and see if it changes the THD.

Ken

[klewis]

Hi Dick,

I ran a frequency response plot (thanks to PMillett) of the Twin-T. Input voltage 1v, Twin - T tuned to 10kHz about -60db null. You can see that it's a bit lopsided. The value at 1kHz is 0db, at 20kHz it's -1.42, at 30kHz it's -0.98db. I ordered some 1% caps today and will see if that helps.

What do you think?

Ken

[richiem]

I think you should repeat this trial with a 1kHz fundamental, just to eliminate any possible bandwidth issues with the active amp in the filter, then check the actual upper bandwidth (-1dB) point of the filter.

I also think that it looks like the filter is being driven from a 600 ohm source, which will tend to cause the lopsided response -- 600 ohms is about the upper limit on source Z for decent measurements. Try driving from a lower impedance source, like the output of a power amp -- just for response checking. Then for distortion readings, you can always re-normalize your "0dB" to the upper passband level resulting from the source Z that you're using, without causing problems, because the products you're measuring are all in the upper pass band of the filter. It is always good to have bandwidth/level plots for the actual measuring conditions that you are working in.

[klewis]

Originally Posted by richiem

I think you should repeat this trial with a 1kHz fundamental, just to eliminate any possible bandwidth issues with the active amp in the filter, then check the actual upper bandwidth (-1dB) point of the filter.

.

Hi Dick,

How do I check the upper bandwidth (-1db) point of the filter?

I ran the 10k and 1k plots through my power amplifier. The 10k improved a bit to -.97db at 20k and -.60 at 30k. Then ran 1k again through the amp and got -.09db at 2k and +.12db at 3k. So, it seem more like a bandwidth thing. Also seems to answer the question about whether the Q is too high (probably not). I can't get as deep of a notch on the 1k setting as I can on the 10k setting. I'm thinking that this is a result of the capacitors not matching as well. I ordered 1% caps and will re-fit those along with moving the trimmers to be adjustable without opening the box.

Found some nice surplus precision 1k pots for 50c each for the trimmers.

Ken

[klewis]

well I know I attached the images...