Trying to improve an active LPF

[atferrari]

Shift register driven by a 12x clock. The 6 outputs loaded with weighted resistors to produce a sine-like waveform.

Network output fed, via a unity gain buffer to a 6-pole LP Butterworth active filter designed (FilterLab) for Fc 1150 Hz.

My intention is to use for a sinewave between 10 Hz and 1 KHz.

After testing I found that below 200 Hz the output is bad, coming to almost copy the step-like input!!

Besides revising the design with FilterLab and the hardware, I simulated it, confirming de bad performance at the lower end. LT's .asc file attached.

The board is already populated and the number of opamps (1 x TL074) thus fixed.

Any chance to improve it? Otherwise I will have to throw the board (veroboard) which took me ages to assemble. Vessels are too demanding ladies...

If anyone recall a similar question of mine from some years ago, please note that I never managed to built that circuit so better to forget that thread and start afresh here again.

Thanks for any help and the comprehension.

Yes, a follow-up would be asking how to actually center the sinewave at 0V but oviously better to solve this first.

Gracias again.

 

[ronsimpson]

Your low pass filter is fixed frequency. You want a output from 10hz to 1khz. The 'noise' is at 12x the output frequency. You want a filter that passes your out put frequency and removes everything above that.
Look into switched cap filters. Example MAX291. It will change frequency response as you change frequency. With the MAX291 and its brothers, there is a clock input that is 50 (or 100) times the output frequency.

If you had a 96khz oscillator, a divide by 8 to get 12khz then your circuit that produces 1khz. A switched cap filter needs the 96khz and 1khz. It will pass the 1khz and reject the 12khz noise. At 10hz the oscillator runs at 960hz, 120hz and 10hz. The low pass filter now is set to pass the 10hz and reject the 120hz noise.

If the switched cap filter is to complex then you need to find a way to change the capacitors in your filter, depending on the frequency range.

 

[atferrari]

Thanks for the reply and your time.

I started to consider that kind of filter.

 

[audioguru]

I have used switched-capacitor lowpass filter ICs for many years. My very low distortion sine-wave generator starts with a square-wave, uses a CD4018 IC to make a stepped sine-wave with 10 steps, filters it with two switched-capacitor lowpass filter ICs (their frequency follows the sine-wave frequency) and ends up with almost no distortion because most of the harmonics are filtered out.

Why don't you use one counter IC instead of many flip-flop ICs?
Why don't you use a switched-capacitor lowpass filter IC?

 

[atferrari]

Hola AG,

Those FF were just for the simulation. In practice I am using a CD4015 with 6 outputs weighted.

Yes, I am going to use a switched capacitor filter. Let me buy them first. Never used one.

Gracias.

 

[audioguru]

Years ago I used National Semi LMF40 4th-order switched-capacitor lowpass filter ICs. They were an improved MF4.
But maybe I was the only person who bought them because they are not made anymore.
Today, Maxim-IC have some.