hwo to test the THD of an active RC filter

[[email protected]]

hi,
I've now designed an active RC filter with working frequency from 20K
to 300K and I also designed the amplifier my self.
Now I want to test the performance of the filter. How can I test the
THD of the filter? For I use a real amplifier that I designed instead
of a model, can I test the THD of the filter if a add a sin wave with
frequency in the passband?
If yes, how can I test the THD, will anyone give me a instruction?
Thanks !

 

[Rene Tschaggelar]

hi,
I've now designed an active RC filter with working frequency from 20K
to 300K and I also designed the amplifier my self.
Now I want to test the performance of the filter. How can I test the
THD of the filter? For I use a real amplifier that I designed instead
of a model, can I test the THD of the filter if a add a sin wave with
frequency in the passband?
If yes, how can I test the THD, will anyone give me a instruction?

What is a THD ? The harmonics squared and
summed or similar. There are now two ways,
either measure the output and do the
calculations, or measure the input and
the filter and do the calculations this way.

Rene

 

[[email protected]]

I designed the opamp myself, so the active component in the filter
isn't perfectly linear.
On the other hand, I also don't know how to test the THD of the opamp.
Looking forward to reply!
thanks
Rene Tschaggelar 写�：

 

[Jon]

I'm assuming that you do not have access to a spectrum analyzer. One
way to measure distortion is to null out the output of your amplifier
with an inverted version of the input. You need to be able to shift
the phase of the inverted input to compensate for phase shift in the
amplifier. The difference between the amplifier output and the
inverted, phase shifted input signal is THD + noise.
Regards,
Jon

 

[[email protected]]

thanks, Jon
I'll try to do this tomorrow morning!

I' ve tried to do dft to the output of my filter,and I' ve found that
if I input a 0.1v sinewave of 5KHz, I can get a spectrum of 99.8mv at
5KHz in the dft plot and the other harmonic waves are no more than
100nv. But when I calculated the THD of the filter, I found that it is
only -63dB. I wonder how many harmonic waves has been calculated into
THD? Why the THD calculation result is so small?
Looking forward to reply.
Thanks!

By the way, I use Cadence Spectre to facilitate my design. When I
calculate THD, I need to input the "range" and "sample number". If my
input is 5KHz, how do I determine these two values?
Jon 写�：

 

[John Fields]

hi,
I've now designed an active RC filter with working frequency from 20K
to 300K and I also designed the amplifier my self.
Now I want to test the performance of the filter. How can I test the
THD of the filter? For I use a real amplifier that I designed instead
of a model, can I test the THD of the filter if a add a sin wave with
frequency in the passband?
If yes, how can I test the THD, will anyone give me a instruction?
Thanks !

 

[[email protected]]

hi,
I've now designed an active RC filter with working frequency from 20K
to 300K and I also designed the amplifier my self.
Now I want to test the performance of the filter. How can I test the
THD of the filter? For I use a real amplifier that I designed instead
of a model, can I test the THD of the filter if a add a sin wave with
frequency in the passband?
If yes, how can I test the THD, will anyone give me a instruction?
Thanks !

Seeing as Google twats lost this lunchtime's treatise, this one is
shorter.
You need to be looking down beyond the 0.01% area.
Good spectrum analysers only get you down to 0.1%.
Audio band THD is straightforward. Sig gens can be pure and it is easy
to knock up a precise notch filter, which is the usual method.

20k and up is bad news. Both for sig gen and active notch POV.
Go for spot measurements. Use passive LC bandpass to clean up generator
output and passive LC notch on filter output.
Suggest 3rd order Butterworths with say 470ohm source/load resistance.
3rd order allows casual coil winds hence lower coil Qs, yet gives THD
down to better than -100dB (.001% 10ppm)
Bad news again ...
1] 3 spot measurements would need total of 18 inductors.
2] Need 3mH inductors at the bottom end. Can't use cores as they add
THD. 3mH of air core needs hundreds of turns.

Monumental task. Suggest just telling people your filter is "low
distortion" but noting your second post I'm now wondering if you grasp
the scale of the problem.
john