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Twin T Notch Filter Anomaly

I

Ian Bell

Jan 1, 1970
0
I have built a 2KHz twin T notch filter to help measure low level
distortion harmonics in audio equipment by removing the fundamental
(2KHz). It uses 510pF (1%)capacitors 150K (0.1%) resistors in a standard
bridged T configuration. However a strange thing has happened. I have
measured the notch response so I know it is 12dB down at 4KHz and 8dB
down at 6KHz etc so I can correct the readings. However, using a low
source resistance oscillator I was getting different distortion readings
with and without the notch and at first I thought I had the calibration
wrong but I rechecked it and is seems OK.

Now I have discovered that attaching the Twin T itself alters the
distortion products and it makes no difference to the fundamental. I
just connected the Twin T across the oscillator output and measured at
the same point. At first I thought this was just a property of the
oscillator. However, I have now tried this on both the oscillator (low
impedance) and the output of one of my 6SN7 mu followers with almost
identical effects namely the 2nd harmonic is reduced by about 10dB and
the third is raised by about the same amount.

I could understand if it happened with perhaps one particular circuit
but not with two entirely different ones.

Any ideas what it is?

Cheers

Ian
 

Darren Holdstock

Jan 20, 2009
5
Joined
Jan 20, 2009
Messages
5
Could be a number of things - need more info. Supply filter circuit, cap type (I'm assuming polystyrene for the time being), load, and measurement setup.
 
I

Ian Bell

Jan 1, 1970
0
christofire said:
Presumably the filter presents a strange frequency-selective impedance to
the output of whatever you connect it to - perhaps the output impedances of
your oscillator and follower are not as low as you thought? You could try
feeding the filter via an attenuator to see if that reduces interaction.

Yes, that's a good idea, I'll try a 20dB attenuator and see if there is
any observable difference.

Incidentally, 'bridged T' is a different configuration from 'twin T'
although a notch filter can be made using either.

Yes, brain fart on my part ;-)

Cheers

Ian
 
I

Ian Bell

Jan 1, 1970
0
bg said:
Ian Bell wrote in message ...

It sounds like your notch isn't where it is supposed to be.

That was my first thought but I checked the response and the notch is at
2075Hz, 4KHz is 11dB down, 6KHz is 7dB down and so one. Pretty much as
expected.

If you are
interested, I can send some data for creating a notch filter using an
inductor, two caps and a pot. It will create notches in the 60db range.


60dB is plenty for what I need. My HP wavemeter can reliably read
components over 70dB down so if I can reduce the fundamental even by
50dB I should be able to read harmonics down to -120dB.
the inductor is made adjustable, you can dial it in exactly where you need
it. The inductor will typically be in the mH range at 2k . Almost anything
you have in the junk box can be made to fit.
bg


That is VERY interesting, especially as it is adjustable. Can you email
me details to ianbellATukfsnDOTorg please?

Cheers

Ian
 
I

Ian Bell

Jan 1, 1970
0
Robert said:
The twin tee is best and works better if driven from a very low
impedance and has a very high impedance load.

At the moment it feeds my HP 3581A wavemeter which has an input
impedance of 1Meg so the load would seem to be OK. Source impedance is
another matter.
So use voltage followers on the input and output.

Yes, but this will need extremely low distortion in order not to
invalidate the results but I guess that is not too hard these days at
unity gain ;-)

Yes, I think I saw that in J. Linsley-Hood's book

Cheers

ian
 
J

JosephKK

Jan 1, 1970
0
That was my first thought but I checked the response and the notch is at
2075Hz, 4KHz is 11dB down, 6KHz is 7dB down and so one. Pretty much as
expected.

If you are


60dB is plenty for what I need. My HP wavemeter can reliably read
components over 70dB down so if I can reduce the fundamental even by
50dB I should be able to read harmonics down to -120dB.


That is VERY interesting, especially as it is adjustable. Can you email
me details to ianbellATukfsnDOTorg please?

Cheers

Ian

Nothing personal, but that is a pretty sloppy notch for my typical
uses. For me a 60 dB notch at 2 kHz; 3 kHz and 1.5 kHz are not more
than 1 dB down.
 
I

Ian Bell

Jan 1, 1970
0
JosephKK said:
Nothing personal, but that is a pretty sloppy notch for my typical
uses. For me a 60 dB notch at 2 kHz; 3 kHz and 1.5 kHz are not more
than 1 dB down.

NP. It is completely passive and that is exactly the sort of perfomance
a passive twin T gives. You can, however, considerably sharpen the notch
using feedback and obtain the sort of figures you quoted.

Cheers

Ian
 
I

Ian Bell

Jan 1, 1970
0
bg said:
It doesn't quit fit into the brickwall category does it?
bg

No, but then you canna change the laws of physics but if you add an op
amp and some feedback you can improve a twin T no end.

Cheers

Ian
 
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