Can anyone tell me what this filter is doing?

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Lee_M

Well-known member
Joined
Nov 23, 2013
Messages
309
Location
Dorset, UK.
Hey guys,

I just picked up an old BBC attenuator module and there was a strange LC filter board fitted inside the module, but not actually electronically connected to anything. The board is labelled with the same BBC code number as the module it was fitted into (AT2/7A), so it seems fair to assume that it's an original part and was designed to be connected to the 600Ω balanced attenuators that make up the rest of the module, but I haven't managed to find any information about it online.

I've drawn up a schematic (see attached image), L2 and L5 are currently unidentified values (the other inductors have hand-written labels on them) so I'd need to desolder them to take measurements. They look identical to the other inductors, so I'd expect them to be in the same ballpark as 1.14mH.

Can anyone tell me what exactly it's doing? It seems to be some sort of balanced notch filter, but C7 and C8 look like they're doing something else.

Cheers!
 

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band pass filter

(could not find a calculator with parallel LC tee network in quick search)

paralleled LC elements are band pass and shunt C is low pass
 
Isn't band pass filtering done with series LC components?

The only time I've seen parallel LC used in BPF circuits is in the shunt part of the filter, as seen in the image attached to this comment.

The only "shunt" components in this circuit seem to be C7 and C8.
 

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Actually I believe that's a balanced and damped band stop plus low pass filter but it's complicated enough that I would have to model it in LTSpice to be sure of the specifics. Caps are high impedance at low frequencies where inductors are high impedance at high frequencies. So high frequencies pass straight through the caps and low frequencies pass straight through the inductors. Middle frequencies do not pass and thus it's a band stop. And the shunt caps clip the highs. Definitely higher order but not exactly sure what degree without a sim.
 
There's a combo box...

abbey road d enfer said:
This calc does not cater for elliptical (Cauer) filters. These filters have parallel LC elements in the series branchs, which result in notches.
 

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Clearly this type of filter has the disadvantage of having a very high group delay near the corner frequency, so that in the filtering of video signals (to avoid ghost images) and also in very selective crossovers (to eliminate phase distortion) it is necessary to correct the GD with the use of suitable all-pass filters.

https://www.researchgate.net/publication/289049366_Correction_of_crossover_phase_distortion_using_reversed_time_all-pass_IIR_filter

https://en.wikipedia.org/wiki/All-pass_filter

cheers
Jaco
 
Thanks for the help, guys. It's much appreciated.

I guess I can safely pillage it for the inductors, as it doesn't seem like the filter will be useful to me as it is.
 
I know BBC made a lookahead limiter, am6/17 I think, that used a bunch of inductors and capacitors to make a really short delay. They put the delay on the audio signal and NOT the sidechain. Resulting in a limiter with a few milliseconds of lookahead.
I’m not at all sure if this is the case here. I’ve never seen a schematic of one.
 
Studio Mollan said:
I know BBC made a lookahead limiter, am6/17 I think, that used a bunch of inductors and capacitors to make a really short delay.
That is an All Pass Filter. Typically take 10-20 inductors and capacitors to generate the required delay. This is not the case here. Group delay is far from being constant.

I’m not at all sure if this is the case here. I’ve never seen a schematic of one.
Google BBC AM6-17. take also the opportunity to search for the EMT266, where an inductorless (active) version is used.
 
I just found the Schemo, here on groupdiy.
Thanks!
/
Emil
abbey road d enfer said:
That is an All Pass Filter. Typically take 10-20 inductors and capacitors to generate the required delay. This is not the case here. Group delay is far from being constant.
Google BBC AM6-17. take also the opportunity to search for the EMT266, where an inductorless (active) version is used.
 
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