Re: Post Detection Filter with buffer


Radu Bogdan Dicher
 

Here's more work on this. 

Group delay measurements enclosed. Mind, you, again, these and not sims, but actual measured values (I assume, for instance, the notches can never go as deep as the sims show in actual measurement, as much as the GD measurements will be lesser than the ideal simulation of the circuit). By sims measures, this circuit (LTSpice file enclosed for anyone interested in tinkering with the circuit) and the original/Ray's are extremely close in group delay behavior, as shown before (BTW, if anyone's willing to lend me their built "Ray Post Detection Filter," I'd be happy to run measurements and make it more apples to apples). 

That said, I think if this one gets' some focused attention in the 70-100kHz range (and I'm excitedly looking forward to feedback from the group) it will be a very beneficial kit given its better frequency response (-3dB @ about 95kHz) and better potential integration in existing tuners/receivers. Very flat GD response all the way to 70kHz, and so in the 23-53kHz range all things are well if that's what really matters. 

An important point, for me, is that one of the further steps here is also employing higher Q inductors - the reason I kicked off that conversation over common mode chokes. I want to get some and try them out with this, though selection of parts to run through the gamut is a bit challenging at this stage. 

Radu. 

On Mon, May 4, 2020 at 5:02 PM Radu Bogdan Dicher via groups.io <vondicher=gmail.com@groups.io> wrote:
Bob,
Thank you for your feedback and points. I'm looking forward to further tweaking this design. 
I enclose what I'm getting from LTSpice for both circuits. This includes levels and group delay on the same plots. I also include a magnified view on the roughly 23-53kHz band. 
Radu. 

On Mon, May 4, 2020 at 4:22 PM newaag via groups.io <newaag=yahoo.com@groups.io> wrote:
here's the 19/38 notch filter simulation and board. On this design, passive de-emphasis was included in the response, hence the constant down slope. 
https://groups.io/g/FMtuners/files/LM4500%20Filter%20and%20buffer

I can likely find the notch simulation for the version without de-emphasis on my computer somewhere. 2 or 3 windows OS versions ago. 
As you can probably figure out, the higher the Q of the components used, the more narrow and deeper the notch is. But the more narrow and deeper the notch is, the harder it is to tune and get on exact frequency.
Bob

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