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2019-02-22   02:35:46

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/aimg-morse-man          the "Advanced"    AF"X"    Filter Circuit  ( early version )

*** ( Read the AFX basic pages before reading this schematic. ) 

*** Filter Topology Design choices: 
*** Multi-FeedBack topology was chosen because : 
*** (1) input vs output impedances match well, intra-stage. 
*** (2) single resistor Frequency Control on each stage. 
*** (3) Frequency Adjustments alters gain by only the square-root of the f(change) . 
*** (4) the Roof-Triad-Differential design is very tolerant of component variations . 
*** (5) Voltage levels are aprox. 1 V range well above the noise floor. 


Note:    All Filter and Differential stages are set to aprox. standard 1V peak 
                      (a) to be above noise 
                      (b) for easy comparison from stage to stage , thus easy visual analysis 

Note:    Power Supply ByPass caps are not shown. 
             OpAmp Null trim pots were not used, in this development with LM-324 quad OpAmp chips. 
             All Filter Stages are MFB, effectively AC audio coupled, thus self-centering.  

Note:    For each Roofing-Triad-Differential Filter (RTD) : 
             On each Differential chip, the (+) 1K to ground should be adjusted 
             between 1K and 5K to allow bias currents to equal out (+) vs (-).  
             The indicated 1 K Ohm was used to satisfy ngSPICE calculations. 
             In this real circuit, for the LM-324 quad OpAmp, aprox. 4K is good.  FET OpAmps may be different. 
             A trim pot and Oscope will allow adjustment of RTD filters 
             to produce a apox. flat-topped 250Hz wide Roofing Shaped PassBand, with NO peak.  

*** The ADVANCED AFX has Near DSP narrow CW filtering. 

*** Preceeded by Three Roof-Triad-Differential Filters. 
*** Hi-Notch = -60 dB and -26 dB under Fx04
*** Dual Notches aprox. +/- 200 Hz from 700 Hz , ie, 520Hz and 920Hz.   Maximum Stop-Band Attenuation is under -98 dB. 
*** Based on Differential Filter, UDIF, and including Dual-Notches, the Band-Pass is 90Hz  
*** Based on the inclusion of the  UQ20 final narrow filter, including the dual-notches, the Band-Pass is 35Hz. 

*** Stop-Band floor is -98 dB, depending on the Filtering selected.  UDIF plus UQ20 provide the extreme deep -98dB to -130dB.

( Advanced AFX  is  further developed in the "Variable AFX 'V' design", which incorporates single User control of f(0) for all filter stages. ) 

***   Advanced "AFX"   with   THREE Roof-Triad-Differential Filters

*** click image for hi-res schematic.    Included here is a final Q=20 filter. 

*** Below : Preceded by Three Triad-Differential Filters :  
*** UDif Hi-Notch = -60 dB and is also -26 dB under Fx04 at 920 Hz. 
*** Q20  Hi-Notch = -80 dB and is also -46 dB under Fx04 at 920 Hz. 
*** Stop-Band floor is -98 dB flat. 

Stop-Band Spice results: 

*** Roof Dif-C       =128uV=-78dB
*** matched by Lmt-Ctl   =-128uV=-78dB
*** Filter 04 =304nV=-130dB 
*** Stage uDif       =3.5uV=-109dB 


*** Experiments show that a 10% component change in Roof-Triad-Differential stages 
*** will have little effect on the final Fx04 or DIF14 stages. 



*** Linear Bode clearly showing  the  very deep  Udif Double-Notches  are below the Fx-04 filter curve.  
*** Also note the slight raising of Stop-Band after the Notches take effect (blue trace).
*** The energy that  is being filtered out by the Notches is pushed into the Stop-Band energy. 
*** Authors opinion is that the -60dB Notch Depth at 920Hz is more important than the final Stop-Band.  


The latest version includes a final Filter "Q20"
which produces much greater attenuation at the dual-notches and the stop-band. 
Notice that there are two traces showing a dual notch filter
First is the UDIF  dual-notch at BW=90Hz and -70 dB.
Second is   UQ20 final filter  at BW=35Hz and -92 dB for double-notch.   


Each of these six Magnitude plots show
the ever deepening dual-notches for the three final filters. 

Look for the "cursor marker" ( big round dot ) 
and read the data in the little window.  
Notches at 922Hz are -46dB,   -70dB,   -92dB
The plots are of : blue Netsf4,   green NetUDif,   orange NetQ20.

*** Look for the Cursor mark ( big round dot )

*** NetSF4 f(925Hz) ( hi notch ) (blue trace)
*** NetSF4 f(1400) ( stop band ) (blue trace)

*** NetUDif f(925 Hz) (notch) (green trace)

*** NetUDIF f(1400) (stop band) (green trace)

*** Last two bodes are for the Q20 filer,
*** applied after Three Roofing Filters + the Quad filters + the DIF filter.  
*** NetUQ20 pushes the stop band down much further.  
*** The important 922 Hz Notch = -92 dB,  and 1400 Hz StopBand = -141 dB. 

*** NetUQ20 f(925 Hz) (hi notch) (orange trace)


*** NetUQ20 f(1400) (stop band) (orange trace)