( Be sure to Refresh your pages, (F5), as web-page construction has been ongoing since 1994, for 25 years
...... and may have changed since your last visit . Remember, the internet browser retains a cached page. )
*** (Home) ***
*** Please wait for large image loading ***
*** ( 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.