Brick wall LPF schematic wanted...

Thanks all, I'll give the Moamps active filter a go if I can find the time...


For some reason, when the transmitters are in stereo mode the side chains of the L and R companders are not linked, what causes heavy stereo image shifting when a panned hi-hat hits them.

Does this have a particular reason; are sidechains never linked on stereo FM devices?


Greetz,


Rogy

I have been working on a similar filter design also. I started out with a schematic I found in a datasheet. That wasn't very effective, but I added some more sections, and played with the component values in PSpice. This is what I came up with:



The complete response plot can be seen here: http://stiftsbogtrykkeriet.dk/~mcs/F4V10.gif

And a zoom of the 1-50kHz range: http://stiftsbogtrykkeriet.dk/~mcs/Not_nice.gif

There's good attenuation at 19kHz, and great above that. But what about the ripple in the audio range? Does that cause (audible) problems?

I tried a different approach also. I started out with a 6th order L-R lowpass filter, and added "notch" caps. That produces a much nicer looking curve: http://stiftsbogtrykkeriet.dk/~mcs/Nice.gif - but it's not nearly as "effective" as the first filter.

The "moamps" passive filter is more effective than my "nice" filter, but still has a nice smooth response curve. How do you calculate a filter like that?

And how much attenuation do you actually need at both the transmitting and receiving end?

And another thing. I found some Toko data sheets. They include a small cheap filter with these specs: -3dB at 17kHz and -35dB at 19kHz! How do you make a passive LC filter that steep?

Best regards,

Mikkel C. Simonsen

[quote author="mcs"]..... How do you calculate a filter like that?..[/quote]

It's an elliptical filter type. I found calculation in the Lancaster's Active Filter Cookbook. Nowadays you can use some filter design software to do this (FiltersCad, for example).

And how much attenuation do you actually need at both the transmitting and receiving end?

Click to expand...

At the TX end the attenuation of 19 and 38kHz is very important (interaction with pilot frequency and preemphasis reason). 60dB of attenuation at 19kHz is OK. At the receiving end attenuation isn't so important (receiver has a deemphasis circuitry anyway), 40dB with a good phase response is just fine.(The old Revox tuners have the LPF filter with 45dB pilot attenuation).

Regards,
Milan

Rogy said:

Hi all,

I'm back on this topic again.

I found an example of a 8-order elliptic filter in this document:

http://users.pandora.be/Rogy/Elliptical HPFs.pdf ; schematic at page 18 of the PDF.

The -3dB corner freq is 10KHz and it attenuates -80dB at 15KHz and above.

I simulated this circuit. I want to change the -3dB point to 15KHz and need to achieve as much attenuation as possible at 19KHz.

I tried to divide all component values by a factor 1.2666 to move the 15KHz point to 19KHz (19/15 = 1.2666) but that results in a -80dB point of 25KHz, so there must be a different scaling trick to be applied.

Anyone who can point me in the right direction?

The 19KHz point does not need to be -80dB down; it's more important that the frequency response remains flat upto 15KHz. So a lower order version of this filter might also do the trick.

Thanks in advance!


Rogy

Click to expand...

I know it's a bit late, but if you scale either the resistors or the capacitors by (1/1.2666), then the reduction will increase the corner frequency by 1.2666. If you scale both R's and C's by that factor, then the frequency will scale by (1.2666)^2. It is proper to hold the capacitors at standard values, and scale the resistors to fine tune the target frequency. That's because resistors happily come in finer increments in values. To move the Fc from 10kHz to 15kHz, multiple resistor values by (10/15). It's that simple.