N path filters

In the mid-60's there was a two-book series on integrated circuit design by Motorola. The first book was on the physics of the thing, the second book on various circuit design techniques. I wrote the chapter in the second book about various "frequency-selective" techniques, including N-path. Not being home right now I can't give you an exact reference.

Probably, though N-path can be a handy scheme. ...Jim Thompson

Thanks for the reply, if at some point you could let me know the title of the book that would be greatly appreciated!

I did a bit more digging and found some basic information; a short article in the book "Analog Circuits Cookbook". Basically, the center frequency of a low-pass prototype N-path bandpass filter is going to be Fclock/n, where n is the number of capacitor stages. The upper and lower 3dB point is then going to be 1/2*pi*n*CR, where R is the input resistor.

So the Q of the filter is going to to be Fclock*pi*C*R. The number of stages doesn't seem to affect the Q but I guess with more stages post-filtering becomes easier, and it relaxes the requirements on whatever is switching the capacitors in and out of the circuit. A Q in the millions looks possible, though to work at RF frequencies it seems one is going to need a really fast clock, shift register, and set of analog switches. I'm guessing it's probably not possible to make it work at anything but low frequencies unless the system is implemented in an IC.

Looks great, thanks!

How about these?

Ect. The list goes on..

Jamie

I made it work at 455kHz in 1964 ;-) ...Jim Thompson

That's like the single-pole RC lowpass, with the cap replaced by a spinning SPnT rotary switch and N grounded caps, no?

That was never very popular. Switched-capacitor filters like the MF10 were more practical, and digital processing even better.

John

MF10's were kinda cool but as I remember, they were pretty noisy due to the clock.

Gimmee a Delta-Sigma A2D and a DSP.

(hey, that rhymes !)

boB

Analysis and Design of Integrated Circuits (Motorola series in solid-state electronics) (ISBN: 0070417237 / 0-07-041723-7 ) Charles S. Meyer

I learned the techniques from several papers by Franks and Sandberg....

Theory and Applications of All-Digital N-Path Filters ieeexplore.ieee.org/iel5/31/23596/01086265.pdf by S Mitra - 1987 - Cited by 4 - Related articles The N-path filter originally introduced by Franks and. Sandberg [l], [2] for the processing of analog continuous- time signals consists of N identical time-invariant ... ?

I _may_ still have those papers ;-)

Back in AZ October 1, and I'll look. ...Jim Thompson

Found it on-line....

They were noisy in their own right, but worse, they aliased all the noise on their power rails, or anywhere else in sight. And they shot spikes out all their pins, so you really needed to pre and post filter with RCs. But they were kind off cool, with the clock tunability. I designed a really spiffy multifrequency, multibaud rate (75 to 600!) FSK modem for the Reuters wire services, just before the Internet blew all that news printer stuff away. Pity.

MF10 was a metal gate cmos part, and National did a better, quieter poly gate thing later, MF100 or some such, but too late, the party was over.

You can get an ARM for under 1 dollar with a mediocre ADC on-chip.

The style is Classical Geek!

John

on

Great, thank you!