Switched capacitive audio filter projects
With W1AW practically in my back yard, I get to play with some expensive toys. I volunteer there from time to time….. and really get to play with some toys! One thing I’ve learned (and try to teach new hams in my classes) is….. spend more time listening than transmitting!
So, on all my non homebrewed rigs, I have spent tons of time learning how to use, or installing filters and figuring out not only how they work, but how they truly help me on the air. Software defined radio has also shown me some really neat things that can be done just playing with bandwidth and CW (especially trying to work a DXpedition!).
To that end, I finally got to play with a few switched capacitive audio filters that really had me thinking, why do I not have one!
Today I’m going to show you a few projects that can get you going. First, the image on the right is from the NEQRP Club and their NESCAF project. The theory of operation, I’ll quote from their website: http://newenglandqrp.org/nescaf
The integrated circuit at the heart of the NESCaf is made up of two CMOS active filters. These filters are extremely configurable (low pass, band pass, notch etc).
We have chosen to set up both filters as Butterworth band pass filters and to cascade the filters. Butterworth filters have the characteristic of constant amplitude in the band pass region, while the cutoff knee is not be as sharp as if the filter were configured as a Chebychev. We considered this an acceptable tradeoff, wanting constant volume out regardless of the bandwidth or center frequency setting of the filter.
There are two on-board trimmer pots. One is used to “calibrate” the center frequency pot. This allows you to adjust the frequency at which the center detent occurs. If you are using a rig with a transmit offset and sidetone of say, 700 Hz, you can use this trimmer to make that the center-detent frequency. The other on-board trimmer adjusts the audio level into the filters output amp. Using this pot, you can set the overall gain of the filter. This can be used to set the filter for unity gain, if desired. This way, the filter could be switched in and out, and still maintain a comparable volume level in the headphones.
Pretty neat! This is a relatively simple kit that the club has been offering for quite awhile. Out of stock now, but will be available again soon and priced really inexpensively – under $35.00. If you want, all the information is available int he schematic and documentation if you want to “roll your own”.
There are some commercial offerings as well, many we have seen advertised like the one from Idiom Press (http://www.idiompress.com/scaf-1.html). This one works VERY well, and comes as a complete kit with an enclosure. There is some good information on the Idiom Press site that shows the response curve as well as why the filter doesn’t use DSP. This kit is a bit more, but has an extremely high quality/professional looking enclosure and sells for $89.95. I can’t afford one now, but when I can, I plan to build one and post details here with audio files (there is an audio file that can be played on their website to demonstrate the audio characteristics).
Here is a great video of NG9D’s build of the SCAF-1:
But, I don’t want to make this just a CW project…… how about AM?
Stewart (“Stu”) Personick, AB2EZ is a ham that wanted to work a bit on his transmitted and received audio. He took the SCAF-1 and modified it in true ham spirit. From his online writeup of the project:
My original objective was to demonstrate, to the AM community, the use of a switched capacitor filter for “brick-wall” bandlimiting of the output of an AM audio chain… in order to limit the bandwidth of the r.f. output signal produced by a vintage high-level-modulated vacuum tube transmitter, or a modern FET-based “Class E” transmitter.
What’s really neat, is this is a relatively easy mod to build in and expand the already excellent capabilities of a great kit/filter! You can find full details of this modification at: http://mysite.verizon.net/sdp2/id14.html I think the Idiom Press site has copied this info on their site as well.
A little more research on the topic led me to “An Adjustable Audio Filter System for the Receiver” by Lloyd Butler VK5BR (Originally Published in Amateur Radio, March 1995).
This is also a “roll your own” project, but gives some detail on the use of the filter in CW, RTTY, voice and other narrow bands. This article was originally published 15 years ago and is still VERY relevant today.
I took a look at the schematic and it’s not entirely too difficult to build this in an evening or two and looks like a very useful and educational shack accessory.
You can find the article and schematic at: http://www.qsl.net/vk5br/SwCapFilter.htm
Hopefully this will whet your appetite and make you want to fire up your soldering iron and build a useful filter for your operating needs. I know after using a few of these filters I’m left to wonder, “how did I NOT learn about switched capacitive audio filters!”.
I have the Idiom press filter and the New England SCAF filter.
To my ears they both seem the same but the NE/SCAF filter lets you tune down to 60Hz or very close to it.
Also the other advantage of the NE/SCAF filter is that it can be built into most rigs but both will for sure zap out QRM
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Anyone know where I can get one or even the PCB? @arrl.net pse
Dead links – Dec 2016