Switched capacitive audio filter projects

Filed Under (Homebrew, Kits, Morse Code - CW, QRP, YouTube Goodness!, YouTube Homebrew Goodness) by Jonathan on 11-11-2010

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!”.

Foxhunt gear – offset attenuators

Filed Under (Antennas, Dayton Hamvention 2010, Homebrew, Kits, VHF/UHF) by Jonathan on 08-07-2010

I attended a great fox hunting presentation at Dayton this year, hence a few posts on the topic.

I had some posts in the past about tape measure beam antennas.  Really neat antennas and they have multiple purpose use (they are GREAT for hitting distant repeaters when you orient them vertically!).  Much like the picture on the right (courtesy of Joe Moell K0OV) they are more useful for fox hunting when you add the active attenuator to your setup.  FYI, Joe is the co-author of the great book “TRANSMITTER HUNTING, Radio Direction Finding Simplified” available where most ham books are sold.  His website has more information on the book at http://www.homingin.com/THRDFSinfo.html and he contributes to CQ and CQ VHF.

So….. just what is an offset attenuator?  Joe explains it on his “Homing in” site as:

An RF attenuator is a device that goes between antenna and receiver to reduce the signal strength down to within the range that the receiver S-meter can handle. Without one, you may think you’re close to the fox when you’re still far away. You won’t be able to get close enough to a camouflaged hidden T to identify it. The amount of attenuation should be adjustable so that you can add just a little when your S-meter first pins, up to a lot as you get within a few feet. Special ARDF receivers used by champion foxhunters have electronic attenuation built in, but ordinary handi-talkies don’t. Adding it would require major micro-surgery in the HT.

His attenuator page is:


I recommend his site in general, many great projects:


On his attenuator page, he has full schematics to make an offset attenuator.

But wait…… there’s more!

Further on his page, you see one made in a sweet Pomona box.  I like this box and thought it was a bit pricey at first, until I did the math and figured out the cost/time to do it myself.  These boxes are shielded with the connector of your choosing (BNC/SMA/259, etc).

They generally cost around $25 or so and are shielded!  Great to have.  When you add the cost of connectors and such, it isn’t really so expensive after all.

I really advise using such a case or a metal case in general, makes things work out much smoother in the end.  More information on this box at: http://www.pomonaelectronics.com/index.php?i=prodsub&parent=BOX&cat=BONCONN&getDetails=

But wait….. there’s even more!

Marvin Johnston KE6HTS is now offering a “semi-kit” for this attenuator on his website.  I’ve seen this kit when I was at Dayton this year and encouraged a friend to pick it up and build.  I may end up running a buildathon here in CT on these attenuators.

The price is really not bad at $22.00.  You can purchase them built for a few dollars more.

Information on the kits and pre-built models are at:

And yep……. there’s even more (again!).

If you would like to “roll your own” from parts you may have on your bench, but don’t want to make a circuit board, you can get one from……. you guessed it…… Far circuits.  I picked one up and am going this route myself.

There are a few boards/projects on the Far circuits website at:


There are a ton of great resources out there on the web, these will really get you going right from the start.  Fox hunting is a really fun and useful part of our hobby and one that doesn’t cost a ton of money to get started in.  If there are no active fox hunts in your area – start ’em!  There are plenty of options as far as transmitters and such and really doesn’t cost a club much money to get started.

QRP/Hans Summers (G0UPL) 30m QRSS Beacon Kit

Filed Under (Dayton Hamvention 2010, Digital Communications, Homebrew, Kits, QRP) by Jonathan on 15-05-2010

Ahhhh, those lovely days in May when a QRP’er can go to Dayton a few days early and enjoy the companionship of his fellow knack sufferers.  One of the nice things about the QRP Amateur Radio Club International (QRPARCI) is their annual Dayton “Four Days in May” (FDIM).

FDIM is a great way to enjoy seminars and discussions on a whole range of topics.  There was a really good presentation by Hans Summers (G0UPL) on QRSS.  Many of you may be familiar with QRSS from podcasts such as “Soldersmoke”.  Essentially, it’s CW so slow, it’s pretty much impossible to decipher by ear, but sent at a VERY low power (if you’re using a full watt, you’re probably just plain rude!!!!

You build yourself a little low power transmitter and usually use some sort CW keyer circuit of sorts to send your call, short message or whatever.  The call is usually received by other hams and shared online.  It’s kinda neat to see your low power signal making it’s way over long distances.  It’s really a neat beacon.

Images are available on the web, but examples of reception are like below (from Hans Summers site):

Hans had a great discussion and was nice enough to produce a kit for FDIM (maybe he was earning his flight home, who knows!!!).  The kits were INSANELY popular and I think he might have just made a huge jump in the use of this unique operating mode.

Hans painstakingly took orders from hams and even programmed their Atmel chips with their calls.  What a nice guy!

He has plenty of information, schematics and such on his website.  If you’re interested, point your browser to:


Seriously great antanna analyzer information from down under (mostly)…..

Filed Under (Antennas, DIY Electronics Projects, Kits, New Ham Primer, YouTube Antenna Goodness, YouTube Basic Electronics Goodness, YouTube Goodness!, YouTube Homebrew Goodness) by Jonathan on 22-01-2010

Man, sometimes a topic starts going through your head, you do a little research and then start going all over the place.  This is one of them – antenna analyzers.  I’m a huge fan of them and run antenna clinics for our local club.  This tool is one quick way to get a snapshot of your antennas performance in a jiffy.

For those that have seen the MFJ analyzers, they basically consist of a low power transmitter and various circuitry to do the math and display the antenna or feedline information on an LCD screen.

Oztales strikes again with a very well done video.  The descriptions should give hams without an engineering background a very strong understanding of what is going on behind this “mystery box”.

While I’m “down under” let me show you a really neat analyzer kit that can be had for a fraction of the cost of the larger analyzers like the MFJ.  If you’re willing to burn a little solder, you can make a very sweet analyzer (I’m thinking of getting this kit as well, seems a good companion piece to my AADE LCR Meter).  As of this writing, the kit mentioned here costs $150.00 AUD for DX orders – which is a bargain!

As you can see, it has many things in common with the higher priced analyzers as far as layout and such.  Just keep in mind, from what I’ve seen in the picture and on their site, this is an HF only analyzer and does not have six meters – strictly 10 and up.

The kit is available from the South Coast Amateur Radio Club and on their website at:



Across the pond, someone built one of these kits and you can get a really good idea of what is inside the kit.  This video is what is now making me consider purchasing this kit:

BYRONLOCAL’s Youtube channel has a few other videos that may be part of the kit (components) but I haven’t watch them yet, so check his channel if you want more information.

While I’m “down under” let me show you a really neat analyzer kit that can be had for a fraction of the cost of the larger analyzers like the MFJ.  If you’re willing to burn a little solder, you can make a very sweet analyzer (I’m thinking of getting this kit as well, seems a good companion piece to my AADE LCR Meter).  As of this writing, the kit mentioned here costs $150.00 AUD for DX orders – which is a bargain!

Oztales has scratched the surface of what you can do with an analyzer.  If you practice your google-fu, you’ll find lots of other great uses/mods for analyzers to expand their capability.

NUE-PSK – Kit building, digital modes, low power….. what’s not to love!

Filed Under (Digital Communications, Homebrew, Kits, QRP, YouTube Digital Goodness!, YouTube Humor Goodness) by Jonathan on 29-03-2009

For me, digital nirvana!

For me, digital nirvana!

This is one of those projects that come along now and then and just make my jaw drop.  I fell in love with this project in 2007 at the TAPR/ARRL Digital Communications Conference in Hartford.

Disclaimer:  I haven’t yet had the chance to build this kit.  I have been hit early on with this economy thing and haven’t been able to afford a kit.  But, I do come from Hartford, CT where we have the Mark Twain home.  Like Tom Sawyer….. I “convinced” a few friends to check it out and had a chance to play with their kits.

Just what is this thing?  It is a PSK modem.  But, it is also a bit more than that.  Unlike PC modems, this one has an LCD screen with all the input/output information right on the modem itself.  You get the kit (or buy it pre-built) and connect it to your radio and a PC keyboard.  Tune up your radio to the PSK action and go from there.  I’ve seen this set up running with the DC to daylight rigs from Yaesu and Kenwood.  I’d like to get one of these and play around with one of the PSK Warbler kits from Small Wonder Labs (http://www.smallwonderlabs.com/Warbler.htm)  QRP and this modem are a PERFECT match.

It is lightweight and runs off a couple of 9 volt batteries.  Unless you’re KE4WLE, who has a video I’ll embed below, who runs his off of solar power!  Like many homebrew projects, since you build it, you are free to find many ways to tinker around and play with the kit.

I recently received permission from George Heron, N2APB to upload a couple of videos to YouTube and link in this post.  The videos below are of the prototype version (the one I got to check out at TAPR) and the battery installation of the new kit.

NUE-PSK prototype video (the final kit looks like the image at the top of this post)

New NUE-PSK kit battery install

What’s the price of admission?  Really rather inexpensive, consdidering.  You can get a full kit for $185.00 and fully assembled modem for $250.00.  If you’re an experienced kit builder, then you have a few other options (purchasing printed circuit boards, partial kits, etc).  Cables are most radios are also available.

Below is the Solar Powered NUE-PSK station of KE4WLE.

Final thoughts:

This is one of my favorite projects available for the radio amateur.  You can purchase the kit or get all the information to roll your own on the NUE-PSK website.  Something I think is really promising.  I have been involved in the creative commons and open source community for almost 20 years now and appreciate when ALL the information is freely available.

This kit is a full functioning modem/display with no personal computer required.  Great for a home station, even more fun for those of us that play radio in the great outdoors.  I’ve gotten my hands on a few of these and it’s always great to help others build them.  I hope to get one of these myself in the future so I can take them on some of my IOTA activations.

More information and ordering options are available on the new NUE-PSK site at http://www.nue-psk.com


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