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!”.

Antique Wireless Association Journal – On-Line

Filed Under (DIY Electronics Projects, Educational / Courses, Homebrew, QRP, Radio/Comm. History) by Jonathan on 01-06-2010

I recently featured some material available from the Antique Wireless Association and had to go a little deeper.

They have a wealth of information available on their site with some of their journal’s online articles.  Great articles on vacuum tubes and such – but much, much more.

I found a great little article called “Working With Crystal Control: A ‘Part 15′ Broadcast Band Transmitter “, the transmitter setup on the left.  Really neat article (I think I must….want to build one of these)………  The image on the left is from that article – not to difficult to build and learn!!!

As I understand it, you can purchase a CD with back copies of this magazine.  I just might look into that.  Kinda sounds like the enjoyment I get when I receive that little journal from the G-QRP club – SPRAT.  When that hits my mailbox, I know it’s getting stuffed in my backpack for enjoyment down the road.

One important thing to keep in mind with the AWA Archives is how well they not only demonstrate radio history, but how you can recreate it and learn from it.  What a better way is there for a budding QRPer to learn where he’s going, but get a hands on demo from where other hams have been?

Below are only a few examples of the neat articles you’ll find on the AWA website:

Key and Telegraph  by John Casale, W2NI
President Taft’s Telegraph Key

Building a 1929 Style Hartley Transmitter  by Scott M. Freeberg, WA9WFA
Need a transmitter for our 1929 QSO Party? Build it in one week-end!

Breadboarding  by Richard A. Parks
More Adventures With Transistors

The Vacuum Tube  by Ludwell A. Sibley
Tube Bases and the Asbestos Hustle

Restoration of Shellac Finishes on Older Radios  by Lane Upton
Don’t Strip That Old Finish–Save it Instead!

A nostalgia trip for the old-timer; an eye-opener for the newbie.

The Beginnings of Radio Central  by Ralph Williams with Marshall Etter, Bob McGraw and Chris Bacon

Pupin and Armstrong lay an egg–An Antique Radio Gazette reprint.

A Solid-State Filter Choke or Field Coil Replacement

Go ahead and check them out at:


http://www.antiquewireless.org/otb/archive.htm –  a direct link to the journal archives

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:


Analog Devices – a company that gets it right

Filed Under (DIY Electronics Projects, Homebrew, QRP, Reference Material) by Jonathan on 24-02-2010

I made a post awhile back about Tektronix and loads of information available on their site with regard to their (sweet!) line of O-scopes.  It was nice to see a company furnish plenty of generic information that is useful for the hobbyist or student.

This time around, it’s Analog Devices (http://www.analog.com).

When I’m hosting a buildathon or teaching a class, there’s always simple projects being built by attendees.  Some manufacturers make really good product data sheets, some don’t (as I recently found out using simple LM386 and similar clones).

Analog Devices not only has loads of great product data sheets and information on their website, they push the envelope one step further.  They publish a newsletter that is available on their website.

Analog Dialogue is the newsletter:


This month alone has a great article for the homebrewer/QRP’er called “Driving PIN Diodes: The Op-Amp Alternative“.  I didn’t look to see if the newsletter is downloadable or not (I hit the site, and start reading an article and then I get sidetracked.  Not that any of my fellow QRP’ers would ever know anything about that!).

They push the envelope one step further with Webinars!!!


I haven’t had a chance to watch some yet (I had to register, and haven’t had the chance yet) but I have plenty on my viewing list.

Here’s a sample:

  • Introduction to Op Amps — This presentation is an introductory discussion of operational amplifiers (op amps) and is intended for users that are new to or unfamiliar with analog design or concepts.
  • Quadrature and DC Correction for Direct Conversion Receivers –This webcast discusses the various wireless communication architectures with a focus on homodyne receiver challenges such as: DC offset, quadrature errors and even order distortions.
  • Amplifier Noise Principles for the Practical Engineer — This presentation will focus on practical noise tips for the board designer, rather than theoretical aspects often covered in academic texts.
  • Introduction to Differential Amplifiers and Design Tools — The differential amplifier function provides precision signal conditioning in many high-performance system applications, such as medical instrumentation and portable test equipment. But how do you apply these devices to insure that your signal chain meets performance expectations? This on-demand technical webinar covers the basics of differential amplifiers, their common applications, and explores several design examples.
  • Differential Circuit Design Techniques for Communication Applications — One of the major challenges in communications system design is the successful capturing of signals with adequate fidelity. This webinar explores the advantages of differential design techniques for today’s high performance communication systems.

I’ve used a lot of their product in the past, this only makes me want to continue knowing how much they back up their products with loads of information on how to use them!

If you know of any other companies products that we use on a regular basis that is very good at educating their customers on how to use their product, please share and post below!

JBOT – An SSB linear amplifier made from Just a Bunch of Transistors

Filed Under (Amplifiers, Homebrew, QRP) by Jonathan on 10-08-2009

Since I lost some time on the site when I broke my foot, I thought I’d drop a few bonuses for you guys this week.

I have an earlier post on the BitX 20 that generated quite a bit of attention.  Farhan has done a lot of great work on that project that has a HUGE online following.

Many have noted that there were some issues with the amplification of this transceiver and it may take a bit to get it going.  Farhan has mentioned this on his site as well:

For many of us, getting hold of high power RF transistors is impossible. They are also expensive and easily blown. In the last decade, hams have pressed the power MoSFETs into use as RF devices. This has been a mixed success. The power FETs (mostly the IRF series for International Rectifiers) have very high input and output capacitance, they need higher drain voltage and they are very non-linear devices. As result, IRF device based RF power amplifiers have remained beyond easy replication for the average homebrewer.

Often, the RF power amplifiers are run flat-out without any attempt at stabilizing the performance, This has to lead to a lot of grief.

The BITX’s linear amplifier painfully illustrates both these issues. The IRF510 takes a lot to get it going. Many of those who scratch build it on their own often found themselves struggling with stability and insufficient power output.

I heard about Farhans’ new project and thought I’d mention it here as it can work pretty well for those of you that have already built a BitX 20.

His JBOT page is at:



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