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!”.

Rigol DS1052 scope hack – NEAT!

Filed Under (Homebrew, Reference Material, Tools, YouTube Basic Electronics Goodness, YouTube Goodness!, YouTube Homebrew Goodness) by Jonathan on 20-10-2010

OK, regardless of what many “old hat” hams think, there is a lot of kit building and homebrew activity going on in our little “geek” subculture.  I don’t bother with the discussions with many of my friends, as they don’t go anywhere to see the truly neat projects people are developing.

To that end, many hams have asked me over the past few months about scopes (even a recent topic on “the zed”).  Basically, you can go one of four routes.

1: Buy a super expensive scope

2: Buy a decent used scope for a couple hundred

3: Buy a USB/computer based scope

4: Buy a brand you may not be familiar with, hack it, and get double what you paid for (in the $400. range).

I’m a fan of 2 and 4.  Both have their merits.  A used O’scope (not really Irish either) is a bargain, you can get a name brand and will cover most of what you need in our hobby.  Used scopes are also great to learn on as well until you figure out what you really may need.

That leads us to a “no name” brand.  The Rigol is truly a neat scope.  I used one on my last job for about 9 or so months and it really had some neat features.  It is portable, can store images to a USB drive, can connect via USB to a computer, the list goes on, but here’s a few:

Model DS1052E
Bandwidth 50 MHz
Analog Channels 2
Vertical Sensitivity 2 mV/div – 10V/div
Real-time Sample Rate 1 GSa/s?each channel??500 MSa/s?dual channels?
Equivalent Sample Rate 10 GSa/s
Memory Depth Mode capture rate common long memory
one channel 1 GSa/s 16 kpts N/A
500 MSa/s 16 kpts 1 Mpts
dual channels 500 MSa/s

250 MSa/s
or less

8 kpts

8 kpts


512 kpts

Standard Interface USB Host & Device, RS-232, P/F Out
Vertical Resolution 8 bits
Math ?, ?, ×, FFT
Max Input Voltage All inputs 1M??15pF 300 V RMS CAT?
Cursor Measurements Manual, Track and Auto Measure modes

Not a bad scope for the $400. range on ebay.

But…… (think Vince from “Slap Chop”)…… if you act now, because we can’t do this all day………

Dave Jones over at the EEVBlog has a neat video on Youtube on how you can double some of the specs…. for nothing.  Be wary of some google searches on the topic, some want you to hack into the scope and do all sorts of nonsense.  This is a simple first start and many people have had great results with it.

If you would like to go to the actual Rigol page and look up the specs and download the manual, go here:


Finally, if you’re still not sure what you are looking for, you may be able to try a scope at a few places.  Some trade schools, community colleges and such will let you come down and just take a look (or attend a workshop).  There is also a growing number of “hackerspaces” around the globe.  A “hackerspace” is a place to explore and learn technology in all it’s facets.  Some vendors (Tektronix for example) have some really good resources on their websites as well.

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.

The QRP-L.org Group Build Project

Filed Under (Homebrew, QRP, YouTube Goodness!, YouTube Homebrew Goodness) by Jonathan on 15-05-2009

Now this is homebrew!!!

Now this is "homebrew"!!!

OK, I slacked off and missed a scheduled post last week, so I’m gonna give you a “twofer” this week with this well documented project.

Somehow, I stumbled on the website of NT7S, Jason R Milldrum.  His site documents the QRP-L project.  FANTASTIC WORK!

If you are interested in this project, there is a full build sequence (several pdf’s) which will make this an easier project for a new homebrewer.  Though, I think this is more of an intermediate project, the process is very well documented.

What is this project?  Let’s look at Jason’s site for more information:

The aim of the qrp-l.org Group Project is to provide members of the group with the opportunity to gain hands-on experience building and testing a QRP CW transceiver with like-minded individuals. The design will be a direct conversion rig with moderate complexity. The entire rig will be built from easy-to-find discrete components. This will make it relatively easy to obtain the required parts or make substitutions. It also gets away from the “black box” concept of some project, where much of the circuit action is enigmatic.

More information is available on his site at: http://www.nt7s.com/index.php5?page=qrplgp

Here is a block diagram of the project that would give you a pretty good idea of what to expect:

Jason has kept the site updated with regular posts regarding the current status of the project and also included audio files of the group EchoLink conference.  Now that’s really neat!

He also posted a YouTube video of his work in progress:

I’ve built several kits and smaller projects from schematics, I might just give this one a shot.  I’m looking at my parts bin and checking to see if I have enough to get started.

Thanx Jason for a well documented project!

Introduction to Oscilloscopes

Filed Under (Homebrew, Reference Material, Tools, YouTube Goodness!, YouTube Homebrew Goodness) by Jonathan on 23-04-2009

Tektronix 465 O-Scope, commonly available online for a fair price

Tektronix 465 O-Scope, commonly available online for a fair price

I know I’ve been tinkering around when I just spent a few hours looking at Oscope information!

I’ve been tinkering around a lot with QRP projects and some PIC based toys.  I’ve been wanting one on my bench for a few years now, but just couldn’t justify it – until now.

For past few years, more and more older analog scopes have been hitting the market in industrial surplus and on e-bay.  As companies are upgrading their equipment, many used scopes have been hitting the market for only a few hundred dollars.

I have found a bunch of decent videos and some good information on the Tektronix site.

First, I’m going to start of with a Wikipedia entry to give you some basic information.

An oscilloscope (commonly abbreviated to scope or O-scope) is a type of electronic test instrument that allows signal voltages to be viewed, usually as a two-dimensional graph of one or more electrical potential differences (vertical axis) plotted as a function of time or of some other voltage (horizontal axis). Although an oscilloscope displays voltage on its vertical axis, any other quantity that can be converted to a voltage can be displayed as well. In most instances, oscilloscopes show events that repeat with either no change, or change slowly. The oscilloscope is one of the most versatile and widely-used electronic instruments.

The rest of the information is available here: http://en.wikipedia.org/wiki/Oscilloscope This is one of the better writeups I’ve seen on Wikipedia.

Afrotechmods has a three part video set on YouTube that give a decent introduction to scopes with some really good advice as far as purchasing used on ebay and what is good for the home tech-bench.

AllAmericanFive radio has a neat intro to scopes.  He does a lot of videos on radio restoration and such.  VERY RECOMMENDED.

Found this neat older video here – still has good info.

Check out the Tektronix download here:  http://www.tek.com/learning/oscilloscopes/ You have to sign up to download from this site, but it is really worth it.  The sixty page booklet called “Oscilloscope Primer” is really worth reading.

There are a lot of sites out there with information on O-Scopes, this will get you started.

As usual, if you find any good links, please share with the rest of us and add a comment below.

**** late note/addition 4.24.09 ****

Based on a thread on the ‘zed, I have a few additions to add to the post.

The initial link from Tektronix (the one you have to register for) I think was for this file (direct download):


And here’s one that is a primer on probes:


Paul Harden, NA5N, has a few good, brief tutorials on his site:


Here is his scope tutorial part 1:


And here’s part 2:



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