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!”.

A little more on Morse Code – beginners and for those who wish to improve their proficiency

Filed Under (Educational / Courses, Morse Code - CW, New Ham Primer) by Jonathan on 01-12-2008

I had a few other Morse code posts in the past.  This time, I want to share with you a few resources for improving your skill or learning.

This will be a little in-depth, since I don’t wan to repeat myself in future posts.

I’m not going to go into the “code/no code” banter here.  That is not my intention in this thread.  I would like to state that I didn’t learn CW until after the requirement was moved in February ’07.  I have since learned and enjoy the mode (though I still slip up from time to time).  But, I am getting better at it!

The following resources are materials I’ve used in the beginning of my journey into learning CW.  Once you have learned the letters, numbers and some prosigns, just get on the air!  Will you mess up?  Most likely.  But you’ll find a lot of hams out there willing to take the time to work with you as you learn.  Just ask them to slow down a bit and offer pointers.

Since you only learn by doing………..

Reading and courses

The Art and Skill of Radio Telegraphy: This is the book that taught me a lot about CW and made me want to learn good code.  I had several learning issues early on, but this book really helped out.  The author, William Pierpont, N0HFF (SK) wrote a great and thorough book on the topic.

From the official site:  A Manual For Learning, Using, Mastering And Enjoying The International Morse Code As A Means Of Communication.

’nuff said.

Chuck Adams K7QO Code Course: Chuck Adams has a great course available including a course that can be burned to CD with audio and a text companion.  Truly a “code course”.  This is also an incredibly popular resource on the web.  Chuck is one of the few people that can copy code at 140WPM – so the guy knows what he’s talking about!

A Beginner’s Guide to Making CW Contacts by Jack Wagoner WB8FSV: Another great source that can be printed out and read over a few cups of coffee.  Easily read and understood.  One thing I like on Jack’s site is the beginner QSO template.  Makes that first contact jitters a little easier to overcome.


Just Learn Morse Code by Sigurd Stenersen, LB3KB: Very popular windows software that uses both the Farnsworth and Koch method.

Some features:

Accurate Morse code timing
Thorough evaluation of code copied
Customizable character set
International characters
Selectable speed, pitch and volume
Optional Farnsworth timing
Creating Morse code audio files
Generate Morse code from text files
Practice selected characters
Practice common words, abbreviations and Q codes
Optional dual pitch for initial learning

G4FON Koch CW Trainer by Ray Goff, G4FON: I’ve used this program the most.  It works really well.  Ray has a bit of information on the styles used in this program, but to give you an idea:

When I returned to Amateur Radio my CW was decidedly rusty. What I felt I needed was a CW Trainer to get my speed back. I looked around the Web and found an article by Dave Finley N1IRZ on a method of CW training developed by a German psychologist called Ludwig Koch back in the 1930’s.

Reading though Dave’s article, I decided that it all made a lot of sense. Basically you start off learning the code at the speed you would like to achieve. Unlike the Farnsworth method which seeks to reduce the gap between the letters as you become more efficient, Koch came up with the idea that you should start off just learning two letters at full speed and add an additional letter once you reach 90% proficiency until you have mastered them all. Since you are only learning one new letter or figure at a time, your frustration is significantly reduced.

Learn CW Online: This site is growing in popularity, so I’m including it here.  Also, because I’m not a Mac user, I wanted to give everyone a web based link.

– Koch Method CW Course
– Highscores — compare your results with others
– Speed Practice (Code Groups, Plain Text Training, Callsign Training)
– MP3 practice files (Download)
– Convert text to CW (does not require a login)
– Forum for user discussions and feedback

AA9PW Online Morse Practice: I’ve used this site A LOT in the beginning (probably when I should’ve been working!).  Simon Twigger, AA9PW offers a lot on his site.  You can get news headlines in CW and a whole host of other features.  The link here is for his morse pages, but you can also look at his FCC license exam information also.  Practice exams are incredibly useful.  I use them with all the classes I teach since when you take the test, the results page tell you what you need to work on.  Really useful indeed.

And then there’s…….

The Myth of Iambic Keying, by N1FN (pdf format).  Let me start off by saying I don’t necessarily agree with N1FN in this article, but some do so I’m including it here. I am an iambic operator.  It just “works” for me.  If it didn’t work for many, you wouldn’t see so many iambic keyer kits on the market.

I learned a long time ago that humans have different learning modalities and to say that one thing is absolute right or wrong just doesn’t make sense.  But, N1FN is an expert in the field, just check out the Morsex site!

He also has a bunch of interesting material in his articles section.

The main thing here is that you just get out there and practice.  If you have other great resources, please list them below in the comments and share with others.

A few interesting links to some morse code goodies online

Filed Under (Morse Code - CW, Uncategorized, YouTube CW Goodness!, YouTube Goodness!) by Jonathan on 30-11-2008

OK, it’s Sunday.  A holiday weekend at that.  Let’s just relax and see a few neat CW items I found online.

Many of us are familiar with Chuck Adams, K7QO.  He has a neat course available online to help you with your morse code skills, and, he can copy code at 140WPM!!!  Below is a bit that the Wall Street Journal did a while back that you may find interesting.

Here is a link to the Wall Street Journal article: http://online.wsj.com/public/article/SB119161604206850468.html?mod=editsend

Here’s a link to his course information: http://k7qo.radiotelegraphy.net/

This next video is for pure morse code entertainment…..

Which is faster:  Morse or SMS?

Just who is the man behind morse code?

Enjoy, and I hope you had a great holiday weekend.

One-valve (ECL82 or 6BM8) CW Transmitter de Jan Axing, SM5GNN

Filed Under (Homebrew, Morse Code - CW, QRP) by Jonathan on 29-11-2008

Hans Summers, G0UPL

Hans Summers, G0UPL's One-valve (ECL82) CW Transmitter

OK, now I wish I had a one of these tubes and sockets.  Hans has a lot of inspiring information on his site. This is his version of Jan Axing’s (SM5GNN) design.  You can find the original information on Jan’s old site here.

Han’s site has a lot of information on the project and modifications to this project.  This can be built as a dual band 80/40M transmitter.  Think about the use of this transmitter on two of the most popular bands!

Unlike many QRP projects on the web, this one has several other neat things to learn like building a power supply.  It is a really good first tube project – BUT USE CAUTION with these voltages.

This project runs about about 10 watts and has a VFO!  Something I need to make in a future project.  Frequency range is about 3,560,000 and 3,560,500 if you follow the original schematic as Hans did in his first build.

The original version on top of Hanss tuner.  Something nice about a whole home made shack!

The original version on top of Hans's tuner. Something nice about a whole home made shack!

From Hans’s page:

The Pierce oscillator runs continuously while transmitting so there is no chirp during key down. The bias was adjusted to -24V during keydown at the junction of the 22K, 220K and 100K (var) resistors, in accordance with Jan’s instructions. The crystal used was a 3.560 MHz QRP calling frequency crystal in an HC49/U style case; I had some worries that this crystal would not withstand the high powers in a tube circuit but so far it hasn’t cracked under the strain.

This project would be great step up from a weekend kit builder.  Well documented so it’s easy for you to do your research and find components (many of which would be in any builders junque box).

The US equivalent would be a 6BM8.  I hope you find this as interesting as I did and enjoy the reading over this long holiday weekend.  And……. if you happen to have a spare 6BM8 and socket you can mail my way………….

OMG, morse code (CW) isn’t dead yet? BBC Video

Filed Under (Morse Code - CW, Opinion, YouTube CW Goodness!, YouTube Goodness!) by Jonathan on 06-11-2008

OK, I’ll admit it.  I am fine with the FCC dropping morse code as a requirement.  I am on the board of a local club in Newington, CT.  I have access to airtime on the repeater to organize a net.  I am also actively involved in ARES (which eats up a lot of time, and I’m a single dad).  So, I didn’t have the time to do it myself as I already volunteer for several radio projects on a regular basis.

Flash forward……

I know several hams that are good friends of mine that were talking about “dumbing” down the hobby.  This, is ridiculous.  Roughly three dozen sounds to memorize isn’t “dumbing” down the hobby/service.  If you removed technical questions on modes, RF safety, electronic theory, then maybe you’re “dumbing” down the hobby.

Now, I asked several of them when they were talking about being bored as of late (not like I have that option with a workbench of radio toys!).  Not one of them volunteered to run a CW training net.  Yet all thought removing it was “dumbing” down the hobby.

So, that was enough to change my mind on the topic completely – good riddance if nobody wants to teach the subject.  I was in a QSO with an Aussie friend after they dropped their code and he told me that CW use was on the RISE!  Now, that got me thinking.  If a ham has a taste of HF, wants to work DX, he’s going to have to use CW either way, so….. it makes sense to me.

Needless to say, I took my General exam, and started learning CW.  I enjoy the mode.  There’s something about how astoundingly relaxing it is to sit back with a 1 watt transmitter you built and carry a conversation with someone a hundred miles a way (all with a 9 volt battery while sitting on the patio!).

Now, I really like CW and operate quite a bit.  If you are reading this and have worked me, thanx!  It is with patience of hams like yourself that I will improve my CW skills.  I’m still working on it though……

If you are reading this and haven’t yet given CW a try, go for it! It takes some time, a little patience, but is really rewarding. I make sure when I teach a class that I tell all the “soon to be” hams to give it a try even though it isn’t a requirement.

I haven’t had a single one tell me they regret the experience.


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