Posts Tagged ‘YouTube Homebrew Goodness’

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!”.

Rigol DS1052 scope hack – NEAT!

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

N.A.

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:

http://www.rigolna.com/products/digital-oscilloscopes/ds1000e/ds1052e/

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.


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