Weekend Radio Click Here for More Electronics Projects and Tutorials By Mike Maynard, K4ICY

A Curiously Narrow CW Audio Filter
By Mike Maynard, K4ICY

       If you're just getting into CW and you are starting off with an older radio or QRP set then you're sure to have run into the problem of trying to pick out any given CW signal from a pileup.  Modern commercial radios come equipped with filters for narrowing the audio passband and the digital signal processing (DSP) models can do every trick allowed by virtual physics to clean up unwanted noise.  The trouble you may be having is due to the bandwidth of your receiver's audio passband which may be too wide, so other nearby signals are competing for your ears.  What you need to do is just pick out one single CW signal.

       This little accessory will add the much needed narrow band filtering to your radio's audio output!

       The circuit is (litterally) textbook, 
inspired from the ARRL Extra Class study manual's (Volume 8) pages on active filters and is an Active RC driven filter using Op amps.  It has a narrow passband that will amplify only a desired range or audio frequencies and reject others that may present QRM & QRN interference.  It's small enough to be tucked away inside of many older rigs and the ouput is enough to drive a pair of headphones or a small PM speaker.  However, it must be formulated and pre-constructed for a set passband frequency range, i.e; 700 Hz.

Example of a finished radio accessory
This project was stuffed into a small Altoids mint tin.
(Click to enlarge photos)

       Building your own station accessories is a time-honored tradition of the Ham Radio hobby and there's more satisfaction and learning that comes from homebrewing a device over that of just ordering an "appliance" from an online distributor.  If you would like to construct your own gear such as this, you should have some basic knowledge of electronics, being familiar with IC's and audio circuitry.  If you're a ham, this should come naturally of course.  A lot of trial and error went into my own construction of my CW filter and I do not profess to being an engineer.


       Operation couldn't be simpler:  One audio input jack accepts a patch cord from the radio and your headphone plug into the other one.  There are two switches, one to select between activation of the circuit and bypass mode, and the other to choose between two and four stages, or rather, ecause of the switch I used, in the "OFF" position the filter is in "Bypass Mode" and only a 220 F capacitor sits on the line.  When activated, the stage selector is kept at the halfway point at the output of the 2nd 741 Op amp as this allows for much filtering but with enough outside audio being passed through to be able to adjust the VFO knob on the rig.  When an incoming CW signal is set at "zero-beat" you can then switch the output to four-stage mode using all the Op amps.  If the received CW audio is clear or even piercing then the filter is working.  The only downside is, since this filter is extremely narrow, some "ringing" (which sounds like noise played underwater) may be produced.

       The provided schematic (below) has component values set to provide a frequency response filter the incoming audio at around 750 Hz and the "shoulder" of the response curve is wide enough to allow frequencies 50 Hz in either direction with audibility. (700-800 Hz)  If you have a different sidetone on your rig other than 750 Hz, you'll first need to zero-beat the signal in bypass mode and use the RIT on your receiver to "fake" the received tone match 750 hz.  This will allow you to enjoy the benefits of the filter without the math.  So far, the low capacity generic 9 volt dry-cell has provided filter power for hours, the circuit's current drain is in the low mA's so I can imagine a good Alkaline battery lasting a long time.  If you use a Lithium 9 v battery, you can escape the risk of battery leakage and have a viable accessory that will stick around for years.

Suggested Steps in Construction

     Study the schematic
     • Calculate the parts needed, and then recalculate.
     • Hunt for spare parts and purchase the ones still needed... 
           Consider the cost of parts together as this may cost more than a pre-built version.
     • Build circuit on a protoboard first as this will allow for changes and help you determine the project's usefulness.
     • Use construction method of your choice, either with a perfboard or with a custom etched PCB from PCBWay.com
     • Test every aspect of the board.  An oscilloscope is preferred but even a multimeter can be invaluable.
     • Enclosure prep, switch mounts and wiring.  Can you make things compact but easy to access and service?
     • Finalization, mounting and cosmetics - this is industrial design and a well-designed product will get used more.
     • Consider additional feature such as adding a gain amp at the end along with a volume reducing resistor network potentiometer before the first capacitor to address operating level deficiencies.

4 Stage, RC Active CW Filter

The layout is actually easier than it looks. There are just four of the same filter cascaded.

Circuit Description

       Standing on the "shoulders of giants," I, of course, can't take full credit for this circuit, I used various reference sources such as the ARRL Extra Class License Manual, and a GL Tab book on Practical OP Amp Circuits.  The filter runs off of a 9 v-15 v power source and unbelievably draws only 3 mA.  You can either use four individual 741 op amps ICs, or in my case, (2) 947D dual op amps, but others Op amp variations will work.  The Op amps are configured with a bandpass topology, using a Twin-Tee Resistor-Capacitor network and the external components are what determine the operational characteristics of the amps and the quality (or effectiveness level) so close tolerance components must be considered.

       C4 in front decouples the incoming audio signal.  R1 determines gain and is compared to the +/- voltage reference input on the non-inverting side (+), those values R4 & R5 must be equal but do not have to be an exact value related to the rest of the circuit.  Cutting R1's value in half to get the value of R4 & R5 will work fine.  C1 & C2 and R3 are the resonance tank that determine the center frequency.  The active bandpass frequency must be determined by R1-R3 so creating a variable frequency response control is out of the question!  R2 will thus determine the bandwidth to a degree and if only one stage is to be used, that may be a nice feature.  You can vary this value to give you a controllable width of the final passband.

       This project uses four stages to hone in the passband and is quite effective with CW signals.  Using more stages will subsequently add more filtering, but in the bargain, a ringing will become more evident and eventually the desired signal will distort beyond usefulness.  The ringing on this one is not too bad and if it is evident it sounds more like a signal that was put under water.  There has to be an increase in out-of-passband signal energy (other signals) to cause this effect.

       As you can see, the voltage comparator network: R4 & R5 is shared between all stages and cuts down on parts and connections.  C3: a 10F electrolytic cap was included for AC signal blocking to prevent a spurious oscillation.  A 100-400F electrolytic cap must precede the output to decouple the output.  As far as functionality and controls are concerned, this is the builder's choice.  I went with a DPDT switch for circuit power, using the other side to bypass the audio when "OFF".  A SPDT switch is used to choose between the output for the two stages and a total four stages.
You can even talor the "envelope" of the passband by allowing the end two phases to have a wider pass, letting in just enough outer-band audio to keep you from getting lost when spinning the tuning dial on your rig.

       Concerning the chosen resistor-capacitor values: first, if choosing smaller cap values (like the 680 pF) will thus cause the resistor values to go up to satisfy the equation.  When the resistance goes up, the audio power going though will be cut and your total volume gain will be small.  You'll then have to add another amplifier circuit such as an LM386 IC based one to re-gain the audio volume.  Higher values of caps, like above 0.2F will give you lower resistor values, allow more audio power to run though, but will cause possible distortion and too much current flow to the op amps.  This is why experimentation is suggested to be done on the breadboard or in a simulation program like LTSpice first.

       I chose 0.01F's since I had a handful of them laying around. This value works fine, but my final volume is a little low, and I cannot allow too much AF volume from my rig before the filter becomes either non-responsive or distorts with large signals.  By using a potentiometer for R2 AND using that part in series with a low value resistor like a 500 ohm (to prevent a short) will allow you to tweak the bandwidth and R2 value to match your sidetone signal - this may allow you to use fixed resistors. You can however use four trim-pots if money is not an issue.

The formulas can be found in the ARRL's 8th Edition Extra Class License Manual on pages 6-7. (2007 issue... not sure about currently)
Also check this info out at Radio-Electronics.com... where you can get some extra theory information.

C will be both C1 and C2
in farads
F   =   Frequency in Mhz
Q   =   (sharpness of the filter skirt) should be less than 10
G   =   (Gain) should be 5 or less
R1   =   Q  /  G x 2 x π
xx C
R2   =   Q  /  (2
Q - G) x 2 x π xx C
R3   =   2 Q  /  2 x π xx C

       • First choose a capacitor value, 500 pF - 0.2 F
       • Choose a frequency close to the "sidetone" of your transceiver. New rigs allow you to set the frequency.
           Applications such as CWget are useful in detecting your side tone's frequency using your PC's sound card.
       • Calculate and choose resistors close to those values. Use series or parallel values where necessary.
       • Start off with one stage on the breadboard first.
       • Use a variable potentiometer for R2 with a value of at least twice the required resistance or more to allow for tweaking.
       • Measure this value with your ohm meter to get fixed values. Use trim-pots for final construction if desired.

On The Breadboard

Single stage set up on breadboard

First one and then two stages were breadboarded and values adjusted with potentiometers.

       On the breadboard is where I delveloped my own version of a very common project.  By using potentiometers on different parts of the circuit, my initial goal was to make the frequency passband variable and not the bandwidth.  However, the nature of the op amp and the twin-tee RC net required that many values separately determine the center frequency.  I had to settle for a close match to the 732 hz sidetone that my current rig produced.  The passband would end up being 60-100 hz and would allow for deviation anyway.

The whole circuit in action with fixed resistors...

Closeup view of the final circuitry on the breadboard
Here is a good time to work out all the kinks.

The entire circuit was re-constructed on the breadboard with fixed-resistor values that match that of the potentiometers when adjusted.

Breadboard being tested with live audio feed
Project shown connected to audio source.

Testing with PC based audio anaylizer...

PC used as an audio spectrum analyzer
This setup allowed me to get the values right where I wanted them.
Though nothing beats the equipment between your ears!

       Testing revealed that the center frequency was finally at 750 hz, there was the -6 dB reduction knee at 718 hz and 782 hz and dropped off very rapidly after that.  There was 40 dB to 60 dB attenuation by 685 hz and 815 hz (roughly) and a 3 dB gain on the center frequency.  It was found that the noise floor or overall noise of the stop bands (ouside range) had a potent effect of desensitizing the filter. Sronger signals caused distortion, so starting with a lower output AF volume setting is suggested.

_spectrum.jpg (119108 bytes)
This is a screen capture of the analyzer.  The shaded area represents the receiver's AF passband, the Red line is sampled from the original audio signal and the Blue line is sampled from the filter output.  The transceiver's sidetone was used as the test tone, and the noise level was such that it very well buries the tone in the original audio.

       Later, another test [on 05/17/08] was done using a place on the 40 m CW band. Click here to listen to an audio sample...
The Kenwood TS-130SE was placed in the 3 kHz wide SSB mode to allow a lot of noise and adjacent signals in.  In the sample you should be able to barely hear the CW letter "D" (-..) in the audio, this signal was pre-tuned to match the sidetone of the rig at roughly 732 hz.  The filter set at 750 Hz would have no problem with the sidetone.  You will first hear the bypassed audio for a few seconds, then with the filter in four-stage operation, then in two-stage to allow more audio in, then back to bypass mode.

The screen capture (below) was derived from parts of the audio sample analyzed in SoundForge.

spectrum2.jpg (235655 bytes)
This is a composite of SoundForge's spectrum analyzer's screen capture and is a more comprehensive analysis of the accumulated audio component amplitudes.  The Red line is sampled from audio that was directly from the receiver in Bypass mode on the filter and the Black line shows the filter in action with all four stages.

       This scan more reflects the more extreme conditions of the audio sample; heavy QRM/QRN and a high noise floor . The level of attenuation is impressive!  There are two curious anomolies that I would like to resolve. 1) The extremely loud "Ker-Chunk" that occurs when the circuit is powered. There's a large capacitor accross the audio output but there must be a way to shunt or initially quell this artifact as it is unpleasant to hear when constantly bringing the filter in and out. A build-up circuit using a 2N7000 MOSFET and and timing capacitor and resistor could initially sink audio to groung for a few ms during power up.  2) There seems to be some type of harmonic filter response that is seen in a broader spectrum, a byproduct of the simple RC configuration.  As with any harmonic, the amplitude decreases as the frequency increases, though this is a harmonic based on band passing and not a present tone and occurs at evey 5 kHz interval, but also at 2.5 kHz & 1.7 kHz.  However this seems to be at and acceptable level as there is no audible evidence perceived.  An additional low-pass filter element could be dded.

Construction of the circuit board...

Kitchen table commondeared as a work bench.
My ex-wife was never too happy when it was time to enlist the kitchen table into serving as a lab bench...

       Every builder must determine what kind of construction and housing methods are best for them and the use of the project.  Good construction techniques learned from "Elmer's" and from the manuals are a must to taking pride in your work.  Proper care must be taken when working around larger 12 v power supplies and RF shielding implemented around the final project.

Finished perf-board project

New circuit board attached to breadboard for testing.

       It's a good idea to run a "smoke test" and test the circuit once soldered to find any mistakes and maybe even adjust component values, as tolerances and values can change during installation, especially in a circuit where there's a bit of interplay between stages.  Shortening lead lengths and placing components closer together can augment stray capacitance values.  The bottom of my prototype board feels like it has about a 1/4 lb of lead!  Note that smaller capacitors can be used along with 1/4 watt resistors.  Amazon and eBay sell massive 600+ piece capacitor sets for both electrolytic and ceramic for as little as $10.  They're not the best by far but can be used in non-timing critical places.

Show here re-connected to rig

Testing back on rig shows that the center frequency moved up 10 hz.
This is to be expected and allows you to go back and augment parts.


Switch work detail - outside  
Switch work detail - inside
Holes drilled for switches and jacks... then the circuit board was wired to the switches.

       I chose a small Altoids mint tin for the enclosure... of course I did, but I really didn't know going into this project, how much space I needed when the project was started.  I purchased the switches at the end for being small and durable and I had plans to use an exterior jack for power with Powerpole connectors but discovered that a 9 v would fit neatly at the end.  The current drain is very low, so a dry-cell works quite a long time.  Ensure that if the container you are using has conductive properties, either made of metal of anti-static plastic (I bet you didn't know that) then use an insulator or a few strip of electrical tape to keep the circuit joints and wire from shorting out.  The outside of this container had some kind of clear paint which made it mostly non-coductive.

All element put together

Everything fit neatly... This was by accident I assure you!

Ready for action

The lid actually closed! Providing an RF shielded compartment.

Works great - job well done!

Labeled and attached for work to the transceiver.

       Now it's ready to use, and work it did!  I was amazed to be able to both zero-beat onto another caller quickly and be able to isolate just who I wanted all while cutting out all kinds of noise, even the extreme noise produced by my computer equipment.

Good Luck on your own projects! 73, De Mike Maynard K4ICY
SKCC #8600

Updated 06/02/20

(c)2020 Copyright - Michael A. Maynard, a.k.a. K4ICY