A(nother) CW Filter

I have held this subject under a self-imposed “D-Notice” for the past few months as George, g3rjv, kindly agreed to publish it in our beloved SPRAT. Today, the Spring 2011 number hit my doormat, with my article on page 20, so I feel free to comment...

There have been any number of attempts to limit the output bandwidth of a CW receiver “correctly” (i.e. early in the signal chain) or as an “afterthought” (i.e. in the audio path). This idea belongs to the latter category. It is a tunable bandpass filter, which I designed with the intention of providing independent control of tuning and bandwidth. The “novelty” of the design – if it has any – is associated with the application of a “gryrator”, which is used to emulate an inductor, and with the manner by which the bandwidth control is achieved.

The “virtual” inductor is resonated with a capacitor to produce a resonant system, tuned to the desired frequency. The bandwidth is established by driving the parallel “LC” network with controllable source impedance; high impedance drive gives a very narrow bandwidth and vice-versa. The SPRAT article gives more detail.

The original development version was the usual pig’s breakfast of tangled, blue spaghetti, as seen in the photo...

Despite the mess, I was sufficiently encouraged by the performance of the filter to write it up for the journal. Here’s the measured frequency response – bold lines show the response with R4 at extreme settings and dashed/dotted lines at manually selected intermediate settings of the R4 potentiometer.


It worked – but it was hardly suitable for everyday use, so I cooked up a more practical version...

Here’s the schematic (identical to that in SPRAT, except for the addition of a “bypass” switch, S1)

here’s a PCB layout ...

and here's the copper side (shown in negative to make it visible against the black background of this blog)...

(any reader foolish enough to reproduce this PCB can scale and invert using image handling software)

The twin-gang potentiometer was chosen from Maplin’s catalog (usual disclaimer) to ensure availability (part number JM84F ) – no doubt an equivalent part is available from “other fine retailers”!

Here’s the finished article...

The tuning control, R6, is reduced to a trimmer. Once set (to 600 Hz, in my case) it is left alone.

The unit sits on the shelf next to my Funster Plus 40m rig, the usability of which it greatly enhances.


One day, I’ll get round to building it into the Funster’s case (assuming it will fit)!

Now for the bad news...

I explained in the SPRAT article how the prototype system makes an ugly noise if you turn it on with the control in “narrowest bandwidth” setting (momentarily backing off R4 stops the oscillation, after which the unit can be used without further problem). When I made the PCB version (above) I noticed some more “issues”...

• Don’t drop the power supply voltage to 9V – keep it at 12, or higher


• Don’t overdrive the unit with incoming audio – too much signal will cause the unit to clip and distort (as the gyrator saturates)


• One particular setting of R4 on the current PCB version (close to – but not at – the “widest bandwidth” setting) can also evoke a brief “buzz”/”fart” sound

Despite these practical challenges, the unit works very well in a QSO. So well that I’m not inclined to sort out the remaining bugs and issues – life is short and there are so many interesting things to do!

...-.- de m0xpd