Pixie Sixite (Part II): The Passive Audio Filter

Today I want to discuss an imperative add-on for the Pixie Sixtie I'm building. This should also apply to any other Pixie kit or similar QRP kit with poor front-end filtering and a wide passband.

Filtering

One of the biggest caveats of a Pixie kit is its wide receiving passband. Most hams claim that it is almost 3KHz wide or even wider! As a CW transceiver, this is more than unacceptable. While it's not impossible to make a QSO using a stock Pixie, I can tell you that it will be miserable. In essence, this means that several CW conversations will overlap one another as each and every signal you hear will try to drown one another out. Even using the on-board pot won't make a difference. I have a Pixie on 7.050MHz and every time I power it up, I'm greeted with digital noises that are several kilohertz above or below the specified frequency.

So how can we fix this problem? Many would say that it's impossible, and even the most rudimentary methods would be futile in trying to solve the overloading problem. However, I beg to differ.

Lots of hams toss aside the Pixie due to its wide passband on receive. There's absolutely no way to fix it, so why bother? It's a less than $10 kit, so its loss can be written off easily. However, with just 2 components, you can make a simple low pass filter that will filter out other CW stations and digital chatter but keep the CW station you are currently listening to. And all it involves is a resistor and capacitor. This configuration is called an RC filter.

Passive Audio Filter

To figure out the exact components we need, we can use the following equation:

1/(2*pi*R*C)

Where R is the value of resistance and C is the value of capacitance.

Just 2 components are all you need for a basic LPF, but you can add more stages on if you want a sharper filter cutoff. For an adjustable audio filter, consider replacing the fixed resistor with a potentiometer. 

For our purposes, we need a filter with between a 400 to 800Hz cutoff. Note that this won't get rid of all signals but it will cut down on the signals who are outside of 800Hz, namely digital and SSB signals. CW signals have pitches between 400 and 800Hz, so these signals will be allowed to pass while other, higher-pitched signals will be heavily attenuated to the point where they should be muted.

A common place to start would be a 22kOhm resistor and a 10nF capacitor. This will give a frequency cutoff of around 700Hz - not bad! Place the resistor in series with the audio output line and place the capacitor on the output side of the resistor to ground. It's just that easy. 

Of course, more poles (more RC pairs) will attenuate signals better, but if you just need a quick fix, an RC filter is the way to go to filter out a wide passband direct conversion receiver.

Stay tuned for more add-ons as I improve and build on the Pixie Sixtie!