Passive LC CW audio filter

Author: Wolfgang "Wolf" Büscher, DL4YHF.
Last modified: January 2014


Not much to say about this filter.. it's a passive L/C filter for aural CW reception, built from junk box parts as an 'ugly weekender build'.
It helped a lot on MW in the presence of QRM, or when very weak signals had to be copied.
The bandwidth is deliberately low (only about 20 to 50 Hz) so if not really necessary, the filter can / should be bypassed with a simple switch. The bypass has a similar attenuation as the filter itself (which is quite high), so that when switching it in or out of the headphone cable, there's no need to adjust the volume level of the shortwave (or, in this case, medium wave) receiver.

A 16-position rotary 'hex' switch (usually labelled from 0..9..A..F, thus 'hex') is used to select the center frequency of the main filter element, which is just a high-Q series tank consisting of a large 740 mH ferrite pot core ('N28' material, made by Siemens und Halske so it's really old).
Each of the rotary switches 4 'binary' outputs adds 2, 4, 8 or 16 nF to the 68 nF capacitor, which (in this case) covered the author's preferred CW audio frequency range (somewhere between 600 and 700 Hz), and it allows tweaking the filter's center frequency while listening to really weak signals.

Schematics and prototype of the passive CW filter
Schematics and prototype of the passive CW audio filter.
The 16-position rotary 'hex' switch is hidden
under the 'potentiometer' knob at the upper right corner.
Click on the image for full resolution.

The second resonant element (LC tank with L1 + C1, tapped at a low impedance point) is connected parallel to the headphone output. It helped to attenuate signals further away from the passband. Since its Q is low, the bandwidth is over one hundred Hz, so there's no need to make the center frequency of this part adjustable.
C1 was selected to resonate L1+C1 around 650 Hz, which is the 'center' frequency, and the default position of the 'hex' switch. In this case, using a smaller pot with 48 mH (+ 24 Ohms DC) from the junkbox:

  fc = 1 / ( 2 * pi * sqrt( 48 mH * 1250 nF ) ) = 650 Hz .

The tuning range of the switchable bandfilter (LC series with L = approx. 740 mH and C = 68 to 98 nF ) is:

  fc_max = 1 / ( 2 * pi * sqrt( 740 mH * 68 nF ) ) = 709 Hz [with hex switch in position '0')
  fc_min = 1 / ( 2 * pi * sqrt( 740 mH * (68+2+4+8+16) nF ) ) = 591 Hz [with hex switch at 'F', i.e. 15)

Your mileage may vary, also depending on the parts in your junk box. The inductor (especially the pot core for the series resonant part) must have a large 'Q' factor. In this case, the coil's DC resistance was about 40 Ohms (actually multiple windings with the same number of turns were connected in parallel), giving an unloaded Q at the operating frequency of approximately

  Q = (2 * pi * f * L) / R_loss = ( 2 * pi * 650 Hz * 740 mH ) / 40 Ohm = 75.5 .
(ignoring the fact that the loss at 650 Hz will be larger than at 'DC').

The loaded Q is limited by the impedance of the receiver's loudspeaker output (which is a few Ohms), the impedance of the headphones (in this case, about 16 Ohms each), and the loss resistance of the coil (here about 40 Ohms). If, for some reason, the loaded Q is too large and the filter is 'ringing' excessively, reduce Q of the series resonator by placing a resistor with a few ten ohms in series, or a resistor with a few ten kilo-ohms in parallel.

Btw, building a similar filter with readily avaliable fixed-value inductors didn't work, because those inductors had a too low 'Q' / too large DC resistance (for example, 70 Ohms for a 100 mH coil results in a maximum(!) possible unloaded Q of about SIX, which is way too low for this application).
With the '740 mH' coil from the junkbox, the frequency response was acceptable, and the pulse response was 'steep enough' to copy faint CW signals (shown further below: aeronautical beacon 'SA' on medium wave).

Frequency response

.. measured by feeding the filter with white noise, using an audio spectrum analyser:

frequency response of the passive audio filter
Frequency response of the passive CW audio filter.

Audio step response (with on-off keyed signal)

.. tested with a weak Morse code signal, using an oscilloscope connected to the headphone output:

amplitude response for a moderately slow CW signal
Amplitude response for a moderately slow CW signal (beacon 'SA' on medium wave)

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