29 June 2013

WB7AEI Audio Filter Bandwidth tests

Test Objective: Test the WB7AEI Audio Filter bandwidth with it integrated into my W1FB DC Receiver. The filter is inserted between the audio pre-amp and the LM386 audio amp.

I used the K2 transmitter set at approx 100mW transmit on the test frequency of 7020KHz.
I leaked some signal through the connection to a TEE piece of a piece of wire approx 12inches long. The RX was connected to a 51 ohm load in order to ensure proper matching of the input stage.

Test information and process.

  1. Headphone DC resistance = 12.3ohms
  2. Signal strength adjusted in the receiver for normal listening levels using terminated K2 as the signal source.
  3. Vcc= 10.9v
  4. Connected 1X probe to the speaker connection and used Channel 1 set at 50mV per division.
  5. Adjusted volume control for a peak to peak signal of 8 divisions while peaking the audio filter and input filter. Adjusted the receiver to give an approx 700Hz tone.
  6. Recorded the peak audio frequency at about 677Hz.
  7. Then tweeked the receiver trimmer for a peak to peak signal of 4 Divisions on the scope.
  8. Recorded the audio frequency at approx 637Hz.
  9. This yields a -6dB point bandwidth of 2X(677-637) = 80Hz.
  10. The design spec of the filter quotes a calculated 3dB bandwidth of 32Hz. This I concluded that the filter is working within spec.
  1. The receiver oscillator is not stable enough to obtain better more accurate readings.
  2. The signal coupling should be through an attenuator to better be able to stabilize the input signal level.
  3. More work is needed to improve the VFO stability for sure.
W1FB Receiver showing connection of 51OHM resistor to the antenna input

Rear of the K2 showing TEE piece and dummy load for signal leak

WB7AEI Tunable Audio Filter. BW at -6dB ~ 80Hz. ie in spec.


  1. Very Informative! This blog is great source of information which is very useful for me. Thank you very much for sharing this!network

  2. Hi Malu thanks for the comment and may you also continue to enjoy the wonders of electricity, radio and electronice. best regards and please keep in touch,,