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R/C Audio Filter – Low pass
Figure 132. Main brick wall filter. 800 Hz active R/C low pass filter.
Other DC phasing receivers have used L/C audio filters as the main receiver bandpass. While these
inductors are shielded, they will pick up stray magnetic fields such as AC hum from any nearby power
transformer. R/C filters do not have this problem, and have the additional advantage of providing gain.
The first three sections of the filter have been set for a voltage gain of 2x (6 db) of gain, for a total of 8x
(18 db) for the stage. The first stage of the audio filter relies on the net output resistance 1K (two 2K
resistors in parallel) of the phasing strip.
A Butterworth low pass filter shape was selected in order to minimize ringing. A tradeoff was made
between the number of stages required to get the desired high end signal roll off rate and the resulting
audio ringing. Typical past receiver designs have emphasized sharp roll off with as few filter sections as
possible. This requires high Q audio sections which tend to cause ringing. This ringing produces an
audio tone that sounds "hollow". An alternative (employed by the NC2030) is to use more sections,
with each having a lower Q in order to get the same signal roll off rate. The longer 9 pole, low Q design
has much less ringing than the shorter five pole, high Q design. The nine pole Butterworth in the
NC2030 has all sections with a Q of three or less, while the alternative Chebychev five pole design
would need at least one section with a Q of nine.
The difference can be clearly heard, especially when listening to the band when it is noisy and in poor
condition. Intense band noise tends to be amplified by a ringing filter yielding a very harsh audio signal
that is hard to listen to for an extended period of time such as a contest or a fox hunt. The lack of ringing
makes the receiver much less fatiguing to listen to.
Norcal NC2030 v5
12-18-05
Page 114 of 128
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