Scaf Variable Low Pass Filter & Headphone Drivers; Figure 134. Scaf Low Pass Filter And Headphone Amplifier - NORCAL NC2030 Manual

harmonic distortion that the clipped waveform produces, making even an amplitude clipped waveform
sound more natural.
The main function of the diode clipper is to limit the maximum signal to the ears when using
headphones. 400 mV pk-pk is a loud signal, but is much better than listening to a solid 3v pk-pk signal
(17 db louder – very hard on the ears) that might be heard if the limiter were not in place. When
distortion due to diode clipping occurs, simply reduce the volume control until the distortion disappears
and the volume is reduced back down to a comfortable listening level.
SCAF Variable Low Pass Filter & Headphone Drivers

Figure 134. SCAF Low Pass Filter and Headphone Amplifier

This SCAF filter is a fifth order Elliptic low pass filter. This filter has an even steeper frequency roll off
th
response than the main filter 9 order Butterworth, but has a more limited stop band response.
However, SCAF filters tend to be fairly noisy and thus cannot be placed early in the gain chain. This
filter works best at the very end of the gain chain, where it introduces virtually no noise.
The cutoff frequency of the SCAF filter is adjustable from approximately 900 Hz down to 300Hz by
changing the voltage across the varactor diode, changing the low pass filter frequency of the filter. The
cutoff frequency of the filter is normally set to a frequency higher than that of the main 800 Hz 9 pole
Butterworth low pass filter. Here it serves to enhance the stop band of the receiver, without introducing
ringing. In addition, for very weak signals, the SCAF filter can be lowered in frequency and thus
narrow up the receive bandwidth.
Norcal NC2030 v5 12-18-05
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