Details and Specifications
for the W3NQN High Performance CW Filter
Modern commercial receivers for amateur radio applications have featured CW filters
with digital signal processing (DSP) circuits. These DSP filters provide exceptional
audio selectivity with the added advantages of letting the user change the filter's
center frequency and bandwidth. Yet in spite of these improvements, many hams are
dissatisfied with DSP filters due to increased distortion of the CW signal and the
presence of a constant low-level wide-band noise at the audio output. One way to
avoid this distortion and noise is to switch to a selective passive filter that
generates no noise! Although the center frequency and bandwidth of the passive
filer is fixed and cannot be changed, this is not a serious problem once a center
frequency preferred by the user is chosen. The bandwidth can be made narrow enough
for good selectivity with no ringing that frequently occurs when the bandwidth is
too narrow. This passive CW filter project was designed, built, and refined over
many years by ARRL Technical Advisor Edward E. Wetherhold, W3NQN.
The effectiveness of an easy-to-build high-performance passive CW filter in providing
distortion-free and noise-free CW reception- when compared with several commercial
amateur receivers using DSP filtering-was experienced by Steve Root, K0SR.
He reported that when he replaced his DSP filter with the passive CW filter
that he assembled, he had the impression that the signals in the filter passband
were amplified. In reality, the noise floor appeared to drop one or two dB. When
attempting to hear low-level DX CW signals, Steve now prefers the passive CW filter
over DSP filters. Steve reports that he still uses his filter almost daily.
The CW filter assembled and used by K0SR is the passive five-resonator
CW filter that has been widely published in many Handbooks and magazines since 1980,
and most recently in Rich Arland's K7SZ QRP column in the May 2002 issue of QST.
If you want to build the high-performance passive five-resonator CW filter and
experience no-distortion and no-noise CW reception, this website will show you how.
This inductor-capacitor CW filter uses one stack of 85-mH inductors and two modified
separate inductors in a five-resonator circuit that is easy to assemble , gives high
performance and is low cost. Although these inductors have been referred to as "88-mH"
over the past 25 years, their actual value is closer to 85-mH.
The actual 3-dB bandwidth of the filters is between 250 and 270 Hz, depending on
the center frequency.
This bandwidth is narrow enough to give good selectivity, and
yet broad enough for easy tuning with no ringing. Five high-Q resonators provide
good skirt selectivity that is adequate for interference-free CW reception.
The image below shows a screen shot of digital audio processing software used
to analyze the W3NQN filter. The total
audio bandwidth from a receiver is shown as the wide lower section. The upper narrow
section results when the filter is switched in. Note the strong interfering
heterodynes indicated by the vertical lines.
Click on the image for a larger view.
Simple construction, low cost and good performance make this filter an ideal first
project for anyone interested in putting together a useful station accessory, provided
you operate CW of course!
The measured 30 dB and 3 dB bandwidths of the 700 Hz filter are about 567 and 271 Hz,
The 30/3 dB shape factor is 2.09. Use this factor to compare the selectivity
performance of this filter with others.
The measured insertion loss of these passive filters with transformers is slightly less than
3 dB and this is typical of filters of this type. This small loss is compensated by slightly
increasing the receiver audio gain.
The W3NQN CW filter is simply hooked up between your radio's external audio output jack,
and an external speaker or headphones of 8 to 16 ohms.
Standard commercial 8-ohm to 200-ohm audio transformers are used to match the filter input
and output to the 8-ohm audio output jack on your receiver - and to an 8-ohm headset or speaker.
Thousands of Amateur Radio Operators and Short Wave Listeners have constructed this five-resonator filter, and many have
commented on its ease of assembly, excellent performance and lack of hiss and ringing.
Information for this page from Ed Wetherhold and the ARRL Handbook
for Radio Communications. - 2004. Thanks to I2PHD and IK2CZL for Spectran.
3 dB Bandwidth - 271 Hz
30 dB Bandwidth - 571 Hz
2.9 dB insertion loss
30/3 dB Shape Factor - 2.09
546 Hz, 600 Hz, or 700 Hz Center Frequency
Passive - requires no external power.
Price $70, Postpaid to your door.( $90 outside US)
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