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The Home of W3NQN's CW Passive Audio Filter

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.
Computer Snapshot of Filter Bandwidth


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!

Performance

The measured 30 dB and 3 dB bandwidths of the 700 Hz filter are about 567 and 271 Hz, respectively.

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.

Operation

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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