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A number of multi-pole high-performance SSB and CW crystal ladder filters were described in TT, July 1994, pp56-67 and January 1994, pp38-39. These used the Chebyshev approach optimised to provide the best stopband attenuation (steepest skirt response), together with the design coefficients needed to determine the value of the coupling capacitors. There are, however, other possible approaches. These include Butterworth bandpass filters designed to provide optimum flatness at the centre frequency, orthe compromise Cohn ('minimum-loss') filter which is optimised to exhibit minimum insertion less when built with practical resonators, while preserving a reasonably good shape factor. Cohn filters, with LC tuned circuits, have been used in the front-ends of a number of receiver projects, Including the G3PDM high-performance, hybrid (valve/semiconductor) receiver of the 1970s.

Some years agog Wes Hayward, W7ZO1, in 'Designing and Building Simple Crystal Filters' (QST, July 1987, pp2429), showed how simple and inexpensive crystal filters that perform well at a fraction of the cost of commercial crystal lattice filters could be implemented using the Cohn approach, without involving capacitor coefficients. All capacitors are of equal value and all crystals have the same resonant frequency.

Although for purists this approach may seem a compromise, one notes that Cohn ladder crystal filters now figure frequently in home-brew receiver projects. The Cohn filters are more symmetrical, making them very attractive for narrow-band CW filters but not ruling out their use as SSB filters. As W7ZOI put it: 'The Cohn filter, crystal or otherwise, is a rather simple circuit. This becomes more apparent when we view the filter using coupled-resonator methods. All normalised coupling coefficients are equal. Moreover, the normalised end-section loaded-Q factor is the reciprocal of the coupling coefficient.

The practical simplification becomes apparent if we examine the generalised crystal filter shown in Fig 1. All capacitors are of equal value. The shunt capacitors are coupling elements while the series capacitors in the filter end sections are included to properly tune the circuit.

Fig 1
Fig 1: Generalised Cohn crystal ladder filter suitable for empirical construction. Note all capacitors are of equal value. All crystals have the same resonant frequency.

"The 1987 QST article included practical designs based on three (Fig 2),'four and six (Fig 3) crystals implemented either by W7ZOI Bruce Williams, WA6IVC ARRL. They used low-cost American NTSC colour-burst 3.579MHz crystals or rather more expensive 4.000Mhz crystals. Terminating impedances for practical CW filters at these frequencies may be as low as 50Ω and 200 - to 500Ω for SSB filters. Fig 4 shows one way in which a filter can be incorporated in a receiver so that terminations can be arranged to achieve the proper filter shapes."

Fig 2
Fig 2. (a) Simple Cohn CW filter using three 3.579 NTSC colour-burst crystals. (b) Frequency response. Dotted input return loss indicates the quality of the impedance match.

Fig 3
Fig 3: (a) Cohn filter with six 3.579MHz crystals. (b) Frequency response. The reference sweep shows the response of the three-crystal filter and this indicates the Improved characteristics at the expense of some extra insertion loss.

Fig 4
Fig 4 Showing one method of connecting a ladde filter, with correct terminations, in a receiver.

Michael O'Beirne, G8MOB notes early professional use of a two-crystal (Chebyshev) ladder filter in the Racal RA1772 (a 1972 design) receiver. A 1.4MHz filter is placed at the input to the 1.4MHz amplifier which feeds the product detectors, and used to select the 7th harmonic from the 200kHz square wave spectrum.