Stephen, If you are doing mostly digital modes and not too much SSB, you might want to change the OP1 filter instead. The SSBCAPKT contains capacitors to make it either a 1.9k, 2.1k, 2.4k or 2.6k filter.
73, Don W3FPR On 7/17/2011 6:28 PM, Stephen Rector wrote: > I ve been using me K2 (s/n 6770) in the digital modes lately and have wanted > to make an improvement in the passband ripple, which is almost 10 dB p-p when > the BW is set at 2 KHz, with the RX audio viewed spectrally. So I simulated > the filter using the crystal parameters given here: > http://www.arrl.org/files/file/QEX_Next_Issue/Nov-Dec_2009/QEX_Nov-Dec_09_Feature.pdf > , using the spice simulator Qucs at http://qucs.sourceforge.net . The > simulation agrees with the measured passband when I set the termination > impedances between 200-300 ohms, with the ripple being more pronounced as the > impedance is lowered. > > I found a reference on the web to a modification of the Cohn filter which > preserves the equal capacitances and crystal parameters for all elements, > documented by G3UUR and described (page 5) here: > http://www.warc.org.uk/filestore/projects/xtal_bpf_203/eDishalHelp.pdf . I > haven't gone into the mathematics of this alteration, but the effect of > placing a crystal in parallel with X7 and X11 is a de-Q-ing of these two > elements and substantial reduction of ripple at the passband edges. This > effect is apparently not unknown to Wayne, who uses it in the BFO resonator > and considered it for increasing the pullability of the PLL reference. I > don't have the 2010 ARRL Handbook, but G3UUR has an article in it describing > his modification of the Cohn ladder response. > > So - the mod I tried in simulation was to place a crystal in parallel with X7 > and X11, and to replace the coupling varicaps D29 and D34 with 4700pF fixed > caps. The varicaps D30-33 are untouched. My simulation results: > > > K2 design: > With Cvc = 44pF: > > 6dB BW = 2.07 KHz, 60 dB BW = 4.55 KHz, ripple = 9 dB p-p, shape factor = 2.2. > > With Cvc = 200 pF: > > 6dB BW = 0.57 KHz, 60 dB BW = 1.52 KHz, ripple = 1.5 dB p-p, shape factor = > 2.67. > > Modified K2 design, G3UUR topology: > With Cvc = 56pF: > > 6dB BW = 2.03 KHz, 60 dB BW = 5.31 KHz, ripple = 3 dB p-p, shape factor = > 2.61. > > With Cvc = 205 pF: > > 6dB BW = 0.57 KHz, 60 dB BW = 1.96 KHz, ripple = 0.5 dB p-p, shape factor = > 3.50. > > So it looks like I get a substantial reduction in passband ripple and a > flatter group delay with the modification, at the cost of a bit of increase > in the shape factor. If I used the K2 strictly for CW or psk31, I might not > want to make a change. But I use wider digital modes like Olivia where group > delay distortion can make a difference in BER. So I think I'm going to try > the mod. Don has suggested in the past to narrow down the passband when using > the digital modes, but this makes enough of a difference above 1 KHz BW (and > I see surrounding signals in the waterfall), that this mod is worth a try. > I'm curious if anyone on the list has considered it. > > Steve NU7B > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
