I don't understand why the IMD figures worsen as the roofing filter narrows. Also I've always been led to understand that the signal spacing likely to cause IMD problems is roughly 0.25 of the roofing filter bandwidth assuming a narrow final passband. On this basis, and even allowing for responses down to the -60dB level (say 5kHz for the 2.7K filter...) none of the quoted signal spacings should be 'getting through' any of the roofing filters listed. So at these spacings the IMD response should be a function only of the pre first mixer circuitry which doesn't explain (to me!) the significant variations with roofing filter.
I'm sure I'm missing something here, perhaps about how these things are actually measured; anyone care to put me right....? Stewart Rolfe, GW0ETF (K3/100 #145) Bill W4ZV wrote: > > > > Jan Erik Holm wrote: >> >> Yes something isn´t right. This was his measurements: >> >> CLOSE-IN INTERMODULATION ON 7MHz band,500Hz bandwidth, CW preamp off >> >> 2.7 kHz roofing 1.8 kHz roofing 500 Hz roofing >> 2kHz +19dBm 101dB +12.5dBm 96dB +2.5dBm 88dB >> 3kHz +19dBm 101dB +12.5dBm 96dB +2.5dBm 88dB >> 5kHz +22dBm 103dB +12.5dBm 96dB +8.5dBm 92dB >> 7kHz +22dBm 103dB +12.5dBm 96dB +13dBm 95dB >> 10kHz +22dBm 103dB +12.5dBm 96dB +17.5dBm 98dB >> 15kHz +22dBm 103dB +12.5dBm 96dB +22dBm 101dB >> 20kHz +22dBm 103dB +12.5dBm 96dB +25dBm 103dB >> 30kHz +22dBm 103dB +12.5dBm 96dB +26.5dBm 104dB >> 40kHz +22dBm 103dB +12.5dBm 96dB +27dBm 104dB >> 50kHz +22dBm 103dB +12.5dBm 96dB +27dBm 104dB >> > > Yes, and also the 8-pole 1.8k should be better than the 5-pole 2.7k. Some > very strange results indeed but I'm sure Peter is reporting what he's > measuring. I cannot find his original Orion review but I recall it showed > the 2.4k 4-pole and 1.8k 8-pole to be much better than the 1.0k 4-pole at > 2 kHz spacings, which again did not make sense unless the 1k filter was > defective. > > 73, Bill > > -- View this message in context: http://www.nabble.com/RSGB-RadCom-K3-review-posted-on-RSGB-Members-web-page-tp17869976p17981961.html Sent from the Elecraft mailing list archive at Nabble.com. _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
