Re: Filter width on DSP radios

Gary Kinsman

Thanks for your explanation, Kevin.

--- In, "dhsatyadhana" <dhsatyadhana@...> wrote:

Hi Gary:

You've got it spot-on: the DSP bandwidth is specified per sideband, not for the whole thing like most filters. So, the 1 khz bandwidth is actually a 2 khz (total) in normal parlance.

At the beach last time listening to split frequency targets, I did some comparisons between DSP filtering and the outstanding 2.4 khz Murata filter that has found its way into a lot of Eton e100's and CCrane SWP's. The Murata is tighter than the 1 khz (2 khz total) filter on the DSP radios, so kudos to Murata for still having the best filtering. For instance, listening to 738 under a domestic on 740, the Murata has less slop when the receiver is detuned by a khz to 737. So, the 1 khz DSP setting might be better compared to a 2.8 khz "regular" filter - still pretty darn good.

Since the 1 khz DSP filter does allow a little more signal through, one can detune by 2 khz (i.e., down to 736) on the Tecsuns sometimes, and get a better overall result than the Murata de-tuned by only a single khz. The limiting factor is whether or not there is still enough carrier frequency to modulate the signal: otherwise, the Donald Duck distorted audio results. This requires either a healthy signal for the desired target (i.e., a near-local), or the use of a regenerative loop like the Super SAT which when tuned to the carrier frequency supplies the carrier that the DSP filter is otherwise largely cutting off. Using a regen loop like this, tight trans-oceanic targets (like 729 under a 730 khz domestic) are much better IMO. The Murata filter is just too tight to allow this.

Thanks - Kevin

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