Re: Filter width on DSP radios


Kevin Schanilec
 

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

--- In ultralightdx@yahoogroups.com, "gkinsman1" <gkinsman@...> wrote:

Hello all,

I noticed that on my PL-380, I can detune up to one filter width and still get good sound quality. For example, if I'm listening to 1070 kHz with the 2 kHz filter, I can tune as low as 1068 kHz or as high as 1072 kHz.

In the past, I've always used the rule that you can detune up to one half the filter width. This is how is works with my Satellit 800 and Icom R75 for example. With the 4 kHz filter selected on the Sat 800, I can detune by 2 kHz.

This leads me to believe that on the Tecsun DSP radios, the specified filter width is half as wide as it is on my other radios. For example, the 2 kHz DSP filter is equivalent to a 4 kHz filter on my other radios.

On most of my radios the filter width is specified using both sidebands; on the DSP radio it appears to be specified using just one sideband. I always thought the former was the standard way of measuring filters. Is this not the case?

Regards,
Gary

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