Re: CW rise time setting
Posted by
k6dgw on
Jun 09, 2020; 6:45pm
URL: http://elecraft.85.s1.nabble.com/CW-rise-time-setting-tp7661876p7661945.html
Which sigmoid function did you model, Al?
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
On 6/9/2020 11:07 AM, Al Lorona wrote:
> I used to think that the rise and fall times of the CW pulse didn't really matter much to the sideband levels; I believed that it was more a function of the waveshaping, especially at the corners of the pulse.
>
> But I just ran a quick simulation of a pulse train going through both a raised cosine and then a sigmoid filter (because those two have been mentioned in this thread) and the rise time definitely does affect the pulse sidebands.
>
> The reduction in the sideband levels (what some folks here called 'clicks'... not sure that's a good name for this) varies depending on where you measure it, but in general the sidebands will drop anywhere from 0 to 12 dB-- sometimes less, sometimes more-- when you go from 2 msec to 8 msec rise/fall times. For instance, arbitrarily choosing an offset of 500 Hz from the carrier, the sideband drops by 11 dB for the longer rise time. That turns out to be a fairly typical value. And by the way, in general the sigmoid does a better job than a raised cosine.
>
> Arbitrarily defining the occupied bandwidth as the -60 dBc points of the spectrum, and using the sigmoid function with an exponent of -1, the bandwidth of the rise time = 8 msec pulse is 420 Hz versus 640 Hz for the pulse with 2 msec rise time. It's not an enormous difference, but it is something.
>
> Anyway, there's another data point for the discussion.
>
> R,
>
> Al W6LX
>
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