[Elecraft] CW/DATA modes: Why you'll want a narrow roofing filter

[Don Wilhelm]

AGC pumping is the problem I notice most with data modes - the roofing 
filter must keep out of passband signals at a sufficiently low level to 
allow copy of a weak desired signal.

Also my opinion on filters used for data modes is that they would 
ideally have good group delay characteristics, and to me that is more 
important than steep skirts or a flat passband. (steep skirted filters 
with very flat passbands usually exhibit poor group delay 
characteristics).  Low group delay also means more pleasant sounding CW.

Steep skirts are not really needed with a roofing filter because the DSP 
can create the desired skirt selectivity.  Again the roofing filter must 
only be narrow enough to keep strong offending signals out of the 1st IF 
passband (or sufficiently reduced to keep the AGC pumping problem at 
bay).  Perhaps a Gaussian to 6dB filter would act as a better roofing 
filter than a steep skirted filter - but I have not done any 
measurements - just thinking out loud.

73,
Don W3FPR

wayne burdick wrote:
> Let's suppose the only crystal roofing filter you have installed is 2.8 
> kHz wide. You put the rig into CW (or data) mode, then dial the DSP 
> bandwidth down to 400 Hz. Everything will be fine -- unless an extremely 
> strong signal appears that's *outside* the DSP's passband but still 
> *inside* the crystal filter passband.
> 
> This can cause what's called "pumping," meaning activation of the AGC or 
> desensing. Many transceivers with I.F. DSP don't have the ability to do 
> anything about this, so they get "trashed" on receive. Such transceivers 
> typically have only a 3 kHz (or wider) roofing filter, allowing strong 
> signals to cause trouble.
> 
> The K3 has roofing filter bandwidths as low as 200 Hz, giving you a 
> means to completely eliminating problems with nearby signals. If 
> installed, these will be automatically selected as you narrow the DSP's 
> bandwidth using the shift/width/hicut/locut controls. Or you can select 
> them manually.
> 
> We'll add this to the FAQ.
> 
> 73,
> Wayne
> N6KR
> 
> 
> 
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