[Elecraft] K-3 Roofing filter comment

[Geoffrey Mackenzie-Kennedy]

Jack,

As Bill W4ZV confirmed one of the AGC loops uses a sampling point which is 
after the roofing filters but before the DSP and its narrow filter(s).

If the unwanted signals are greatly attenuated by the DSP's filter, but they 
are either within the roofing filter's passband or in the roofing filter's 
skirt/ near stopband regions, the level of their third order products could 
still be of concern.  Depending on the frequencies of the fundamentals some 
products could get through the DSP's filter.  Their level will of course be 
determined by their spacing, and the design of the front end and its gain, 
the design of the roofing filters and the quality of the crystals used in 
the filters, to name just some of the areas where "Pre DSP IMD" is 
generated.

73,

Geoff
GM4ESD


On July 15, 2011 at 05:43 Z, Jack WA9FVP wrote:



> Actually the AGC is generated by the DSP in software.  The signal VIFGAIN1
> coming from a D to A converter (AD5302A) if fed back to the IF amplifier 
> to
> control the receiver's gain.  After the DSP filter reduces the bandwidth,
> the AGC is adjusted by the DSP.  It's all done using mathematical
> calculations.
>
> You can have a situation where the DSP filter is narrower then the roofing
> filter but if the DSP engineer does his job, the unwanted signal will be
> outside the DSP filter bandwidth.  The third order products will be at a
> minimum and the AGC will not be affected.
>
> -----
> Jack WA9FVP
> Willco Electronics