Chances are it is actually both. If it is designed like most
all of the other narrow-ish roofing filter mixer scheme
radios then some of the AGC is inside the DSP. And the
remainder of the overall AGC range is analog and ahead of the
ADC. Of the designs like that I have studied closely the
first 60-90 dB or so of AGC action is usually done in DSP.
But when the signals get really strong then there is a level
where HW based AGC starts reducing gain to protect the ADC
from over ranging.
The only radios that I'm aware of that have the AGC wholly
contained whithin the DSP is the FlexRadio SDR-1000 and
Flex-5000's. And as far as I know all of the various direct
sampling RX's like the SDR-IQ, SDR-14, HPSDR Mercury, QS1R,
Perseus, plus some others that I can't think of at the moment
are too.
Duane
N9DG
--- John Geiger <n5ten@yahoo.com> wrote:
> I have read both the QST and RSGB product reviews, as
> I have been thinking about buying this radio.
> However, it gets confusing as the reviews seem to be
> in direct conflict with each other in one very
> important. In the QST review, when it discusses the
> DSP on the radio, if states:
>
> "The NOTCH, SHIFT, and WIDTH functions clearly operate
> within the AGC loop (it's possible to view their
> effects via the S meter)."
>
> However, in the RSGB review of the FT450 they state:
> "The AGC circuitry senses signal levels upsteam of the
> channel filter where the bandwidth is determined by
> the first IF roofing filter. AGC is applied to the
> signal frequency RF and first IF amplifier stages. As
> a consequence, strong signals up to 10khz off channel,
> outside the receiver passband but inside the roofing
> filter bandwidth, will result in desensitizitation."
>
> These 2 statements seem to conflict each other. The
> RSGB statement is depressing to read because, if
> correct, means that the design of the FT450 would make
> it useless in contests and crowded bands because the
> receiver would be constantly getting desensed by
> signals you couldn't even hear!
>
> Do you know which is correct? It seems confusing to
> have two reviews which conflict each other so much.
> What is your experience with the FT450 overall? Is it
> useful in a strong signal environment?
>
> 73s John AA5JG
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