The term 'blowby, as I understand it, refers to an effect where
some of the undesired signal will leak around the main filter.
The issues here are the ultimate rejection level of a filter
and the mounting method used for the filter. If a good filter
is
improperly mounted, some stray capacitance will exist between the
input and output and allow a small amount of signal to get around it.
This is why you see some radios have the main filter on a seperate board.
We are talking a situation where you need to maintain 90-100db of
isolation with just one filter. With two filters, the job gets easier.
Dave
WA0ZZG
On Thu, 14 Feb 2002, Nate Bargmann wrote:
> * Al Fansome <alfansome69@yahoo.com> [2002 Feb 14 20:06 -0600]:
> > I spoke to Inrad when I was considering filter mods in
> > addition to those done by the previous owner. Inrad
> > said there's an inherent blow-by flaw in the 920 that
> > their filters wouldn't eliminate.
> >
> > Being a CW newbie, exactly what is blow-by and what
> > would it sound like using a 500 Hz filter?
>
> It's been about a year since I last used my '920 (still in the box from
> moving...gahh!), however I recall that it manifests itself as stray
> signals clearly beyond the filter passband. You will also notice it as
> hearing the signal "on the other side" of zero beat. I found that
> simply switching in the DSP even at its widest bandwidth eliminated the
> blowby. INRAD is correct, the 400 Hz filter I bought from them did not
> eliminate the blowby, but it sure allows the DSP to do its thing.
>
> The first time I encountered the blowby I was quite disappointed. But I
> think Yaesu opted to keep the radio in a lower price range and clean up
> the effect with the DSP. With this in mind, I've come to enjoy the
> capabilities of the '920.
>
> 73, de Nate >>
>
>
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