[INRAD] Roofing filter and IMD?

[Mike Schatzberg]

Hello Randy:

Actually, the new filter is in series with the old wide first IF filter, and
the new IF amp must make up for the full insertion loss of the new filter.

There indeed does appear to me to be a few db of net gain after
installation.  But you can lower the IF gain in the Mark V menu, and
equalize the gain to be approximately the same net gain as before.

This is why I think that perhaps Tom's values are as they are.  Perhaps he
didn't lower the gain after installation?  His numbers do not agree with
several other sources such as the RSGB and ARRL lab measurements.

Just my thoughts here.

73 and Happy DXing,

Mike
W2AJI

----- Original Message ----- 
From: "Randy Farmer" <w8fn at tx.rr.com>
To: "Joe Subich, W4TV" <w4tv at subich.com>; "'Floyd Sense'"
<fsense at earthlink.net>; <list at inrad.net>
Sent: Saturday, July 21, 2007 4:48 PM
Subject: Re: [INRAD] Roofing filter and IMD?


>
> >With 3 dB more signal from each tone, the IMD will be
> >proportionally larger (memory wants to say the sum of the
> >increase in each signal plus 3 dB but I'm not sure).
>
> Actually, for 3rd order IMD the rule is 3 dB for every 1 dB of tone
> power increase (a 3:1 slope on a log-log plot). Thus, 3 dB more power
> (per tone) into the nonlinear device would increase the IM power by 9
> dB. The roofing filter board amplifier should provide only enough
> gain to recover from the additional loss incurred from the filter
> substitution. That is, if the new roofing filter has 2 dB more loss
> than the original filter the amp should have 2 dB gain to bring the
> signal level up to the same as it would have been with the original
> filter. More gain at this point in the receiver lineup does more harm
> than good.
>
> 73...
> Randy, W8FN
>
>
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