[INRAD] Roofing filter and IMD?

[Joe Subich, W4TV]

Thanks, Randy. 

> 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. 

That's it ...  3:1 comes from the 2F1-F2 coefficients of 
the IMD expression (which was what I was searching for but 
could not remember clearly).

73, 

   ... Joe, W4TV 
   


> -----Original Message-----
> From: Randy Farmer [mailto:w8fn at tx.rr.com] 
> Sent: Saturday, July 21, 2007 4:48 PM
> To: Joe Subich, W4TV; 'Floyd Sense'; list at inrad.net
> 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 
> 
> 
>