[INRAD] Roofing filter and IMD?

[Joe Subich, W4TV]

I suspect the amplifier gain was relatively high - if I recall, 
the amp provides 3 or 4 dB more gain that the filter insertion 
loss.  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).   

The whole point ... a 2 KHz IM DDR test is testing the IF after 
the filter - at least one if not both tones are inside the 
filter passband.  If you increase the tone level, you will 
reach the IMD point faster - just as if you had increased the 
level from the signal generators. 

73, 

   ... Joe, W4TV 
    

> -----Original Message-----
> From: list-bounces at inrad.net [mailto:list-bounces at inrad.net] 
> On Behalf Of Floyd Sense
> Sent: Tuesday, July 17, 2007 5:54 PM
> To: list at inrad.net
> Subject: [INRAD] Roofing filter and IMD?
> 
> 
> I was looking at a receiver test matrix at w8ji.com today and 
> noted that 
> the FT1000MP MK V with Inrad roofing filter tested 13 dB 
> worse for 2 KHz 
> IMDR than the same transceiver WITHOUT the roofing filter.   
> Anyone know 
> why that is?  Here's the URL of the subject chart: 
> http://www.w8ji.com/receiver%20IM3%20sorted.htm
> 
> Is there a problem in the design of the transistor amp used 
> in the filter?
> 
> 73, Floyd - K8AC
> 
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