[Elecraft] start without ANY extra roofing filters....

[Jan Erik Holm]

This I have noticed during my own IP3 measurements on my
FT-1000D, not as much as 16dB but around 3 dB.
This was with the INRAD roofing filter and 2 kHz offset.
in any case this was enough for me not to use the filter.

I´m eagerly waiting for measurement figures on the K3.
Beats me why they can´t be presented, that I don´t understand
at all, after all it´s no rocket sience. This will also help
people select the roofing filter.
Maybe I just have to by one and measure myself, then atleast
I know it´s done right.

/SM2EKM
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Bill Tippett wrote:
> 
> 
> I wrote:
> 
>  >          Bottom line:
> 
> 1.  Narrower is not always better (Ten-Tec experience)
> 2.  8-poles is not always better than 5-poles (per Inrad)
> 3.  Let IMD and BDR measurements be your guide
> 
>         More evidence below to support waiting for IMD/BDR measurements 
> before ordering any roofing filters.
> 
>                                 73,  Bill  W4ZV
> 
> http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html
> 
> There is a fascinating article describing IMD tests on the IC-7800 by DC4KU
> in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset
> degrades by an astounding 16 dB when switching from the 15 kHz to the 6 kHz
> roofing filter. This degradation is due to passive IMD in the filter, and
> possibly also to IMD in the filter driver amplifiers caused by mismatch 
> when
> the filter is excited outside its passband. I can send you an
> English-language summary of the relevant part privately, if you wish.
> 
> It is highly significant that professional receivers manufactured by the
> likes of R&S, Rockwell-Collins, Racal and Harris have a single roofing
> filter. This filter is typically 12 to 16 kHz wide, to pass multi-channel
> ISB, VFT (multiplexed teletype) and high-speed crypto, all of which have
> extremely stringent in-band IMD requirements. To quote a British engineer
> who used to design shipboard HF receivers for the Royal Navy:
> 
> The up-converting architecture, with a roofing filter at a first IF above
> the highest RF frequency, allows the designer to limit the bandwidth
> presented to the first IF chain and second mixer. The bandwidth of this
> filter is a trade-off. Its 3 dB BW must be sufficient to pass the widest
> emission the receiver is required to handle, but not so narrow that IMD and
> temperature-drift effects in the filter become a concern.
> 
> Cheers for now, 73,
> Adam VA7OJ/AB4OJ
>