[ICOM] New Inrad Roofing Filter in 756Pro3

[Adam Farson]

Hi Carl,

AGC pumping (swamping) will not occur on the Pro3, as the AGC derivation point is a process in the DSP algorithm, after the IF filter process. This means that signals outside the DSP IF passband will not develop AGC voltage. 

http://www.ab4oj.com/icom/ic7700/agc.html

The stopband attenuation of the DSP IF filter is close to the theoretically infinite value, and the ADC will over-range long before an out-of-band signal can get through that filter.

Where the narrower roofing filter may help is in reducing the statistical likelihood that strong undesired signals will generate IMD products in the second mixer. The limiting case is where IMD generated in the filter, or in its insertion-loss make-up amplifier, becomes significant.



-----Original Message-----
From: icom-bounces at mailman.qth.net
[mailto:icom-bounces at mailman.qth.net]On Behalf Of Carl Vangsness
Sent: February 16, 2008 18:08
To: ICOM Reflector
Subject: Re: [ICOM] New Inrad Roofing Filter in 756Pro3


Back when I had a FT1000MkV (which got traded in for a 775DSP!) before I 
installed the 4 KHz roofing filter I would notice strong AGC pumping 
from strong signals 10 KHz away or more, SW broadcast stations on 40 
meters extremely wide signals, almost made the nightly OMISS 40M ssb net 
unuseable. After the roofing filter was installed, NO more AGC pumping 
on CW at all, and almost embarassingly no interference from SW stations 
while the rest of the net was complaining. Check out the CW portions 
during any contest and you will be very happy with what you don't hear 
any more.

73, Carl WC0V

PS the 775 was the best traditional rig I ever owned. Made the FT1KMkV 
look sad with or without the roofing filter.


Ekki Plicht (DF4OR) wrote:

>Hello all,
>
>today I installed the Inrad roofing filter in my 756Pro3, with the help of a 
>friend (Stefan, DL2OCB) who has good measuring equipment and good eyes, hi.
>
>The procedure went well, it took about 1.5 hrs. It's not really that difficult 
>if you can work with SMD components, but if you can't it's impossible :-) The 
>installation requires the removal of one tiny SMD resistor and then soldering 
>two traditional components to the solder pads where the resistor used to be. 
>That's something I can't do anymore, but Stefan could.
>
>The rest is not that difficult and I did that myself (removing the RF-A board, 
>removing a shielding case, drilling holes in that shield, installing the 
>small coaxial cables etc.).
>
>Soldering was a little bit strange at first due to the new 'RoHS' leadfree 
>solder. You need much higher temperatures to get a good flow, we used two 
>soldering irons to heat the shielding case.
>
>Before the installation we measured the required input level to show an 
>arbitrarily selected s-meter value on three bands (80, 20 and 10m). The same 
>measurement was repeated after installation. We saw that the S-meter now 
>shows about half an S-level more than before, at same input levels. So the 
>amplifier on the filter board very slightly increases sensitivity, but 
>without discernible increase in noise level.
>
>
>So what is the effect now?
>
>As expected, it's hard to tell. The built-in roofing filter has a bandwidth of 
>15kHz, the new one 5kHz. To compare between the two roofing filters I 
>set 'Dual Watch' to on, on the same frequency (no split). With this setting I 
>can use the balance control to fade between both receivers, Main (balance 
>full counter-clockwise, new 5kHz filter) and Sub (balance full clockwise, old 
>15kHz filter).
>
>To see if it has any effect at all I first listened to an AM broadcast 
>station, with 9kHz IF filter. There was a distinct difference between 
>Main/Sub, now I can be really sure that the filter works at all :-)
>
>It's much harder to hear a difference in SSB or CW. There is a CW contest 
>going on right now, the 40m band is fairly crowded with strong signals. Do I 
>hear a difference? Well, a definitive maybe. 
>I have the impression as if the Main receiver is slightly quieter, slightly 
>less mush at the bottom, slightly clearer signals than on the Sub receiver at 
>same frequency. 
>But this is only a very subjective impression after only one hour of testing, 
>and certainly influenced by wishful thinking after spending US$200 on the new 
>filter. Only time will tell, like in the next contest. 
>I think this modification will show it's effect in the rare 3% of a DXers 
>life, when really the last dB in IMD and BDR counts. For the remaining 97% of 
>my QSOs I probably could do without the mod. Still I love to have it 
>installed.
>
>Due to lack of equipment we did not do a proper IMD or BDR measurement 
>(calculation), both figures should improve significantly with a narrower 
>roofing filter, Inrad claims up to 15dB better IMD. I will try to round up the 
>needed equipment (and expertise) somewhen in the future.
>
>73,
>Ekki, DF4OR

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