9 Mhz filter bypass

[Blaine N1GTU]

I just finished up my softrock kit and got it connected to my Icom 746, it works faaabulous.
I was able to tap off the 455IF off one of the unused filter slots.
but this is what the 746 has...
9 mhz 15kc with 2 more open filter slots/optional
455khz 2.4 with one open filter slot/optional

in the radio menu i "added" an optional 2.8kc filter (nothing there just my IF tap) so when i select that filter it goes to the softrock.
question is...
the 9Mhz filter is only 15kc and i can see it limiting my receive signal, can I just jump a 10 ohm (or whatever value) across the empty filter slot to get the full spectrum going into the 455? if so what value should i use? can i use a straight wire?
I would assume a filter has a certain impedance/resistance.

[w3jn]

I'd try 600-1200 ohms.

[WA1GFZ]

I would make sure there is a series cap in the picture either add it or make sure there is one in the circuit so as not to mess up any diode switching.

[kz0e]

Blaine,

I asked Tony for a 455khz version, and then a 9MHZ version for my IC706 IF, and an 8.8MHZ for a TS430 I have.

He shipped the 455khz version right away, got the 8.8MHZ a week or two later (yesterday). So I'm thinking I may get one for the Icom 9MHZ IF here shortly. Something to consider, if he kits mine I'd imagine he'll have more. I'll let you know if and when it gets here, you could save the 455 unit for a boatanchor.

That Tony is a great guy.

Larry

[KA2QFX]

Hey Blaine,

[KA2QFX]

Hey Blaine,

Haven't heard you in a while. I expect you'll be at NEARFest?

Anyway, the filters used in these rigs today are pretty low impedance at center frequency, usually around 500 Ohms. But you don't need to concern yourself with that since the input and output circuits are designed to see the same I/O Z and will feed straight through just fine. Simply connecting them with a small cap (.1uF) will work.
If you really want to get fancy you could make a 500 ohm T or Pi pad with a dB or so attenuation. But it won't buy you anything. And as GFZ mentioned a coupling cap will still be needed to keep the input and output diode switch currents seperated.
73,
Mark

PS Sorry about the double entry. I hate computers sometimes.

[w3jn]

Connecting straight thru, via a cap, might mess up the gain distribution in the radio.

Frank's right, though, it's best to have DC isolation between both ends of the filter so as not to induce any interaction with the PIN switching diodes.

[Blaine N1GTU]

hey thanks for all the input.
I tried the 600 ohm with a .1 in series, it was pretty much spot on with the gain as the built in filter, nice guess on that value
the only thing is I didn't really see any difference on the spectral display(or hear any)
there is probably some caps/filtering doing rolloff somewhere down the line.
still beats the stock receiver hands down though, neat stuff.

[w3jn]

Lo voz de experiensa