[1000mp] MP Roofing Filter from INRAD

[Ian White, G3SEK]

John wrote:
>Turn off the DSP ......
>
>It hiss like a snake. Actually almost all the annoying noise in the MP 
>is in the 455
>IF section...and specifically in the 455 notch circuit. I have bypassed 
>the Manual
>notch in my MP and it is quiet as a mouse...... of coarse I lost the manual IF
>notch...but I use the DSP one anyways.

When the manual notch is switched on, you can hear it introduces a lot 
of extra noise within the IF passband. Maybe this is not surprising, 
because the manual notch is a regenerative (Q-multiplier) type of 
circuit, half-way to being an oscillator. It certainly seems to generate 
noise sidebands like an oscillator  - strong near the center frequency, 
and weaker farther out.

The cause of the hiss problem seems to be that the manual notch circuit 
is never truly switched off. It is only de-tuned outside the IF 
passband, and I think we're still hearing its noise sidebands.

John's modification suggests that the real cure for the hiss problem 
would be to bypass the manual notch completely when it's not in use 
(preferably using relays, so it can be switched back in when needed).


-- 
73 from Ian G3SEK