[1000mp] Re: Bypassing the manual notch

[John Becker]

Hi John,

Please tell us the details of how you bypassed the Manual notch in your 
MP. I'd like to give it a try too.

73,

John, K9MM

> Date: Tue, 18 May 2004 10:08:31 -0400
> From: "John" <hydroaction at cfl.rr.com>
> Subject: Re: [1000mp] MP Roofing Filter from INRAD
> 
> 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.
> 
> 73 john
> ------------------------------

> Date: Tue, 18 May 2004 15:56:44 +0100
> From: "Ian White, G3SEK" <G3SEK at ifwtech.co.uk>
> Subject: Re: [1000mp] MP Roofing Filter from INRAD
> 
> 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.
> 
> 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).