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Re: [TenTec] filter

To: geraldj@weather.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] filter
From: JOSEPH DAVIS <iloveantennas@dishmail.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Fri, 20 Aug 2010 13:09:16 -0400
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
Thanks for ur complete answer.  Clears many things for me.   jjdavis

On Fri, Aug 20, 2010 at 11:42 AM, Dr. Gerald N. Johnson <geraldj@weather.net
> wrote:

> A roofing filter is the first filter the signal reaches that is similar
> bandwidth to the signal in the receiver. The closer it is to the antenna
> the least effort the following stages have in handling strong adjacent
> frequency signals. Ideally that roofing filter can be the same bandwidth
> as the final filter, but that can reduce the receiver versatility, e.g.
> with a CW bandwidth roofing filter, copying SSB becomes impossible. And
> if the noise blanker pulse detection takes off after the roofing filter,
> it won't work with that narrow a roofing filter.  So there are trade
> offs in the selection of the roofing filter.
>
> In some receiver designs there are three mixers (always the most
> limiting stages for noise generation and maximum signal handling) before
> significant selectivity. Modifying those designs with a 15 KHz bandwidth
> roofing filter (so FM will pass) up front, providing some of the fine
> tuning isn't done with a later oscillator makes the receiver handle
> stronger unwanted signals better.
>
> For decades, TenTec receivers have had the main selectivity filtration
> right after the first mixer, the normal location for a roofing filter.
> That's not the case in the DSP based receivers so they add a roofing
> filter. Many other brands put the main selectivity two or three mixers
> later than the first mixer and need some sort of a roofing filter up
> front, but its hard to get tight selectivity with the first IF at 45 MHz
> so those receivers have much better intermod performance beyond 20 KHz
> than close in.
>
> 73, Jerry, K0CQ
>
> On 8/20/2010 9:47 AM, JOSEPH DAVIS wrote:
> > What is a roofing filter and its advantages. thanks jjdavis
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