>From the TT model 588 manual:
"We refer to the roofing filters in the OMNI-VII
as "distributed" because filters are spread
across both the first and second I-F stages.
The conversion stages in the OMNI-VII are
70 MHz first I-F, 455 kHz second I-F, 14 kHz
(DSP) third I-F. A monolithic filter at 20 kHz
bandwidth is present at the first I-F stage
between the first and second mixers.
Selectable second I-F filters at bandwidths
of 20, 6, 2.5 kHz, 500 Hz (optional) and 300
Hz (optional) can be cascaded with the first
I-F monolithic filter at 20 kHz bandwidth.
Bandwidth filtering is done in DSP at the
third I-F and is controlled by the BW encoder
on the transceiver front panel.
The net effect of using 455 kHz second I-F
filters is to increase blocking dynamic range
over what the receiver would be capable of
without the second I-F filters installed. Third
order intercept point essentially remains
constant. The front-end AGC in the
transceiver is after the 455 kHz I-F filters;
having them installed prevents the radio
from attempting AGC action on a signal that
is outside the bandwidth of the 455 kHz I-F
filter.
SSB operators will not require additional
filters; additional filters can be installed
optionally by CW or digital mode operators."
73,
Gary - AB9M
--------------------------------------------------
From: "Dr. Gerald N. Johnson" <geraldj@weather.net>
Sent: Friday, August 20, 2010 10:42 AM
To: <tentec@contesting.com>
Subject: Re: [TenTec] filter
> 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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>
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