[TenTec] filter

[CSM(r) Gary Huber]

>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 at weather.net>
Sent: Friday, August 20, 2010 10:42 AM
To: <tentec at 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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