[Elecraft] Variable bandwidth crystal filter

[Steve Ellington]

Howardz:
There's little one catch with the Flex test results. There's a control on 
the PowerSDR screen called AGC-T which stands for Automatic Gain Control 
Threshold. When signals are strong, you must manually adjust that control 
much like you would adjust an RF Gain control so the receiver doesn't 
overload. When they run all those test, they fiddle with that AGC-T for best 
performance with any given signal. I've used the Flex 5k several times and 
it always seems to crude to be forced to adjust AGC-T to compensate for 
bands, band conditions and signal levels. If Flex had roofing filters then I 
doubt that this manual adjustment would be necessary because the A/D 
converter would be protected. So when those dynamic ranges test are run, you 
can bet that someone tweaks that AGC-T control, otherwise the results would 
be horrible.

Steve
N4LQ
N4LQ at carolina.rr.com
----- Original Message ----- 
From: "HowardZ" <HowardZ at howardz.com>
To: <elecraft at mailman.qth.net>
Sent: Thursday, August 06, 2009 5:47 PM
Subject: Re: [Elecraft] Variable bandwidth crystal filter


>
> It appears that the only way the K3 achieves its high test scores in 
> dynamic
> range is when the noise is outside the roofing filter's bandwidth.  Only 
> the
> roofing filter yields this high performance.
>
> For example, if your roofing filter is 2.8Khz, but you hear an unwanted
> nearby signal, and you use DSP to narrow it to 2.0khz, the reduction of 
> the
> unwanted signal will not be as great as if you switched to your 2.0khz
> roofing filter (which you had not purchased yet).  This is because the
> offending signal is within the roofing filter's 2.8khz bandwidth.
>
> Thus following this example, the Flex 5000 has the same dynamic range
> regardless of the DSP filter bandwith khz, and will outperform the K3 when
> the offending signal is within its DSP bandwidth.  So with the Flex, 
> you'll
> have 96db of dynamic range regardless of the DSP bandwidth selected.  But
> with the K3, such high dynamic range is only achieved when the offending
> signal is being blocked by a roofing filter.
>
> (When the offending signal is 20khz, 50khz, etc away - the K3's dynamic
> range will be superior to the Flex, since the Flex is always 96db 
> regardless
> of how far away the offending signal is.)
>
> Loading up the radio (and second receiver) with roofing filters makes the
> expensive K3 radio even more expensive.
>
> Thus the interest in the "variable bandwidth crystal" roofing filter as a
> major money saver - if it ever comes to fuition.
>
> Howard
>
>
>
> Chuck Guenther wrote:
>>
>> J.R. Groeger wrote:
>>
>> "I seem to remember talk some time ago about a variable bandwidth
>> crystal filter becoming available for the K3 at some stage. Anyone know
>> the status of its development?"
>>
>>  With the upoming 10 Hz resolution DSP upgrade (Firmware 3.23 being
>> talked about), I certainly wouldn't see much need for a variable BW
>> roofing filter.
>>
>> 73,
>> Chuck  NI0C
>> K2, K3
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