[Elecraft] Poor Low audio frequency response on JT65 mode with my K3 ?

[Joe Subich, W4TV]

Iain,

 > To give genuine JT65 signal reports, you want the full 2kHz of audio,
 > as the signal report is dB value relative to the "noise figure" across
 > that 2kHz.

JT65 "signal reports" are dB relative to the total noise in a 2.5 KHz
bandwidth (see Joe Taylor's web site).

 > I have a software macro for setting up for JT65, which includes
 > "MD6;DT0;BW0200;IS 1270;"

That's a bit narrow ... the DF is relative to 1270 and represents the
sync bit (the lowest tone) and the JT65A signal is just under 200 Hz
wide.  You really need BW0240; to pass the highest possible signal
(DF=1000) with JT65-HF by W6CQZ and WSJT9 by Joe Taylor decodes a full
3 KHz (-1200 to +2800 Hz) which means "IS 01500;BW0300;" is not
unreasonable.

Still, the original report has some validity ... if I transmit with
DF, <-800 it takes more audio drive (increase line in or sound card
TX level) to maintain proper ALC level.  It's not a lot ... from Line
In = 10 at DF=0 to Line In = 20 at DF= -1000.  The bigger problem is
that the gain is very sloped across the 200 Hz when operating at
DF below -850 and the power output will vary almost 3 dB between the
lowest and highest tone in the JT65 constellation.  For that reason
I do not like to use a DF less than 800 (lowest tone about 470 Hz).

It does seem that the K3's low frequency roll off is a bit severe in
DATA A.  It may be roofing filter related.  Although I'm using a 2.8,
if the center is at 1500 Hz the -6dB shoulder would be around 100 Hz
which would explain the 3 dB slope below 400 Hz +/-.  The roll off
might be even more pronounced with the 2700 Hz (5 pole) filters
since they are less flat than the 2800 (8 pole) filters).

It might be nice if there were an option to move the roofing filter
center to 1300 Hz for transmit in DATA A.

73,

    ... Joe, W4TV


On 11/14/2012 1:49 AM, iain macdonnell - N6ML wrote:
> Don,
>
> JT65 spots include an offset from the center of the (2kHz) audio
> passband, which is nominally centered at 1270.46Hz, so a "DF" of -500
> would be at audio frequency ~770Hz, and DF-1000 would be ~270Hz.
>
> I have a software macro for setting up for JT65, which includes
> "MD6;DT0;BW0200;IS 1270;"
>
> To give genuine JT65 signal reports, you want the full 2kHz of audio,
> as the signal report is dB value relative to the "noise figure" across
> that 2kHz.
>
> 73,
>
>      ~iain / N6ML
>
>
>
> On Tue, Nov 13, 2012 at 9:47 PM, Don Wilhelm <w3fpr at embarqmail.com> wrote:
>> Cliff,
>>
>> The K3 defaults to a 400 Hz bandwidth when selecting DATA A mode. unless
>> you have previously set some other width.
>>
>> I am not sure about the negative bandwidth values you have indicated,
>> but I am going to assume that those are just "dashes" and not negative
>> signs.
>>
>> I suggest you look at the width and shift (or HiCut and LoCut) to obtain
>> the filter bandwidth that you desire for digital modes.
>>
>> When working with a waterfall display, you may want a wide bandwidth,
>> but if you are tuning for a single signal with the K3 VFO knob, you will
>> want a more narrow bandwidth.
>>
>> 73,
>> Don W3FPR
>>
>> On 11/13/2012 7:04 PM, Clifford Hynds wrote:
>>> Hope someone has an answer,
>>>
>>> I am a recent owner of the K3 having just bought a K3, P3 and KPA500
>>> station from another ham.
>>>
>>> Probably you would have to be a regular user of the JT65 mode to notice but
>>> I seem to get little or no output when I am below about -500hz or so on the
>>> waterfall on JT65HFand nothing really at -1000Hz, fine from there up.
>>>
>>> Probably affect other data modes also ?
>>>
>>> I am using line in/out on the K3 and DATA mode so no equalisation,
>>>
>>> Had no problems with other rigs FT847,FT817 and TS950SDX had no problems
>>> with the low end.
>>>
>>> Could be as simple as increasing the input capacitor for the line audio or
>>> as the audio is controlled by the DSP perhaps an upgraded DSP unit. The K3
>>> is in the < 1000 Serial numbers 718 if i remember not in front of it at the
>>> moment.
>>>
>>> Hope someone has an answer,
>>> thanks
>>> Cliff VK2CCJ
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