[Elecraft] [K3] Filters for mixed JT65 and JT9 HF

Tue Oct 8 11:34:23 EDT 2013

Bob,

On 10/8/2013 10:28 AM, Bob wrote:
 > Hi Joe,
 >
> I disagree with your adamant public statement as the -10 dB points
> even with the filter center shifted up to 2000 Hz are at 300 Hz and
> 3800 Hz -- That's 3500 Hz edge to edge, not 4000.

I believe your measurements are incorrect.  Using an XG3 at -73 dBm
for a signal source and an audio spectrum analyzer connected to the
Line Out of K3 S/N 622 with an *unmodified* DPS board and original
transformers on the KIO3, the audio is -4 dB at 300 Hz/-15 dB at 200
Hz and -5dB at 4000 Hz/-14 dB a 4200 Hz.

Put another way, the measured -6dB points are 263 - 4016 Hz (BW[6 dB]
= 3753 Hz) and measured -10dB points are 222 - 4142 Hz (BW [10dB] =
3920 Hz).  Those numbers are close enough to 4 KHz for me to be adamant
that the K3 works very well in "broadband mode" with either the FM or
AM filters in WSJT-X.

Moving the LF limit down to 50 Hz as in the case of SSB isn't going to
make a significant difference because the transformers have a good deal
of effect there.  Duplicating the LF measurement in SSB with LO=0.00
moves the -10dB point down by only 50 Hz (170 Hz).

That the K3 is down less than 6dB at 4KHz is the key - could it be a
bit better, yes but at what cost?  Audio frequencies below 300 Hz have
little value for WSJT-X receive (they're more than -1KHz from the old
1270 Hz "0 dF") and even with the roll off WSJT-X performs very well
at -200 Hz.  Split handles any transmit bandwidth issues in any case.

73,

    ... Joe, W4TV

On 10/8/2013 10:28 AM, Bob wrote:
> Hi Joe,
>
> I disagree with your adamant public statement as the -10 dB points even
> with the filter center shifted up to 2000 Hz are at 300 Hz and 3800 Hz --
> That's 3500 Hz edge to edge, not 4000.  This has also been confirmed by
> Lyle himself, who agrees that widening the bandwidth 400-600 HZ would be a
> good thing to do.
>
> I've also confirmed this with two other K3 owners.  If your DSP response is
> wider then consider yourself lucky because others don't experience the same
> thing.
>
> 73, Bob, WB4SON
>
>
> On Mon, Oct 7, 2013 at 4:51 PM, Joe Subich, W4TV <lists at subich.com> wrote:
>
>>
>> On 10/7/2013 4:39 PM, Bob wrote:
>>
>>> Just be aware that current DSP firmware in the K3 limits the receive
>>> bandwidth to something around 3400 Hz. So even if you have a wider
>>> filter, like the 6 KHz AM filter, you will not be able to utilize the
>>> full 4 KHz bandwidth of WSJT-X for JT65/MT9 modes.
>>>
>>
>> Absolutely *not* true ... I regularly receive 200 to 2400 Hz in DATA A
>> with the FM (or AM) filter.  It is simply a matter of adjusting the
>> High and Low cut frequencies to 200/4200 or shift/Width to 2200/4000.
>>
>> I have been doing that ever since K1JT offered me a pre-release look
>> at the "wideband" decoding support in WSJT-X with no issues other than
>> some CAT problems with the interface to CI-V Commander from DXLab
>> Suite which K1JT and AA6YQ resolved quite quickly.
>>
>> 73,
>>
>>     ... Joe, W4TV
>>
>>
>>
>> On 10/7/2013 4:39 PM, Bob wrote:
>>
>>> Just be aware that current DSP firmware in the K3 limits the receive
>>> bandwidth to something around 3400 Hz.  So even if you have a wider
>>> filter,
>>> like the 6 KHz AM filter, you will not be able to utilize the full 4 KHz
>>> bandwidth of WSJT-X for JT65/MT9 modes.
>>>
>>> Lyle is aware of this issue and it is on his list to work on in the
>>> future.
>>>
>>> And even if you are in Split mode, rest assured that you know exactly
>>> where
>>> you are transmitting -- on the exact audio offset indicated on the
>>> display.
>>>    Operating Split is the way to go.  The only drawback I've seen is that
>>> WSJT-X does nothing to make sure the K3 is in the Split mode, it just
>>> assumes it is.
>>>
>>> 73, Bob, WB4SON
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