[Elecraft] K3 - AFSK anamoly? No replies, try again

[Mike Murray]

After doing some more testing in last weekends RTTY contest, it appears
that I made an erroneous statement as to what I was hearing and where.
What I found is that as I tune down in frequency and find a signal that
will decode, if I then tune down another 170 Hz I'll hear the inverted
signal.  Still using dual passband DSP filter at 400 Hz and 400 Hz, 8 pole
roofing filter as before.  I have a brief video on uTube showing the effect
at:

*http://tinyurl.com/hxykq9c <http://tinyurl.com/hxykq9c>*

Anyone have additional thoughts as to what's causing this or where I should
look next?

Mike - W0AG

On Thu, Jan 7, 2016 at 9:41 PM, Wes (N7WS) <wes at triconet.org> wrote:

> Assuming that when the OP says "tuning" he is turning the VFO knob, then
> here's my take.  All of the worry about roofing filters (gad how I hate
> that term) is for nothing. The crystal filters in conjunction with
> so-called hardware AGC are there to protect the delicate 2nd mixer from
> overload in the presence of strong signals. (Conventional wisdom says
> "protect the DSP" but the mixer is way weaker than the DSP. )
>
> Absent that condition, the demodulation BW is set in DSP and non-hardware
> AGC is developed in that same bandwidth.  Depending upon the strength of
> the desired signal and AGC threshold and slope, some amount of AGC will be
> developed when the desired signal is within the DSP BW.  Hence there is
> some amount of gain reduction in play.  When you tune to the opposite
> sideband, that AGC gain reduction goes away, raising the amplitude of the
> unwanted sideband with respect to the desired signal.  In other words, you
> cannot measure opposite sideband rejection unless 1) the gain remains
> constant or 2) some amount of known attenuation is included and accounted
> for in the calculation.  This is pretty much what Lyle was talking about
> earlier.
>
> One more thing.  If we understand that a superhetrodyne receiver is
> nothing more than a narrow BPF that can be tuned over the spectrum then
> that "roofing filter" that overlays the DSP filter tunes right along with
> it.   The belief that somehow when you tune "below zero beat" the crystal
> filter stays put while the DSP passband moves is silly.
>
> Wes  N7WS
>
>
>
>
> On 1/7/2016 3:24 PM, Mike Murray wrote:
>
>> Joe, et.al.,
>>
>> I finally got back to the shack to check the setup.  It looks like I was
>> actually using a 1.8 kHz SSB roofing filter in conjunction with the 500 Hz
>> dual PB DSP filter.  I also checked the filter offsets which were all at
>> 0.0 (all 8 pole) and I was using AFSK A mode.  Is the effect I'm seeing
>> due
>> to use of the 1.8 filter or do I have more research to do?
>>
>> Thanks for everyone's input and patience - seems like the learning curve
>> is
>> getting worse with age.
>>
>> Mike - W0AG
>>
>> On Wed, Jan 6, 2016 at 6:14 PM, Joe Subich, W4TV<lists at subich.com>
>> wrote:
>>
>> Since I'm new to RTTY, I probably didn't describe my question in
>>>
>>>> correct terminology - I mainly only operate CW. I'll try again....
>>>>
>>>> And I'll ask my questions again ... what *ROOFING FILTER* and what DSP
>>> bandwidth are you using?
>>>
>>> As Eric said in reply to your posting, this sounds like the offset is
>>> incorrect on the roofing filter and allowing you to tune "past" zero
>>> beat.  This is particularly true if you are using the SSB filter and
>>> a wide DSP setting instead of the more typical CW filter (500 Hz)
>>> centered on the mark/space tones (915/1085 Hz in your case).
>>>
>>> Note: RTTY is typically lower sideband with MARK being the lower audio
>>> tone and space being the higher audio tone because of the "reversal"
>>> that occurs in the audio to RF translation.  In the K3 AFSK A is LSB
>>> but DATA A is USB ... if you are using them interchangeably that may
>>> also be causing you some confusion.
>>>
>>> 73,
>>>
>>>    ... Joe, W4TV
>>>
>>>
>>> On 1/6/2016 6:47 PM, Mike Murray wrote:
>>>
>>> Bob, Joe, Bill & Mike,
>>>>
>>>> Since I'm new to RTTY, I probably didn't describe my question in correct
>>>> terminology - I mainly only operate CW.  I'll try again....
>>>>
>>>> I'm using DATA A/AFSK A, setup for low tones (915/170) and the RTTY
>>>> dual-PB
>>>> filter.  To illustrate what I'm hearing/seeing, imagine tuning down from
>>>> 14.090 until you hear and can decode an RTTY signal.  Then continue
>>>> tuning
>>>> down 915 Hz (to what I probably erroneously referred to as zero beat)
>>>> and
>>>> then down another 915 Hz, I can now hear the same signal but inverted
>>>> (slightly weaker, but definitely there).  It is not decodeable, but it
>>>> is
>>>> definitely there.  I assumed (yeah, I know) that the opposite sideband
>>>> should be suppresed, but I think that's what I'm hearing.  Any advice
>>>> will
>>>> be appreciated.
>>>>
>>>> Mike - W0AG
>>>>
>>>> ______________________________________________________________
>>>>
>>>
>>>
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