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

[Wes (N7WS)]

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
>>>
>>> ______________________________________________________________
>>