Are you *absolutely sure* you are using the 400 Hz IF filter? By ear,
it sounds as if you have a wider IF filter that is centered on the
carrier and allowing the image to reach the DSP.
Since your display shows FL4, is it possible that you have all your
filters reversed?
73,
... Joe, W4TV
On 2/18/2016 11:56 AM, Mike Murray wrote:
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) <[email protected]> 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<[email protected]>
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
______________________________________________________________
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[email protected]
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [email protected]
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[email protected]
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [email protected]
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[email protected]
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [email protected]
