[Skimmertalk] [RBN-OPS] Attn: Red Pitaya 16-bit users decoding FT8

[Alexander Ranaldi]

I am suspicious of the exact 100hz offset.  The FT8 freq deviation is
6.25Hz.  100/6.25 = 16, or 2^4.  Almost seems like some sort of digital
thing, perhaps on the transmit or receive side.




On Wed, Jul 31, 2019 at 10:03 AM Björn Ekelund, SM7IUN <bjorn at ekelund.nu>
wrote:

> I have seen this several times also with "normal" receivers so I believe
> it is generated at the transmit side.
> But never this strong. The cases I have seen the "ghosts" have been
> 15-20dB below the "main" signal.
>
> If the signal is inadvertently modulated by 50Hz hum leaking into the
> audio path, it would give exactly this effect.
>
> Björn SM7IUN
>
> Den ons 31 juli 2019 kl 14:21 skrev John Ackermann. N8UR <jra at febo.com>:
>
>> I've seen that a few times with WSJT-X 2.1.0 using an IC-7300 as the
>> radio, so it may not be related to the RPi.
>> On Jul 30, 2019, at 5:33 PM, N4ZR <n4zr at comcast.net> wrote:
>>>
>>> I'm noticing a strange phenomenon using WSJT-X to decode FT8 messages.
>>> Fairly often, I see multiple spots of the same station in the same
>>> transmission period.  Invariably, the highest-frequency spot is the loudest
>>> (and always quite strong, typically over 0 dB), and at 100-Hz intervals
>>> below it progressively weaker spots of the same station appear.  Here's an
>>> example:
>>>
>>> My setup:  Carolina windom to Clifton Labs 11 dB preamp to AM and FM
>>> broadcast band filters, then to an RP-16.  Software-wise I'm running
>>> Demin's SDR receiver to CWSL_Tee to CWSL_SSBWave.exe to WSJT-X.
>>>
>>> I'd like to know if anyone else is noticing this.  I've only seen it
>>> since I started using the RP-16.
>>>
>>> --
>>>
>>> 73, Pete N4ZR
>>> Check out the Reverse Beacon Network
>>> at <http://reversebeacon.net> <http://reversebeacon.net>, now
>>> spotting RTTY activity worldwide.
>>> For spots, please use your favorite
>>> "retail" DX cluster.
>>>
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