Date: Sat, 17 Feb 2018 09:56:39 -0330 (NST)
From: jcraig@mun.ca
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Subject: Re: LF: Testing Audioinjector Octo with RPi3
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Always good to hear about these adventures.  20 nS jitter is amazing.

Meanwhile on this side of the pond, the aerial is iced and the current is
quite low since last night but the transmissions on 8.2700075 kHz 
continue.

73
Joe VO1NA

On Sat, 17 Feb 2018, DK7FC wrote:

> ...so it could be an EMC issue. One of the first things i've done is removing 
> the ribbon cable and the audio jacks and use RG174 instead.
> To prevent a galvanic ground loop maybe it is worth to try to use a few turns 
> RG174 through a ferrite core? It is quickly done...
>
> 73, Stefan
>
> Am 17.02.2018 12:31, schrieb Paul Nicholson:
>> 
>> I had a bit of time to play again with the Octo, to investigate
>> where the excess timing jitter comes from.
>> 
>> The stereo audio injector is giving around 20nS jitter on the
>> PPS from the neo-m8t.
>> 
>> With the stereo card replaced by the Octo, all else the same,
>> the jitter varies between 300nS and over 1uS.
>> 
>> A close look at the shaped PPS seen through the Octo shows some
>> distortion or interference
>>
>>  http://abelian.org/vlf/tmp/180217a.gif
>> 
>> Not sure if these are additive or little step changes in gain.
>> 
>> Here's worse example
>>
>>  http://abelian.org/vlf/tmp/180217b.gif
>> 
>> Always the interference is on the rising and/or falling slopes
>> about half way.
>> 
>> Not present on the PPS when viewed on a scope at the Octo inputs.
>> 
>> Similar artifacts are visible on a plain old sine wave fed into
>> an Octo input.
>>
>>  http://abelian.org/vlf/tmp/180217c.gif
>> 
>> For some reason on the sine wave the artifacts are always near
>> the turning points.
>> 
>> Meanwhile I monitored the 49.152 MHz master oscillator on a
>> SSB receiver and it was giving a nice pure note with no obvious
>> frequency or phase jitter.
>> 
>> So it looks like the timing jitter on the Octo is due to its
>> analog inputs, not the A/D conversion timing.
>> 
>> Strangely though, there's no sign of these artifacts when there's
>> no input to the channel.  It isn't just background interference.
>> 
>> I think the next step might be to bypass the Octo's input board
>> and take the PPS straight to the pins of the ribbon connector.
>> 
>> Wondering if anyone else sees this behaviour on a PPS or sine wave.
>> -- 
>> Paul Nicholson
>> -- 
>> 
>
>