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Date: Wed, 2 Mar 2016 11:51:32 +0000
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From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: MF: EbNaut
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Consider using a Tee input low-pass filter, so the PA sees a series
inductor first, rather than a shunt C

A shunt C input will give large current spikes when hit with the square
wave from a voltage mode Class-D PA

Andy  G4JNT


On 2 March 2016 at 11:44, Paul Nicholson <vlf0403@abelian.org> wrote:

>
> I wrote the other day about seeing a transient that I thought
> was bouncing off the primary center.
>
> Now after dismantling and measuring a few things I find
> that transient was reflecting from the LPF which follows the
> transformer.  The transient width was about the same as the
> FET switching time and it came back to the drain of the off
> transistor at a voltage many times higher than the supply.
>
> Pinging the transformer itself I can see no sign of any
> transmission line behaviour up to 50MHz, the turns are just
> too tightly coupled.  In fact the coax cable with outer as
> primary and inner as secondary has a very tight coupling.
> 351uH per winding and 1uH leakage inductance.  I wont be
> using coax again though, the inability to tap the secondary is
> a severe limitation.
>
> So I learn that it's no good just looking at the bandpass
> and bandstop of the LPF, one might have to consider transient
> reflections because they end up back at the drain.  Almost all
> the departure from idealness of the drain waveform was down
> to the LPF and not the transformer.  I still haven't accounted
> for the excessive heat of the windings when the primary center
> is AC grounded.
>
> The divided down 477.69kHz multiplier output was compared
> with a GPSDO reference and the RMS jitter was less than 2
> degrees over timescales up to 60 minutes.  Here is a plot of
> the 2-sample RMS jitter over a run of eight hours
>
>  http://abelian.org/vlf/tmp/160302a.gif
>
> The plot means for example, that the average difference
> between two samples of the phase taken 512 seconds apart is
> 1.04 degrees.  The plotted phase is scaled back up to the
> tx frequency.  The jitter at the soundcard output of EbSynth
> is one ninth of the plotted value.
>
> This pretty much convinces me that the spreading seen by Markus
> was unlikely to caused by my tx.   Some doubt remains because
> I'm using the same GPSDO (Trimble) to reference both EbSynth
> and the phase comparison.  Also the PA and antenna were not
> in the loop.
>
> The phase comparison is done using vtsid from vlfrx-tools,
> sampling the phase 100 times per second.
>
> --
> Paul Nicholson
> --
>
>

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<div dir=3D"ltr"><div>Consider using a Tee input low-pass filter, so the PA=
 sees a series inductor first, rather than a shunt C</div><div><br></div><d=
iv>A shunt C input will give large current spikes when hit with=C2=A0the sq=
uare wave from a voltage mode Class-D PA</div><div><br></div><div>Andy=C2=
=A0 G4JNT</div><div><br></div></div><div class=3D"gmail_extra"><br><div cla=
ss=3D"gmail_quote">On 2 March 2016 at 11:44, Paul Nicholson <span dir=3D"lt=
r">&lt;<a href=3D"mailto:vlf0403@abelian.org" target=3D"_blank">vlf0403@abe=
lian.org</a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote" style=
=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br>
I wrote the other day about seeing a transient that I thought<br>
was bouncing off the primary center.<br>
<br>
Now after dismantling and measuring a few things I find<br>
that transient was reflecting from the LPF which follows the<br>
transformer.=C2=A0 The transient width was about the same as the<br>
FET switching time and it came back to the drain of the off<br>
transistor at a voltage many times higher than the supply.<br>
<br>
Pinging the transformer itself I can see no sign of any<br>
transmission line behaviour up to 50MHz, the turns are just<br>
too tightly coupled.=C2=A0 In fact the coax cable with outer as<br>
primary and inner as secondary has a very tight coupling.<br>
351uH per winding and 1uH leakage inductance.=C2=A0 I wont be<br>
using coax again though, the inability to tap the secondary is<br>
a severe limitation.<br>
<br>
So I learn that it&#39;s no good just looking at the bandpass<br>
and bandstop of the LPF, one might have to consider transient<br>
reflections because they end up back at the drain.=C2=A0 Almost all<br>
the departure from idealness of the drain waveform was down<br>
to the LPF and not the transformer.=C2=A0 I still haven&#39;t accounted<br>
for the excessive heat of the windings when the primary center<br>
is AC grounded.<br>
<br>
The divided down 477.69kHz multiplier output was compared<br>
with a GPSDO reference and the RMS jitter was less than 2<br>
degrees over timescales up to 60 minutes.=C2=A0 Here is a plot of<br>
the 2-sample RMS jitter over a run of eight hours<br>
<br>
=C2=A0<a href=3D"http://abelian.org/vlf/tmp/160302a.gif" target=3D"_blank" =
rel=3D"noreferrer">http://abelian.org/vlf/tmp/160302a.gif</a><br>
<br>
The plot means for example, that the average difference<br>
between two samples of the phase taken 512 seconds apart is<br>
1.04 degrees.=C2=A0 The plotted phase is scaled back up to the<br>
tx frequency.=C2=A0 The jitter at the soundcard output of EbSynth<br>
is one ninth of the plotted value.<br>
<br>
This pretty much convinces me that the spreading seen by Markus<br>
was unlikely to caused by my tx.=C2=A0 =C2=A0Some doubt remains because<br>
I&#39;m using the same GPSDO (Trimble) to reference both EbSynth<br>
and the phase comparison.=C2=A0 Also the PA and antenna were not<br>
in the loop.<br>
<br>
The phase comparison is done using vtsid from vlfrx-tools,<br>
sampling the phase 100 times per second.<br>
<br>
--<br>
Paul Nicholson<br>
--<br>
<br>
</blockquote></div><br></div>

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