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Date: Fri, 11 Dec 2015 15:29:20 +0000
Message-ID: <CAA8k23RSZR2aUVUowh9i7b7s5Ye8Qnsc96ZoT28GuN_xcxwSng@mail.gmail.com>
From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: DC spike at 1 kHz
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I think I now know what the DC spike is - it's leakage at 1kHz from the
RS422 driver sending data to the PC.  On my main shack desktop machine, I
use a proper RS422 balanced feed and on a reference signal from DCF77 the
zero spike is -35dB.

For most PC work in the evenings I use my laptop in the living room (so as
to be able watch telly at the same time).  As I don't have a long twisted
pair run, I just take an unbalanced feed from RS422 negative against
ground, feed it down a piece of coax that does go around the house and into
the laptop via an opto isolator.   On that system tested just now, the DC
spike is only 19dB below DCF77.  So it's clear the spike is being caused
from leakage of the serial data stream, which contains a strong 1kHz
component into the audio amp / filter before the digitiser.
The cure will be to go back to having an RF amplifier and less gain at 1khz
- and of course, not driving an unbalanced data link.

Meanwhile, 60kHz DX looks more interesting - there really is a fair bit of
multipath present


Andy G4JNT

On 10 December 2015 at 10:52, Markus Vester <markusvester@aol.com> wrote:

> Hi Andy,
>
> presumably the digital 1 kHz-to-baseband conversion is done in an FPGA or
> DSP in realtime. The associated periodic processing activity or register
> content could lead to a 1 kHz component radiated by the FPGA itself or it=
's
> DC power lines. This could then leak into the analog audio input to the
> ADC. Try sniffing around the FPGA with a capacitive or inductive probe.
>
> Downconverting from a soundcard on a PC in software usually does not
> suffer from such coherent leakage, as there are variable delays due to
> buffering and multitasking which effectively remove the periodicity in th=
e
> CPU activity.
>
> Best 73,
> Markus
>
> -----Urspr=C3=BCngliche Mitteilung-----
> Von: Andy Talbot <andy.g4jnt@gmail.com>
> An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
> Verschickt: Do, 10 Dez 2015 11:23 am
> Betreff: Re: LF: LF EbNaut test from JN80 on 137370
>
> ...
>
> My LF receiver appears to give a small DC spike in the centre of the
> spectrum , which since it is taking input at 1kHz and multiplying in
> firmware with I/Q square waves is a bit odd. That DC term wasn't
> noticeable when I had the RF amplifier previously (the one that popped
> with strong signals) but now gain has been moved to 1kHz IF, the line
> can be seen. Perhaps an RF amplifier is necessary, but one that can
> safely be overloaded
> ...
>

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<div dir=3D"ltr"><p>I think I now know what the DC spike is - it&#39;s leak=
age at 1kHz from the RS422 driver sending data to the PC.=C2=A0 On my main =
shack desktop machine, I use a proper RS422 balanced feed and on a referenc=
e signal from DCF77 the zero spike is -35dB.</p><p>For most PC work in the =
evenings I use my laptop in the living room (so as to be able watch telly a=
t the same time).=C2=A0 As I don&#39;t have a long twisted pair run, I just=
 take an unbalanced feed from RS422 negative against ground, feed it down a=
 piece of coax that does go around the house and into the laptop via an opt=
o isolator.=C2=A0=C2=A0 On that system tested just now, the DC spike is onl=
y 19dB below DCF77.=C2=A0 So it&#39;s clear the spike is being caused from =
leakage of the serial data stream, which contains a strong 1kHz component i=
nto the audio amp / filter before the digitiser.</p><div>The cure will be t=
o go back to having an RF amplifier and less gain at 1khz - and of course, =
not driving an unbalanced data link.</div><div><br></div><div>Meanwhile, 60=
kHz DX looks more interesting - there really is a fair bit of multipath pre=
sent<br></div><div class=3D"gmail_extra"><br></div><div class=3D"gmail_extr=
a"><br></div><div class=3D"gmail_extra">Andy G4JNT</div><div class=3D"gmail=
_extra"><br></div><div class=3D"gmail_quote">On 10 December 2015 at 10:52, =
Markus Vester <span dir=3D"ltr">&lt;<a href=3D"mailto:markusvester@aol.com"=
 target=3D"_blank">markusvester@aol.com</a>&gt;</span> wrote:<br><blockquot=
e class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;padding-left:1ex;=
border-left-color:rgb(204,204,204);border-left-width:1px;border-left-style:=
solid">
<div style=3D"color:black;font-family:arial;font-size:10pt"><font color=3D"=
black" face=3D"arial"></font><span style=3D"display:inline-block"></span></=
div>

<div>Hi Andy,</div>

<div> =20
<div>=C2=A0</div>
 =20
<div>presumably=C2=A0the digital 1 kHz-to-baseband=C2=A0conversion is done =
in an FPGA or DSP in realtime. The associated periodic processing activity =
or register content could=C2=A0lead to a=C2=A01 kHz component radiated by t=
he FPGA itself or it&#39;s DC power lines. This could then leak into the an=
alog audio input=C2=A0to the ADC. Try sniffing around the FPGA with a capac=
itive or inductive probe. </div>
 =20
<div>=C2=A0</div>
 =20
<div>Downconverting=C2=A0from a soundcard on a PC in software=C2=A0usually =
does not suffer from such coherent leakage, as there=C2=A0are variable dela=
ys due to buffering=C2=A0and=C2=A0multitasking=C2=A0which effectively remov=
e=C2=A0the periodicity in the CPU activity.=C2=A0=C2=A0</div>
 =20
<div>=C2=A0</div>
 =20
<div>Best 73,</div>
 =20
<div>Markus</div>
 =20
<div>=C2=A0</div>
 =20
<div>-----Urspr=C3=BCngliche Mitteilung----- <br>
 Von: Andy Talbot &lt;<a href=3D"mailto:andy.g4jnt@gmail.com" target=3D"_bl=
ank">andy.g4jnt@gmail.com</a>&gt;<br>
 An: rsgb_lf_group &lt;<a href=3D"mailto:rsgb_lf_group@blacksheep.org" targ=
et=3D"_blank">rsgb_lf_group@blacksheep.org</a>&gt;<br>
 Verschickt: Do, 10 Dez 2015 11:23 am<br>
 Betreff: Re: LF: LF EbNaut test from JN80 on 137370<br>
 <br>
 <tt>...</tt></div>
 =20
<div><tt></tt>=C2=A0</div>
 =20
<div><tt>My LF receiver appears to give a small DC spike in the centre of t=
he <br>
 spectrum , which since it is taking input at 1kHz and multiplying in <br>
 firmware with I/Q square waves is a bit odd.   That DC term wasn&#39;t <br=
>
 noticeable when I had the RF amplifier previously (the one that popped <br=
>
 with strong signals) but now gain has been moved to 1kHz IF, the line <br>
 can be seen.   Perhaps an RF amplifier is necessary, but one that can <br>
 safely be overloaded=C2=A0<br>
  ...</tt></div>
  </div>
 </blockquote></div><div class=3D"gmail_extra"><br></div></div>

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