Re: LF: DC spike at 1 kHz

[Andy Talbot <andy.g4jnt@gmail.com>]

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 the CPU activity.  

 

Best 73,

Markus

 

-----Ursprüngliche 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 
...