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 <[email protected]>
An: rsgb_lf_group <[email protected]>
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 
...