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Re: LF: LF EbNaut test from JN80 on 137370

from [Andy Talbot] [Permanent Link][Original]
To: [email protected]
Subject: Re: LF: LF EbNaut test from JN80 on 137370
From: Andy Talbot <[email protected]>
Date: Thu, 10 Dec 2015 22:20:13 +0000
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The PC decimation / FFT / file storage software needs a fine tune LO.  That will allow the DC spikes to be removed from any plot s/ files , as well as allowing finer tuning resolution than the 0.01Hz steps the receiver hardware offers.

I'm plotting / recording Joe at 0.2441Hz sampling rate - he highest decimation I built into the PC software, not really expecting to be using it.  A a bit of a bodge as I'm sure the FIR filter in the input to the decimator won't be very effective at that bandwidth - but there don't seem to be any strong signals around to get in the way and Joe is the only (genuine) spike in the passband

The DC spike is very strong at these bandwidtths - the FFT bin size is 17.5 minutes - so the Rx has to be detuned 50mHz to place Joe's carrier  off to the side of it.

I think this is a record here - never done anything serious before at sampling rates below 1Hz.

Inline images 1

(Spot the GUI modifications)

BTW, Wolf, if you're reading this...
I've nicked some of the palette tables from the Speclab folder - saves reinventing wheels.   Are there any copyright / restrictions on their use :  The one above is Horne's 'Gram' Palette; still  one of my favourites.

Andy  G4JNT

On 10 December 2015 at 22:00, Paul Nicholson <[email protected]> wrote:

Those Zoom FFT pages show -sin() as the multiplier for Q.

But a lot of other sources show +sin(), as I use in vlfrx-tools.

Andy's hardware is using +sin and the spectrum is correct in
Spectrum Lab but is reversed with vlfrx-tools/EbNaut.

And RTL2832 appears to use -sin() because I have to use -Q
in vlfrx-tools.

Wolf wrote:

> When loading Andy's file in a curve editor, I see the
> 2nd channel leads the 1st by 90 degrees.

That sounds like Q is using +sin() then. It's a puzzle why
Andy is having to give the opposite frequency offset to the
decoder.

Perhaps I'll sleep on this one.  I/Q signal processing is
always easier when wide awake.

--
Paul Nicholson
--


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