To: rsgb_lf_group@blacksheep.org
References: <56686FA0.4050707@gmail.com>
 <CAA8k23RQGU=hgYNB0fQOh-HnU9B-ZCW6_Oki0RcWKBHDxksV7w@mail.gmail.com>
 <56695B15.6010203@abelian.org>
 <CAA8k23TqCUokxj9GoN0oPq9rbJWsS2w+-e8e0gCTReNNwzBmGg@mail.gmail.com>
 <56696950.1040300@abelian.org> <5669B7CE.1090601@freenet.de>
 <CAA8k23RhRdjUvQKzL6i--44=6Nc9LLDZe2ZhGOwmcbzm7w74Qg@mail.gmail.com>
 <5669E1E3.6070008@abelian.org>
 <CAA8k23QrY0B6KY-KkzcRt3OSCkQTHoPR8vDSdWzGJfbPPVRF0A@mail.gmail.com>
 <CAA8k23TYTBmTZpibfgV=ZdY974OpUdCVE3+HXUeH59prfSw1BA@mail.gmail.com>
 <CAA8k23R89jt5+PyfGRt-ofWHQhXneG03pRsN6TfYRKPssB9x-Q@mail.gmail.com>
 <566D6597.4070901@freenet.de>
 <CAA8k23Tvo7T9i83o+ab-kYbnujrPkfRNpUZgO_djkWvDucb7jw@mail.gmail.com>
 <566D73CD.1070805@freenet.de> <566D7992.2070207@abelian.org>
From: =?UTF-8?Q?Wolfgang_B=c3=bcscher?= <dl4yhf@freenet.de>
Message-ID: <566D9F95.1000808@freenet.de>
Date: Sun, 13 Dec 2015 17:40:53 +0100
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:38.0) Gecko/20100101
 Thunderbird/38.4.0
MIME-Version: 1.0
In-Reply-To: <566D7992.2070207@abelian.org>
Subject: Re: LF: LF EbNaut test from JN80 on 137370
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 8bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Status: O

Ok Paul, looks like we need a little adjustment then. I will add the 
option to save wave-files in SL in the EbNaut / VLF-RX tools compatible 
mode (details further below).

DCF77 recorded with the LO (NCO) set to 77490 Hz, i.e. signal 10 Hz 
*above* the oscillator, produces a wave file with the following content:

http://www.qsl.net/dl4yhf/t/dcf77_SDR-IQ_recording_NCO_at_77490Hz.png

Channel 1 = "I",  Channel 2 = "Q" .

This 'I / Q wave file format' was introduced by SDR-IQ / Spectravue, and 
in turn this may have been inspired by the AD6620 (digital downconverter 
used in the SDR-IQ), which multiplies the input with
  cos( omega * t) for the "I" branch, and  ...tada...    -sin( omega * 
t) for the "Q" branch.

( this can be seen in the block diagram on the first page of the 
datasheet :

http://www.analog.com/media/en/technical-documentation/data-sheets/AD6620.pdf

The same convention was applied for the wave files in SpectraVue, and 
later also for SL when I added native support for that radio )


With a "DC" input signal (0 Hz, Vin = const = 1), the output from the 
I/Q downconverter would equal to the oscillator signal (complex NCO, 
ignoring the chain of decimators), i.e.

i(t) = cos( omega * t)
q(t) = -sin( omega * t)


I had 'trimmed' the wave file export function in spectrum lab to produce 
wave files compatible with the original SpectraVue software. But I will 
add the option to produce wave files with the 'q' component so SL can 
write files with both conventions (for vlfrxtools / EbNaut: Q (2nd 
channel) peaking 90° before I,  and for other SDR software I (1st 
channel) peaking before Q.

Those flags (options) will be set by default in the configurations 
'EbNaut_Rcvr' and 'EbNaut_Rcvr_SDR_IQ' (just in case someone also wants 
to try with an SDR-IQ).

Of course users could just enter a modified RX frequency in the 
commandline for the decoder because there is no absolute phase anyway, 
but for the sake of simplicity, it should not be necessary to 'mirror' 
frequencies at the LO frequency for the decoder.

All the best,
   Wolf .


Am 13.12.2015 um 14:58 schrieb Paul Nicholson:
>
> If the signal is on the high side of the oscillator,
> Q should lead I after the mixer,
>
>  s(t) = sin(wc * t);   // Signal, wc = 2 * pi * Fc
>
>  i(t) = sin(wc * t) * cos( wo * t);  // LO, wo = 2 * pi * Fo
>  q(t) = sin(wc * t) * sin( wo * t);
>
> expands to
>
>  i(t) = 1/2 * { sin((wc+wo)*t) + sin((wc-wo)*t) }
>  q(t) = 1/2 * { cos((wc-wo)*t) - cos((wc+wo)*t) }
>
> which after LPF to remove the wc+wo component leaves
>
>  i(t) ~= 1/2 * sin((wc-wo)*t)
>  q(t) ~= 1/2 * cos((wc-wo)*t)
>
> which has q(t) leading i(t) by 90 degrees.
>
> The above are the polarities used by vlfrx-tools and ebnaut.
>
> On the scope, signal at 77500 Hz and LO at 77490 Hz looks like
>
>  http://abelian.org/vlf/tmp/151213a.gif
>
> with Q (the 2nd channel) peaking 90 deg before I.
>
> -- 
> Paul Nicholson
> -- 
>