Direct sampling HF with rtl2832 (!)

[Bob R]

Hi folks,

I was just perusing the rtl-sdr git repository and noted this branch:

    http://cgit.osmocom.org/cgit/rtl-sdr/commit/?h=steve-m/direct_sampling 

In it, Steve M. left this note:

[experimental] enable direct sampling for frequencies < 30 MHzsteve-m/direct_sampling

This is highly experimental code for using the RTL2832 as a

direct sampling receiver. The mode is only enabled when the

sample-rate 2.048 MS/s and a center frequency < 30 MHz

is used.

This mode disables the tuner, and by attaching a long wire to

the In-phase ADC input (pin 1 or 2 of the RTL2832, whereas

pin 1 is at the molded dot) it is possible to listen to shortwave

radio stations. The coupling capacitors can be left in place,

but for better results they should be removed.

Signed-off-by: Steve Markgraf 

Has anyone tried this yet?  Is it as simple as soldering in a jumper from the antenna to 'pin 1 or 2'?

[Ton Machielsen]

Now it gets interesting!!!

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[henry zuckerman]

I don't know if you have looked inside one of these dongles - the pins on both the e4000 & rtl2832 are quite small & closely spaced.  You'd better have a steady handy for this soldering job!  Nevertheless - very intriguing possibility for additional SDR goodness. 

[Adam Nielsen]

> I don't know if you have looked inside one of these dongles - the pins on both
> the e4000 & rtl2832 are quite small & closely spaced. You'd better have a
> steady handy for this soldering job! Nevertheless - very intriguing
> possibility for additional SDR goodness.

There might be something else connected to that pin you could solder your
antenna onto instead - maybe the coupling capacitor mentioned in the commit note.

If the RTL device is set to 2MHz sample rate though, it would seem it would
only get you listening to broadcasts between 0Hz and 2MHz. Not sure why the
commit message says this mode becomes active when you tune under 30MHz then,
you would think it would only need to activate when tuned to 0Hz!

Cheers,
Adam.

[Laurent Haas]

Hi

On Wednesday, May 30, 2012 1:32:31 PM UTC+2, Adam Nielsen wrote:

 

If the RTL device is set to 2MHz sample rate though, it would seem it would
only get you listening to broadcasts between 0Hz and 2MHz.  Not sure why the
commit message says this mode becomes active when you tune under 30MHz then,
you would think it would only need to activate when tuned to 0Hz!

Actually, the code of this fork modifies the way the RTL demodulator works (not Zero-IF anymore).

OTOH, only one ADC seems to be used, instead of two (quadrature).

Wait and see ;-)

 

[Guido]

Hello Adam,

Yes, it is sampling at 2MSPS with one ADC instead of two, but looking in the code patches it seems that the IF frequency of the RTL2832 can be set anywhere below 30MHz. This is remarkable as Realtek specifies:Supports multiple IF frequencies (4.57MHz or 36.167MHz) and spectrum inversion.

I am thinking of the following improvements:

  1. disabling the tuner (think currently it is still enabled in direct sampling mode), and 

  2. program a steep narrow band FIR filter to attenuate strong out of IF band broadcast stations.

Anyone tried the code yet?

73, Guido

+ if ((freq < 30000000) && (dev->cached_rate == 2048000)) {

+ if (!dev->en_direct_sampling) {

+ rtlsdr_init_direct_sampling(dev, 1);

+

+ /* for some reason the output rate is different

+ * when using only one ADC, setting 2310000 results

+ * in almost exactly 2048000 samples/second */

+ rtlsdr_set_sample_rate(dev, 2310000);

+ dev->en_direct_sampling = 1;

+ fprintf(stderr, "Enabled direct sampling mode\n");

+

+ if (dev->tuner && dev->tuner->exit) {

+ rtlsdr_set_i2c_repeater(dev, 1);

+ r = dev->tuner->exit(dev); /* deinitialize tuner */

+ rtlsdr_set_i2c_repeater(dev, 0);

+ }

+ }

+

+ if_freq = ((freq * TWO_POW(22)) / dev->rtl_xtal) * (-1);

[Adam Nielsen]

> Yes, it is sampling at 2MSPS with one ADC instead of two, but looking in the
> code patches it seems that the IF frequency of the RTL2832 can be set anywhere
> below 30MHz. This is remarkable as Realtek specifies:Supports multiple IF
> frequencies (4.57MHz or 36.167MHz) and spectrum inversion.

Interesting. I think the E4000 supports non-zero IF as well. I wonder
whether changing this and using a higher IF (for normal reception) might help
reduce the noise?

Cheers,
Adam.

[imo]

Yes you can place an IF filter between e4k and realtek then - ie. 10.7MHz one (~150kHz wide bandpass) or narrower when required :)

i.