Re: [SDRSharp] Re: From a SDR Grapevine...


Alberto I2PHD
 

On 8/14/2013 4:01 AM, alan_r_cam wrote:

Wait... huh? No, no, no.

Yes, put a 20MHz IF on the SDR. BUT - you don't send all that to the PC. You do a FFT, then send a smidgen of data to the PC for your "waterfall" display.

THEN you select the band you want to listen to. That selection goes back to the SDR, and you filter JUST THAT PART and send it back to the PC. Even the I/Q decoding can be moved into the SDR. You shouldn't need more CPU, you should need LESS. Ultimately, you should aim for a standalone SDR with no PC processing at all - just display of status & playback of signal.

Wait... huh? No, no, no.

That is not not how an SDR program works.... have you ever written one ?

The ADC on the SDR samples, let's say, at 122.88 MHz. Those samples are passed to an FPGA which, using a NCO usually implemented
with the Cordic algorithm, does a downconversion that brings the center of the chunk of the spectrum of interest to zero Hz.
In doing this, the formerly real signal from the ADC is transformed to analytic, i.e. I and Q.

Then, with a series of CICs and FIRs the signal is downsampled let's say from 122.88 to 6.144 MHz.
The downsampled signal is then sent to the PC through the USB interface, and the SDR hardware doesn't see it anymore....

The software in the PC takes those 6+ millions of samples per second, does a further downconversion bringing to zero Hz the
specific signal of interest among those contained in the entire chunk received, and for this it uses a NCO this time not implemented
with a Cordic, but with a coupled quadrature oscillator (with level stabilization), then the signal is filtered, usually with the
fast convolution method (but there are those who still prefer FIRs...), then it is demodulated, etc. etc.
In a parallel thread, using a different core of the CPU, the spectrum and the waterfall are computed and displayed.

There is no ping-pong between the PC and the SDR hardware as your message would imply....
The bulk of the processing is done inside the PC, the SDR hardware just downconverts and downsamples the samples acquired by the ADC.
The SDR hardware does not do any FFT.

Or, as the latest product of Elad, the FDM-DUO, shown at the Friedrichshafen Ham Fair, the PC is not anymore needed....
What formerly was performed by the PC, in that product is performed by an ARM Cortex M4F CPU, filtering, demodulation,
waterfall computation and display, etc. etc.�� Also the TX part is implemented as a DUC coded in the ARM processor, which
in turn drives an Analog Devices AD9957 interpolator, quadrature modulator and DAC.

That transceiver is completely stand-alone, no pesky PC is anymore needed... perfect for a field day.....�� :-)

--
73 Alberto I2PHD

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