Re: [Discuss-gnuradio] Re: How to scan the whole ISM band?

[Johnathan Corgan]

Martin Braun wrote:

> If you just want to find spectrum holes, you might be able to teach the
> FPGA...

*If* you are going to make a custom FPGA build, one could do as follows:

Since there are two ADCs operating on quadrature downconverted samples
at 64 Msps, the spectral width is actually 64 MHz entering the FPGA.
(This is the reason the minimum decimation rate in the CIC/HB chain is
8, in order to reduce the sample rate/frequency content to 8 MHz to fit
the USB capacity.)

In practice, the anti-aliasing filters on the RFX limit the usable
passband width to approximately 50 MHz.  So you can cover the whole 2.4
GHz ISM band with two different center frequencies.

As discussed in a recent thread, one could collect samples at this full
rate in the FPGA for a period of time, stream them to a FIFO, then drain
the FIFO at 1/8 the clock rate over the USB.  You would be limited to
collecting something like 2-4K samples at a maximum duty cycle of 12.5%.

The host software would take these time domain "frames", perform
windowing, FFT, and bin-over-time smoothing, resulting in a spectral
estimate across 50 MHz.  This is almost identical to what the existing
usrp_fft.py script does, except now the time domain frame decimation
would be happening in the FPGA instead of in the host, to operate over
the full ADC bandwidth instead of over a maximum of 8 MHz.

After "enough" frames have been received, one would switch the center
frequency to the second one and repeat.

(Much of the above FPGA processing is already implemented in the RX
portion of the monostatic radar implementation, gr-radar-mono.)