Re: [Discuss-gnuradio] gr.firdes_hilbert () usage

[cswiger]

On Wed, 24 Nov 2004, Matt Ettus wrote:

>
>
> Feed a real signal to the hilbert filter to form the Q part of a complex 
> signal.
>    Feed a delayed version of the original signal to the I part.  The delay
> should be equal to  .5*(number of taps of the hilbert filter - 1)
>

Thanks Matt - Apologies if this is off topic theory but I do want to
implement a phasing ssb detector in gnuradio. I can see gr.firdes_hilbert
generated taps for gr.fir_filter_fff does create a 90deg phase shift
for all frequencies. However what I can't see is the theory part about
shifting positive frequencies by +90 and negative frequencies by -90.

Trying this block diagram:

                   LO Cos
                     |
              |-----(X)------[Hilbert 37]---|
              |                             |
signal in ----|                            (+)------ signal out
              |                             |
              |-----(X)------[delay 18]-----|
                     |
                   LO Sin

If my LO is 455Khz and my signal in is 455.5Khz I get a nice 500Hz output.
However if the signal is 454.5Khz there's also 500Hz out, so I'm getting
both sidebands. Expecting USB only. Nice clean detection tho!


Anyway I'll study it closer and see what's going on / missing.


def build_graph ():
        local_osc = 455e3
        signal = 454.5e3     # 500hz lower sideband
        sample_rate = 8e6
        fg = gr.flow_graph ()

        src = gr.sig_source_f (sample_rate,gr.GR_SIN_WAVE,signal,1.0,0)

        lo_90 = gr.sig_source_f \
           (sample_rate,gr.GR_COS_WAVE,local_osc,1.0,0)
        lo_0 = gr.sig_source_f \
           (sample_rate,gr.GR_SIN_WAVE,local_osc,1.0,0)

        hilbert_coeffs = \
           gr.firdes_hilbert ( 37, gr.firdes.WIN_HAMMING)
        hilbert = gr.fir_filter_fff (1, hilbert_coeffs)

        del_taps = [0, 0, 0, 0, 0, 0, 0, 0, 0, \
                    0, 0, 0, 0, 0, 0, 0, 0, 0, 1]
        delay = gr.fir_filter_fff (1, del_taps)

        mixq = gr.multiply_ff ()
        mixr = gr.multiply_ff ()
        sum = gr.add_ff ()

        dst = gr.file_sink (gr.sizeof_float, "ssb_out.dat")

        fg.connect (src, (mixr, 0))
        fg.connect (lo_0, (mixr, 1))
        fg.connect (mixr, delay)

        fg.connect (src, (mixq, 0))
        fg.connect (lo_90, (mixq, 1))
        fg.connect (mixq, hilbert)

        fg.connect (hilbert, (sum, 0))
        fg.connect (delay, (sum, 1))

        fg.connect (sum, dst)

        return fg

--Chuck