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Re: [Discuss-gnuradio] help -- how to implement transmitting periodic on

From: Steven Clark
Subject: Re: [Discuss-gnuradio] help -- how to implement transmitting periodic on/off tones
Date: Mon, 29 Nov 2010 13:35:10 -0500

Think of the on/off part as a control stream consisting of 1's and
0's.  Generate the control stream, and multiple the control stream by
the carrier stream.

Don't try to start and stop the graph or anything like that from

You can probably generate the control stream with a
gr.vector_source_f([<my-pattern>], True) followed by a dumb
interpolator that will just replicate values.

 my_pattern = [1, 1, 0, 0, 1, 0, ... ]
 interp_factor = 1000   # scale the pattern up in time to match signal

 ctrl_pattern = gr.vector_source_f(my_pattern, True)
 ctrl_interp = gr.interp_fir_filter_fff(interp_factor, interp_factor*[1.0])

 signal = <generate carrier>

 mult = gr.multiple_ff()

 sink = <something-downstream>

 tb.connect(ctrl_pattern, ctrl_interp, (mult, 0))
 tb.connect(signal, (mult, 1))
 tb.connect(mult, sink)

One would think something like this would work, but I've noticed that even if you're sending 0's to your usrp sink, the transmitter still puts out some amount of power (plenty strong enough to be detectable via a spec-an). This power goes away if you disable the transmitter via software. Does anybody know anything about this phenomenon?


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