Ettus Users: Hopefully someone can explain the behavoir I'm seeing when modulating a square wave with a carrier frequency. I'm using the supplied Ettus example software, tx_waveforms, that generates a waveform for the LFTX daughter board. With the following command: ./tx_waveforms --rate 1e6 --wave-type SQUARE --freq 0 --wave-freq 1e3 --ampl 0.2 I see a beautiful square wave of exactly 1 KHz, with an amplitude of -0.1 for 500us and 0.1V for 550 us on my scope. But when I add a carrier frequency to modulate the square wave as follows: ./tx_waveforms --rate 1e6 --wave-type SQUARE --freq 50e3 --wave-freq 1e3 --args --ampl 0.2 I see unexpected behavoir. What I expect is a square wave where 500 us is modulated with the 50 KHz carrier and then a phase shift due to the voltage sign change, as the 50 KHz carrier continues. What I get is odd: +/-0.2 V for 250 us, followed by 250 us of +/-0.3V, then +/-0.2V for another 250 us and finally 0V for 250 us before the wave repeats at 1 KHz.
Can anyone enlighten me as to why I see this behavoir and not the expected wave form? I simulate this wave form, using what should be the same math under Octave, and I get what I expect, so I'm confused. Thanks in advance, GB
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