GB, What you are seeing I think is the Square wave generated in quadrature as a complex signal….imagine 2 square waves with pi/4 phase offset. Also I think you will find the square wave is [0,1] not [-1,1]. So when your modulated square waves are summed…you get what you see.
Build it for your self in GRC as a simple flow graph using: a Signal Source and Null source both set to float output, and Float To Complex block to combine them and drive a UHD Sink. Like this you will see a OOK waveform if the signal source is configured as Amplitude:1, Offset:0. If you then re-configure as Amplitude:2 Offset: -1 you will see the PSK style signal you desire. If you drive the Signal Source as a complex signal straight to the UHD sink then you will see the same waveform you currently see. -Ian > On Oct 20, 2018, at 10:18 PM, G Broten via USRP-users > <usrp-users@lists.ettus.com> wrote: > > 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 > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com _______________________________________________ USRP-users mailing list USRP-users@lists.ettus.com http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
