Wednesday, 29 January 2014

Trouble with aliases

2k mode using 2048 iFFT
2k mode using 4096 iFFT

The top picture is 2k mode DVB-T being generated with a 2048 point iFFT
notice the aliases either side. The lower picture is the same DVB-T mode
but this time being generated using a 4096 point iFFT only the centre bins
were used, the others filled with zeros. Of course to get the same bandwidth
(in this case 2 MHz) the second iFFT has to be clocked out at twice the
frequency.  The difference in the far out noise is down to me not setting the
levels the same in both traces, but I wanted to show you the aliases.
I learn a new DSP trick every day!

Side-band Suppression
This picture shows the DATV-Express board being modulated with a minus 1MHz
tone. The peak at the left is the LSB, the one in the middle is the un suppressed carrier
at 437 MHz and the one just above that is the unsuppressed USB. The one to the far
right is a harmonic of the modulating tone.


  1. If I remember reading before, I think you said your I/Q amplitudes are fairly low dynamic range. I want to say 8-bits, but maybe I'm off. Are these 16-bit samples?

  2. The DAC is 14bits. I can truncate the samples across the USB2 to 8 bits the output spectrum looks little different.