Tuesday, 19 March 2013
Yet more, you guessed it express!
Yes another photo, this time of my HP spectrum analyser. What you are seeing is
a 2MS/s 32APSK 3/4 FEC DVB-S2 transmission with a 95 tap compensated root
raised cosine response will 0.35 rolloff. The filter takes the DVB-S2 symbols and
interpolates them by a factor of 8 to put the aliases outside the LC Nyquist filter
Believe it or not 6 Mbits/s of video is crammed into that 2 MHz piece of spectrum
DATV-Express will operate at symbol rates below 2MS/s but the alias responses start
to creep in. That is what comes of having a fixed analogue filter. Still if people need
lower symbol rates a new FPGA file can be created with x16 interpolation. It has taken
me a few days to get this to work but I have also re-arranged the code to make it more
readable and easier to change in the future.
The blip on the left hand side at 1.24 GHz is probably a multiple of 20 MHz reference
clock signal, it remains stationary when I change the operating frequency.
I am now going to take a step back and think a bit more as to where I go next with the
FPGA code. I am modularising the code as much as possible so I can use the same
modules for different variants. They all have some parts that are common and those
are the bits I am concentrating on first.