22-07-2016, 11:24 AM
(This post was last modified: 22-07-2016, 11:26 AM by ppppenguin.)
Generating subcarrier. Just the DTO and sine table so far. One trace is from these tests, the other, drifting past slowly, is subcarrier from my lab SPG. By sheer luck they're only about 1ppm apart in frequency.
I've put a high quality low pass filter in the output of this test. Things look pretty grim with no filter so it will need a simple LPF. Probably just an L and 2 Cs. I'll calcualte it as a 10MHz 3 pole Butterworth initially, then cut and try to optimise transient performance while giving equal response at subcarrier and low frequencies.
Just remembered sin(x)/x correction will be needed. Not too bad for 4.43MHz sampled at 25MHz but will need a slight boost somewhere. Hope I've got enough range on the DAC to do it by upping the SC gain values. Could easily go over 255 and wrap around to 0.
If all goes well it shouldn't be too far from making colour bars. I've written almost all the VHDL, just got to try it out.
I've put a high quality low pass filter in the output of this test. Things look pretty grim with no filter so it will need a simple LPF. Probably just an L and 2 Cs. I'll calcualte it as a 10MHz 3 pole Butterworth initially, then cut and try to optimise transient performance while giving equal response at subcarrier and low frequencies.
Just remembered sin(x)/x correction will be needed. Not too bad for 4.43MHz sampled at 25MHz but will need a slight boost somewhere. Hope I've got enough range on the DAC to do it by upping the SC gain values. Could easily go over 255 and wrap around to 0.
If all goes well it shouldn't be too far from making colour bars. I've written almost all the VHDL, just got to try it out.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
