BATC Forum

Hi all.

I have been doing some comparisons of the output of my Jetson Nano + Lime Mini and an SED Starcast DVB-S modulator. Both devices were set to output 2Mbit/s streams on 1700Mhz. I am waiting for a DVB-S2 license for the SED, they have promised me one FOC but it has been a few months so I am limited to DVB-S at the moment.

This is the output from the SED Starcast which is pretty-much perfect I guess?
I tried various settings on the Nano, first was using 1 x upsampling and the default gain of 0.8. (using the special FPGA firmware)
Next I tried 2x
Followed by 4x. This definitely improves the signal!
Next I tried reducing the gain setting to 0.6 and this had a dramatic effect. I would say it compares pretty favorably with the SED?
Not sure what conclusions to draw from this test yet but definitely keep the gain down and use upsampling. Also not sure what the peak at the centre frequency is as it is present in all outputs from the Lime?

73 Phil M0VSE

Incidentally, I have just done the test with 2x upsampling and gain at 0.6 and it is almost identical to the 4x upsampling one so this is probably the 'sweetspot'?
Also no peak at the centre frequency???

73 Phil M0VSE

Hi Phil,

Good plots with very low shoulders - can be hard to maintain the signal purity by the time you reach the powers needed at work through the satellite !

Not sure about the spikes - maybe needs some IQ calibration ?

73 Tim

Hi Phil

I would suggest that the spike is because the Lime Mini needs calibration. It shows up on the 4 times upsampling because the main signal power output is 12 dB lower than with no upsampling.

If you are using limesdr_dvb, run it once without the -q switch (but watch the power spike). That will then save a calibration file for you, and you can then try again with -q and it should improve.

Dave, G8GKQ

Just a basic question, am I correct in seeing a 42.6 dB drop from peak to shoulder. I thought it was 10 dB per vertical division initially so thought I would check?

Adrian

Hi Adrian,

Yes it is a 42-43dB drop, I had auto-scale turned-on which makes it a bit difficult to calculate. This is compared to the 49-50dB drop of the SED Starcast.

One thing I did wonder, as the SED is about 6dB better SNR compared to the Lime, does this mean that I would need 1/4 of the power to get the same signal?

Thanks for the suggestion about calibration Dave, I had been playing with the -q switch so I guess something isn't quite right?

73 Phil M0VSE