LimeSDR Mini QPSK Phase Noise at 3.4 GHz

Projects LimeSDR
1

I have been using a LimeSDR Mini to generate DVB-S QPSK 333 KS/s digital TV signals at 3406 MHz. Initial results were not good, with a really bad received QPSK constellation showing imbalance and phase noise, but as the LimeSDR Mini warmed up, it improved a lot. I reproduced this by cooling the LimeSDR in the fridge and it happened again.

Today I have done more tests with 2 units. I can reproduce the problem on both hardware versions 1.1 and 1.2. The time from poor phase noise to very good is about 5 minutes on the bench - it took a lot longer on a cold windy hilltop where I first noticed the problem. There is no issue at 2405 MHz (or below), it just seems to occur at the very top of the frequency range.

I am not going to spend much more effort on investigating this issue, as I now know that I need to let the unit warm up before (very occasional) use at 3406 MHz. However, I wanted to let other users know that the issue has been observed and ask if anyone else has seen similar problems or solved them.

Dave, G8GKQ

2

It’s bee suggested that this is probably a calibration issue, as calibration will be to some extent temperature dependent. @Zack, do you have anything to suggest here?

3

Thanks Andrew

I have also had other suggestions that it might be some stored factory calibration (as opposed to the pre-transmit calibration routine) that does not adequately account for temperature at the extremes of the frequency range. @Zack could this be a possibility?

Either way, there is no big issue - just so you know that we are pushing this amazing chip to it’s limits! Once it had warmed up, the DVB-S TV pictures on 3.4 GHz were received 28 km away.

Dave

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4

Nice! Was that with a PA? What sort of antenna?

5

Yes, I use a Stealth SM3437-43L-1Q power amplifier which has about 50dB of gain and will give 10 Watts of clean DATV at 3.4 GHz whilst draining a 12V leisure battery. The aerial was a 60 cm prime focus dish - seen in the photo below with a different (10 GHz) feed. Dave4%20Stealth%202

60%20cm%20Dish
60 cm Dish.jpg576×752 115 KB

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