Signal Generator error with Lime Mini

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G3GJA
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Signal Generator error with Lime Mini

Post by G3GJA » Fri Mar 04, 2022 2:56 pm

I think I have found a bug in the sig gen. My Portsdown has the latest update installed. I have both the Pluto and Lime Mini SDRs installed.

Whilst checking the Portsdown sig gen frequency accuracy using a counter with a GPS disciplined clock, I noticed a problem when using the Lime Mini. The output is exactly 1MHz low of the displayed frequency; I used 1275MHz. It was actually a further 173Hz low, but that's just down to the Lime's onboard clock.

If I switch to the Pluto, it comes out on the correct frequency, ignoring the -14kHz offset due to the Pluto's 40MHz clock error.

My Lime is on frequency in DVB-S/S2 modes.

Can someone confirm this issue? Not a high priority for Dave, but I know some of us have been using the sig gen to align gear.

Clive

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Re: Signal Generator error with Lime Mini

Post by G8GKQ » Fri Mar 04, 2022 4:03 pm

Hi Clive

Do you have the LimeDVB firmware loaded on your LimeSDR?

If so, that would account for the issue. The SigGen sets the Lime LO to 1 MHz below the desired frequency and then uses the internal oscillator to generate a 1 MHz signal that is applied as USB to the LO. This allows better control of the output amplitude.

The 1 MHz LO is not available with the LimeDVB firmware, so the only output is the LO; 1 MHz below the desired frequency. Unfortunately, I have been unable to find a reliable way of detecting the loaded firmware, otherwise I would implement a warning.

There is no problem with switching between the firmwares; I checked with the designers - it's an EEPROM with a 10,000 write cycle life.

It could also be that you haven't calibrated after a frequency change - this leads to poor LO suppression.

Dave

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G3GJA
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Re: Signal Generator error with Lime Mini

Post by G3GJA » Fri Mar 04, 2022 5:11 pm

Hi Dave

Yes, I do have the Lime DVB firmware installed.

I have GPS locked HP sig gen to 2.06GHz and the not so accurate Pluto to provide a signal up to 4.4GHz, so I won't bother with switching the firmware unless I'm desperate for something more accurate above 2GHz.

I'd probably be better off using the second harmonic of the HP!

Thanks

Clive

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Re: Signal Generator error with Lime Mini

Post by G8GKQ » Fri Mar 04, 2022 5:23 pm

Thanks Clive

I've updated the Wiki with this information. https://wiki.batc.org.uk/Portsdown_4_Si ... put_Device

Dave

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