External 40MHz tcxo by Phil Crump

Projects LimeSDR
1

I am using my limesdr-mini for Oscar-100 satellite tx/rx.
As it is ssb mode on 2400MHz the internal tcxo is drifting a little bit from cold to warm.
I see that Phil Crump uses an external 40MHz tcxo without modifications on de limesdr-mini.
How is that possible? I thought you have to re-solder a 0 Ohm resistor before connecting an external tcxo.

So, my question: where did he connect the 40MHz signal on the limesdr-mini circuit?

See: https://github.com/philcrump/limesdr-vctcxo
I found a photo of it but you cannot see the connections to the limesdr-mini board.

2

I’ll point you at this thread: LimeSDR Mini external clock reference config permanent? and I’d recommend reading the two links on the last line of my post in this thread LIMESDR on SRSLTE : unable to find network on LTE device

And maybe check the 3rd and last page of the schematic for a LimeSDR mini.

P.S. That wire that in the photo that they are using to connect the clock, must be generating a nice amount of RFI at 40MHz (and all the harmonics from a 3.3v CMOS square wave). My guess would be that they removed the the U.fl (J8) from the Mini (and moved the 0 ohm resistor) . Or another possibility is that they left the 0 ohm resistor alone and removed the RTX5032A chip and are connecting to the “ground” (pin 4) and “RF out” (pin 6) pads from that part.

Thinking about it a bit more at 40MHz the wavelength would be ~7.5 meters, so probably not that bad at all. Even the 3rd harmonic at 120MHz (~2.5m) would not bad either. Nor the 5th harmonic at 200MHz (~1.5m), because it would not be a very efficient antenna. And by the time you get up to say the 37th harmonic at 1480MHz (~ 0.2m) the power at that harmonic would be so low that it is negligible. Just because the length of wire used is so short (~0.02m) it is probably not a problem at all.

3

Hi,
For the 40MHz connection to the limesdr I removed the 0R in R59 and then used both pads on R62 to solder the end of the enamelled wire. The “no reconfiguration” refers to not having to reconfigure the LimeSDR to accept another clock frequency (such as 10MHz.)
Yes the bare wire I used between the boards in my unit is ugly and I don’t like it. You can see from the board design that I started with good intentions for SMA sockets, but temptation to get the station up and running quickly with a lack of suitable cable got the better of me. I’ve not seen any adverse effects in the current use as a QO-100 NB/WB RF driver and NB IF receiver and the die-cast box shields it well from other kit, but when it gets back on the bench then it’ll likely be replaced with a U.FL pigtail.
Phil M0DNY

1 Like
4

Thanks MZS and Phil. Now i understand where no reconfiguration refers to. I will move the 0R resistor to R62 and i have ordered a 10MHz ocxo print with 4x multiplie-er at ali-express. Lets see how it will work.
73’s PE1RTC

5

I think that 10MHz should work. It just means that any software that expects the clock to be 40MHz may need to be modified, which could end up being a lot of work. Unless you plan on just using one application for your modified device. The reason I mention that is because the phase noise (or clock jitter) on a cheap clock multiplier (usually one with the part numbers sanded off) might be very bad.

6

Yes, it will work with 10MHz but then every time i start the lime sdr i have to tell the software that i am using 10MHz. The 10MHz external ocxo with 4x multiplier is cheap so i first gonna try that also because i have good experience with a 10MHz ocxo i bought from ali express somewhere in the past.
If the clock jitter is too bad i always can go for the circuit Phil is using or buy a better 40MHz ocxo or gpsdo.