I am currently working on a project to time synchronize multiple RTL-SDR's. For capture frequency hopping GSM.

I have read that connecting the output of a single TCXO to multiple dongles only works up to 3 dongles due to increased inductance of the circuit. I need to have at least 4 to be time synchronized for my project.

How do I do that?

I am currently a student of Electronics 2nd year. I tried to figure it out online. Most of the circuits use fan out clock buffers, and I don't have much experience working with them.

So I am looking for a way to get 4 outputs from a single output of a clock to feed to my clock input of different dongles.

  • \$\begingroup\$ I’m really surprised distributing a 28.8MHz clock is that troublesome. 28.8MHz is not a very high frequency! Anyway, if you can eventually make it public when you have finished your project, I’d love it if I could read about it. \$\endgroup\$ – user2233709 Mar 19 '17 at 9:48

A dedicated clock buffer IC is the most reliable method at the frequency you have. I have used IDT's before. Have a look at the IDT74FCT3807/A, a 1-in 10-out device for a quid.

Inherent in your question is that you'd be putting this on a PCB rather than a breadboard. The latter would give you such skew problems that you might as well put the clock into the inputs of a 74-series hex inverter and take your clocks off its outputs.

Follow the standard good PCB design guidelines for distributing your clocks. Make sure that the tracks are over a ground plane and of similar length (+/-10% or better). Put a series resistor (e.g. 22 ohm but look into this) between the incoming clock and each dongle at the point closest to the load.

But the same is true of the other signals communicating with these dongles. You're working hard to get everything's clocks close to truly synchronised here. You need to ensure that other signals between these devices are of suitable quality and layout. It's hard to detail further on that without knowing more about your application.

  • \$\begingroup\$ Note to OP: Remember to terminate unused and enabled outputs (or EMI could cause all manner of interesting problems). \$\endgroup\$ – Peter Smith Mar 19 '17 at 14:26
  • \$\begingroup\$ @PeterSmith, thank you and spot on - I meant to put that in and forgot. OP, put a dummy load on each unused clock output e.g. 1 K to GND. \$\endgroup\$ – TonyM Mar 19 '17 at 14:30
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    \$\begingroup\$ Thank you so much man !! Helped a lot. And Thanks for being so thorough. \$\endgroup\$ – Kushagra Dixit Jul 17 '17 at 11:14
  • \$\begingroup\$ @KushagraDixit, you're very welcome, glad it was useful :-) \$\endgroup\$ – TonyM Jul 17 '17 at 16:33

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