Hooking up an Si5351A Voltage Controlled Oscillator to the Local Oscillator Input on an RTL-SDR

With most low cost RTL-SDRs the provided 28.8 MHz local oscillator is of low quality and will often have a sizeable frequency offset and temperature drift. To fix this the oscillator can be replaced with a higher quality one.

Over on his blog Milan has showed how he used a Si535A voltage controlled oscillator on a breakout board from etherkit.com to replace the RTL-SDR’s low quality oscillator. The Si535A is a clock generator circuit which is capable of generating almost any frequency up to 200 MHz with 0 ppm offset from a 25 MHz TCXO oscillator. To connect the Si535A break board to the RTL-SDR, Milan soldered an edge mounted SMA connector to the RTL-SDR and connected it to the RTL-SDR’s clock input with a 10nF capacitor.

Milan also writes how the Si535A has options for different drive currents and writes that by lowering the drive current from 8 mA to 2 mA the spurious signals found at multiples of 28.8 MHz in the RTL-SDR are reduced. He also writes the the Si535A also allows you to detune the frequency a bit which can help to tune around any spurious signals. This could be useful for example when using rtl_power as you could create a spurious free plot.

Using a Si5351A to replace the local oscillator on an RTL-SDR.
Using a Si5351A to replace the local oscillator on an RTL-SDR.
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I was just wondering, if you could control the SI5351 BoB from within the SDR software. If you could do that , then PSDR would be fun and cheap to use.

Has anyone tried it ?

John Silver

The trouble with the Si5351 is that you have to re-program on power up? Unless there is a way to hard program? Having to re-program the si5351 to use the dongle everytime you use it, is pretty inconvienent 🙁


There is NVM memory on the chip but you need the silicon labs/now skyworks programming dongle ($$$)

Tim Havens

How long is the warm up time on the TCXO? My Nooelec RTL dongle with a TCXO seems to drift for hours (albeit slowly).

I’ve recently built an OCXO based upconverter, and was thinking that if the TCXO in THIS tends to behave similarly maybe I could replace the TCXO with an OCXO and finally get where I really wanna be.



Did anyone already try programming the Si5351a with the same 28.8 MHz freq to two outputs for two dongles and see quality of coherent radio done in this way ? With the TCXO it should be better than previous experiments with the dongle oscillator like in https://www.rtl-sdr.com/passive-radar-dual-coherent-channel-rtl-sdr/


I also did this when my 5351’s came in earlier this year. I also use one of the other outputs as a L.O. (along with a mixer, filters, and a switch) to make an upconverter. It all fits nicely in one little box.

Kevin McCormick

Oh I love this. The Si535A is relatively cheap as a kit with TCXO.