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.

6 comments

  1. charudatt

    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 ?

  2. 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 🙁

  3. 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.

    Ideas?

  4. KD0CQ

    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.

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