"Where's the SMA connector?"

A project log for SineStick

What is the opposite of an RTL-SDR? This.

carbonCarbon 07/20/2019 at 16:140 Comments

Good question.

Here is the problem: the Si569 only offers output up to 3 GHz when using a differential signal output (specifically LVPECL, LVDS, CML).

The majority of projects I've seen that us SiLabs oscillators use the CMOS output variants which supports single ended operation, but can only output a signal up to a few hundred MHz. (For this particular chip, only up to 250 MHz.)

(By the way, if you're interested in what all those output modes are, I highly recommend "Application Report SLLA120 - Interfacing Between LVPECL, VML, CML, and LVDS Levels" by Nick Holland at Texas Instruments.)

Here is the other problem: I'm not an actual RF engineer, I just play one on the Internet. I really don't know how to convert a high frequency differential signal into a single ended signal - or if I even should in this case.

I briefly looked into LVDS converter chips but couldn't find any that were obviously fast enough. At this point I think I'm supposed to use a balun or signal transformer of some sort but this is outside my experience right now.

So, what to do?

Make it somebody elses problem! :D

Okay, seriously though, I'm hoping I can provide the base unit and somebody with more experience than I can take this thing a step further and make a circuit that an antenna will attach to. To do that, though, I have to export the signal from the board. The the idea that the average hobbyist would be using these units, I needed something that easy to use, durable, inexpensive, widely available, and with the ability to support high speed differential signals.

I assume the majority of you are familiar with the famous Andrew "bunnie" Huang - I was reminded of his Novena oscilloscope project where he faced a similar problem. His solution was using common SATA cables - these hit all of the above requirements, so I stuck a SATA connector on the end of the SineStick.

I used B channel for the differential output of the Si569. I wanted to push I2C through channel A but there was no way to also push power as well (not enough pins). I decided having power at the other end the connector was more important than I2C control, so the A- channel is attached to the (fused) USB 5V rail. The otherwise unused A+ channel is broken out to a test point so end users can make use of it.

The only downside is nobody seems to make board mount SATA plugs - the cable ends. There are very short SATA cables available, however, so it's not a huge issue.

If this device catches on I hope to see an ecosystem of filters, amps, and antenna connector boards.