I'm in the midst of designing a small WSPR tracker based on SMD technology for use with Pico Balloons. I hope to attempt a circumnavigation with my design and home built envelope.

My tracker will be using a Silicon labs Si5351 CMOS clock generator. Per the datasheet, the clock generator is spec'd to an 50 ohm output impedance. Note, that there are different Si5351 parts. The "-B" parts seem to be the ones you can find now, and they have 50 ohm termination. The earlier non-"-B" parts, were rated at 85 ohm.

Looking at the output of an Adafruit Si5351 protoboard on a spectrum analyzer, the output signal would definitely benefit from some filtering.There are some sizable harmonics.

**First pass at a filter plan**

I'm new to RF design, but as I understand it, I should probably have a DC Coupling Capacitor as well as a filter network. I'm going to begin with a simple Pi Lowpass filter and see how that goes.

In this design, C22 is the coupling cap, and the Pi filter is comprised of C2, C5 and L3.

**Coupling Cap**

The target output frequency is on 30m, at about 10.1 Mhz. I found the following gem on the interwebz, so it must be true:

Substituting in 10.1Mhz into the formula, I get a value ofYou generally want the capacitor reactance (impedance) to be much less than the circuit impedance.

The reactance of a capacitor is Xc = 1/(2*pi*f*C) where f is the frequency of operation.

Thus the higher the frequency and the larger the capacitor, the smaller the reactance.

Thus for a typical 50Ω circuit impedance, you probably should have the capacitive impedance no more that about 1% of 50Ω or 0.5Ω.

So, for example, at 2.4GHz frequency, the capacitor should be no smaller than C = 1/(2*pi*2.4GHz*0.5Ω) = 132pF.

**31.5nf**.

**Filter values**

I found a Chebyshev Pi LC Low Pass Filter Calculator on the web. I plugged in values as documented below:

**Unanswered questions**

I'm not an RF guy, so I have no idea if I've done any of this correctly. In particular, I have these questions:

- I put the cutoff at 11 Mhz, since WSPR is very low bandwidth. Is that too aggressive?
- Answer: Probably. I'm rethinking using 20m rather than 30m for flight.
- Is the coupling cap in the right place? Should it be
*after*the Pi filter? - Answer: Yes, according to a colleague who is an RF engineer.

I'll update this into more of a "This is what I did" document as I finalize my design.

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