Saturday, February 20, 2016

Measuring output power of Adafruit Si5351 board

Since I'm planning to use a Silicon Labs 5351 CMOS Clock Generator to make WSPR signals on my new tracker, I've been doing some testing with an Adafruit Si5351 protoboard.  I can see it generating a reasonable signal, but my Service Monitor (IFR FM/AM-1200S) isn't capable of measuring output power in the milliwatt range.

Unfortunately, the DB numbers in the display are relative, and not actual for low power input like this.  Instead, I tried to make measurements using my oscilloscope.  My scope doesn't have a 50ohm input, so I wired it up through a splitter and attached the ends to my scope probes.

Feeding the output directly into my oscilloscope, I could see that it was on frequency, and generating 2.46v peak-to-peak.

Output power is measured as:   P = Vrms^2 / ohms.  We have 2.46 peak to peak.  Vrms = Vp2p/2 * .707.  So, Vrms = 0.870v.  The Si5351-b outputs at 50 ohm impedance, so P = 0.870v^2 / 50ohm = 15mw.   That's curious, as other Pico tracker builders are reporting an output power in the 20mw range.   It's possible that all the harmonics are accounting for the difference in power.  I'm not sure.

The Adafruit protoboard has it's own LDO regulator, and will operate anywhere between 3.3-5v. During my testing, I was feeding it 3v3.   With my multimeter, I could see it was providing 3.11 volts to the 5351 chip on the board.

My tracker will be running at 2.5v.  Presuming (and that's a big presumption) that the output power will be proportionally diminished, then my board would be making (2.5v / 3.11v) * 15mw = 12mw. That seems pretty low.  I'll want to do more experiments with my own board when I get it together.

1 comment:

  1. Vrms = Vp/sqrt(2) is only valid for sine waves. Your signal looks more more like a triangle, so Vrms should be Vp*0.577. So with Vpp 2.46 this would be (.577*2.46/2)^2 / 50 = 10 mW.
    ( Vrms for sine: *.707, triangle:*.577, sqr:*.5 )

    I've been amazed by the incredible power levels many report. Main error may be by not terminating the output with 50 ohm. The 5351 outputs 3,3 V max, as the supplly is 3,3 V max. With an output impedance of 50 ohm and terminated with 50 Ohm the output voltage could not exceed 1.6 V, ~11dBm at low frequencies where the signal is a nice square.
    Further more the power measured is the sum of all harmonics that form the square. After filtering those out the base frequency has even less power.