Message	Date	From	Subject
14128	2017-07-02 13:27:44	Dana Myers	Si5351A output drive waveform and observations
I connected a 54 ohm load via a 100nF capacitor to the output of an Si5351A (one of Hans' QRP-Labs boards). The Si5351A is programmed for 1MHz CLK0 output with Drive Strength = 8mA (54 ohms as 2 x 27ohm resistors because they were handy, and it's close enough) Here's the scope capture of this: From this, I calculated output resistance of the Si5351A as 26 ohms. (54 ohms * (3.3V - 2.24V) / 2.24V) The CMOS output is sourcing 20.7mA from the Vdd rail, then sinking that 20.7mA into the ground rail. With a 50% duty cycle, you'd expect 10.3mA average DC current flowing from Vdd to ground. I measured the current used by the Si5351A+TCXO with no load as 23.8mA and with the 54 ohm load as 33.8mA; giving 10mA as the "average load current". Slightly less than I expected but well within the accuracy of my measurements :-) In this case, the measurement both shows that the CMOS output is reasonably modeled as a Vdd source with a 26 ohm series resistor, and that the output will source/sink ~20mA. I don't really know how good for the long-term reliability of the output it is to run at 20mA though :-) - I'd buffer it to drive a 50 ohm load. 73, Dana K6JQ

I connected a 54 ohm load via a 100nF capacitor to the output of an Si5351A (one of Hans' QRP-Labs boards). The Si5351A is programmed for 1MHz CLK0 output with Drive Strength = 8mA (54 ohms as 2 x 27ohm resistors because they were handy, and it's close enough) Here's the scope capture of this: From this, I calculated output resistance of the Si5351A as 26 ohms. (54 ohms * (3.3V - 2.24V) / 2.24V) The CMOS output is sourcing 20.7mA from the Vdd rail, then sinking that 20.7mA into the ground rail. With a 50% duty cycle, you'd expect 10.3mA average DC current flowing from Vdd to ground. I measured the current used by the Si5351A+TCXO with no load as 23.8mA and with the 54 ohm load as 33.8mA; giving 10mA as the "average load current". Slightly less than I expected but well within the accuracy of my measurements :-) In this case, the measurement both shows that the CMOS output is reasonably modeled as a Vdd source with a 26 ohm series resistor, and that the output will source/sink ~20mA. I don't really know how good for the long-term reliability of the output it is to run at 20mA though :-) - I'd buffer it to drive a 50 ohm load. 73, Dana K6JQ