开云体育

Mike,

I haven't made any such measurements, so you're ahead of me on that.
I have heard in other forums that crosstalk and perhaps other forms of distortion
creep in as the Si5351 output buffers get loaded up beyond a few ma.

With capacitive coupling, the Si5351 is? effectively driving your
50 ohm load into 3.3/2=1.65 volts, which reduces the currents by half.
So if you see 1.6v pk-pk, that's 0.8v/50ohms=16ma, which is not just too far
from the programmable 8ma max for the output buffers of AN619.
But those CMOS buffers are working awfully hard.

The datasheet says that the Si5351 has an output impedance of 50 ohms.
So the hypothetical 3.3v square wave voltage source inside the Si5351 is driving
50 ohms internal plus your 50 ohm load.? So a 1.6v square wave makes perfect sense.

I agree, if you are seeing a 1.6v pk-pk square wave into 50 ohms, then that is 11 dBm.
Some of that energy is in the harmonics, but we're in the correct ballpark.
When a mixer says it wants 7 dBm, I believe that's defined as a source that can
drive an 11 dBm sine wave into a 50 ohm resistive load, even though the non-linear
diodes will make that waveform very un-sine-like in both voltage and current.?
Whether a diode ring mixer is better off with?a square wave is perhaps up for debate,

There is only one pin for the 3.3v rail to power the output buffers on the Si5351,
so with two or three outputs loaded up like that you may get worse results.

Your results justify the uBitx approach of driving a resistive pad directly from
an Si5351, and the uBitx does seem to work well enough.? Though it has its birdies.??

Jerry, KE7ER