开云体育

Eric,

A free reference level is an excellent addition.
Accurate measurements across 1 to 30 mhz is sufficient for many of us.

Page 12 table 7 of the Si4432 datasheet shows that the GPIO outputs
can be programmed to four different levels.
That table also shows that with the drive level set to its maximum of Iomaxhh,
it might be able to deliver only 1.8 ma, or as much as much as 5.4ma.
So GPIO's on some Si4432 chips might have 3x more available output current than others?
due to process variations,?die location on the wafer, or phase of the moon.
That's 3*3= 9 times more power, almost 10dB.
We will probably never see it that bad, but even 2 or 3 dB variation is not welcome here.

I suggest trying the following network at the GPIO pin, where "out" goes off to the 250mhz LPF:
? ? GPIOpin-----0.1uF-----1.5Kohm-----out-----50ohm-----GND

The series cap allows the GPIO pin to balance the load between the high and low side drivers,
reducing the work it must do considerably.? It also centers the "out" signal around ground level.

The 1.5k resistor limits the GPIO current to around 1ma.

The 50 ohm resistor establishes the impedance that any reflections coming back out of the LPF see.
(The impedance seen is the parallel combination of 1.5k and the 50 ohms, so 52 ohms would be ideal.)

The 1.5k resistor is driving the parallel combination of our 50 ohm resistor and the 50 ohm impedance
of the 250mhz LPF, or 25 ohms.? Assuming a 3v square wave into the 1.5k resistor,
the voltage at the output will be reduced to a pk-pk square wave of? ?3.0*25/(1500+25) = 0.0492 volts.
Assuming the fundamental has a peak voltage of half that square wave,
the fundamental signal out is? (0.0492/2)/1.414? =? 0.0174 volts rms
and has a power level into the 250mhz LPF of? 0.0174*0.0174/50 = 6.055e-6 Watts, or 0.006055 mW.
In dBm, that's? ?10*m.log10(0.006055) = -22.2 dBm.

If we don't need a -22dBm signal, then increase the value of the 1.5k resistor.
This will reduce any dependence on device charactreistics.
A 4.7k resistor (or even 10k) may give more consistent results.

Eric has apparently just put a 50 ohm load on the GPIO pin and measured the same -22dBm signal that I have calculated.
Without the 1.5k limiting resistor, I would have expected considerably more power in his measurement.
However, if he did not have the DC blocking cap, that could well be a loss of 6dB right there.
?
There's almost certainly a few errors in what I am proposing above, let us know if you see them.

Also, I'm showing far more precision in my calculations than needed,
we'll be doing good if my figures are within 10%.

Jerry, KE7ER