Question: Why does is the transfer function of the AD8345 (SSB Pout vs. Baseband Differential Input Voltage) not a straight line? Shouldn't the gain of the device be constant vs. input level. Also as the baseband input level doubles from 0.5Vpp to 1Vpp, the output level increases by 7 dB instead of the expected 6 dB. What is going on?
Answer: The SSP Pout line is curved because of the logarithmic scale on the right. If you were to change the horizontal scale to logarithmic volts, you would see a straight line until the onset of compression (aternatively if you were to make the vertical scale on the right, linear, it would also yield a straight line).
When we plot this data on our newer datasheets, we use a logarithmic voltage scale on the horizontal axis (see example plot below). This, I think, is more useful because it is much easier to identify the device's ouput compression point.
You note that as the baseband drive level doubles from 0.5 Vpp to 1Vpp, the output power increases by 7 dB instead of the expected 6 dB. I agree that this is unexpected; it should indeed be 6 dB. I can only assume that it is a slight plotting error. Note that in Figure 16 of the AD8345's Rev. B datasheet, the scale for SSB Ouput Power is off by approximately 3 dB.