Analyzing Transmission S-Parameters

For S-parameters, transmission is a bit more nuanced than reflection as there aren’t explicit rules of thumb to follow. A “good” transmission value is heavily dependent on the device or application. For example, for an active +3dB amplifier stage, you want a +3dB in the passband whereas for a post VNA calibration check with a through standard from port 1 to port 2, you want 0dB (+/-0.1dB or better depending on the VNA) spanning the measured frequency band. However, if you look at transmitted power percentages, you can gauge what any LogMag transmission plot is telling you regardless of the device or application. It’s not perfect, but it gives you some information as to what you’re looking at.

Here’s a transmitted power percentage table you can reference.

-40dB (0.01% power transmitted)

-30dB (0.10% power transmitted)

-20dB (1% power transmitted)

-10dB (10% power transmitted)

-3dB (50.1187% power transmitted)

0dB (100% power transmitted)

+3dB (199.5262% power transmitted)

+10dB (1000% power transmitted)

+20dB (10,000% power transmitted)

+30dB (100,000% power transmitted)

+40dB (1,000,000% power transmitted)

It’s worth noting that you’re not magically multiplying the percentage of input passive power, but the active DUT has provided gain, drawing energy from its power supply.

As you can see, this curve isn’t linear! The further you climb in LogMag transmission dB, the power ratio increases exponentially (especially above 0dB). Every 10dB increment represents a 10x increase in the power ratio. The inverse goes for signals attenuating (below 0dB). Every 10dB represents a 10x decrease in the power ratio. The transmitted power percentage approaches 0%. This logarithmic relationship applies to both reflection and transmission S-parameters.