Another way to put it in perspective-the Anritsu FDR's bottom out at 54 dB RL. That's as far as it can measure. So 40 dB is really good, and should be enough for just about anybody.
toggle quoted message
Show quoted text
On Fri, Jan 29, 2021 at 08:52 AM, <roncraig1@...> wrote:
The required ultimate return loss measurement accuracy depends on what you
want to do. For just looking for 50 ohm antenna matching.25-30 dB return loss
measurement capability is sufficient. For measuring low or high component
impedance, the more ultimate capability of the instrument the better. The
farther away you get from 50 ohm the worse the measurement accuracy. 5 ohms
to 1000 ohms is the realistic usable impedance range for a 50 ohm reference
bridge. Look at a 50 ohm referenced Smith chart and see the return loss
difference between a 500 ohm and 1000 ohm impedance.
The accuracy of the resistors in the nano input bridge circuit is first item
effecting accuracy. The bridge resistors shunt parasitic capacitance reduces
higher frequency accuracy. Nano PCB also contributes some accuracy
limitations. Harmonic sensing has less signal reducing ultimate capability.
Calibration nulls out some of these effects but calibration compensation
degrades the farther away from 50 ohms, short, and open calibration points the
load impedance is.
Spend $10-$15 on a good 50 ohm calibration load from a reputable manufacturer
if you are working above HF bands. Some cheap ones are really bad at higher
frequencies. Calibration is only as good as your references.
