DELAY has to be understood here as "Group Delay". It highlights the transit time of a signal through a DUT (Device Under Test) versus frequency. It is usually currently related to transmission measurement (s21).
Its calculation is the mathematical derivative of the phase with respect to frequency. If there is PHASE distortion, i.e. a breakout on its monotonous variation according to frequency, the group delay remains no more at a constant value.
Group Delay = -(1/360)*(phi2-phi1)/(f2-f1)
For the NanoVNA, (f2-f1) is the elementary frequency step (Hz) between two measurement points, which depends of FSTART, FSTOP and choosen number of measurements points (101 ... 401). It means that (phi2-phi1) is the phase value difference (Degrees), between two neighboring measurements points.
Group Delay is a relevant parameter, when for example you consider a non-sinusoidal signal passing through a filter. If Group Delay is not constant, the shape of the signal will be altered at the output of the device, due to a possible time shift on its harmonic components.
A last word about Group Delay when related to reflection measurement (s11). It's less intuitive, but we can consider this example with a DUT which is an opened coaxial cable (length 2 meters), one of my favorite educative DUT ! Here the Group Delay (around 18,2 ns, see snapshot) highlights the total transit time at Port 0 level, i.e. a round trip of the signal through the coaxial cable. From it you can calculate the electrical length of the coaxial cable, and if you know the velocity factor, its physical length.
Round trip : 300.000.000 (m/s) * 18,2 E-09 = 5,46 m
Electrical length : 5,46 / 2 = 2,73 m
Physical length : 2,73 * 0,73 = 1,99 m 0,73 is the velocity factor value for RG58A/U cable type
As advanced firmware for NanoVNA now includes automatic calculation for coaxial length (thanks to DiSlord) based on Smith chart, the previous method is not necessary the most accurate or relevant. It is just for an explanation of Group Delay in s11 measurement context.
Jean-Roger / F6EGK
