ANSI C63.25.1-2018 pdf download.American National Standard Validation Methods for Radiated Emission Test Sites, 1 GHz to 18 GHz .
5.2.3 Measurement procedure
a) Set the start and stop frequencies on the VNA to frequencies below and above the desired validation frequencies. The start frequency shall be set to 500 MHz. The stop frequency shall be set to a value equal 10% of the validation frequency span above the desired upper frequency of interest. For example, for the frequency range from 1 GHz to 6 GHz, the start and stop frequencies of the VNA shall be set at 500 MHz and 6.5 GHz, respectively. For the frequency range from 1 GHz to 18 GHz, the start and stop frequencies of the VNA shall be set at 500 MHz and 19.7 GHz, respectively. The additional frequency ranges above and below the validation frequency range are needed to account for the gating edge effects where the data at the band edges are unreliable; see Annex A for additional information. NOTE- Although the start and stop frequencies indicated in this section may exceed the specified antenna bandwidth, the data at band edge for time gating in the following steps is improved by including the data outside of the specified band. A transmission measurement shall be performed and the measurement parameter S21 (in decibels) selected on the VNA. The maximum frequency step size is 1.5 MHz (for example, an acceptable setting is start/stop frequency of 500 MHz and 19.7 GHz, with 16 001 measurement points; this yields a frequency step of 1.2 MHz). The sensitivity of the test set-up shall be verified and an appropriate IF bandwidth and output power setting shall be used. Lower IF bandwidth or/and higher output power, or a pre-amplifier, may be needed to improve the signal to noise ratio. A signal to system noise ratio of at least 20 dB shall be achieved.
b) Perform a VNA response calibration at the ends of each cables used to connct the transmit antenna and the receive antenna. It is acceptable to perform a response (thru) calibration if there are no significant internal cable reflections, for example, if the cables on each side do not have interconnections. A full two-port calibration may be used to remove any internal cable reflections. and additional systematic errors of the VNA.
c) Table 2 shows an example set-up for a quiet zone diameter of 2 m, and Table 3 shows an example set-up for a 1.5 m quiet zone. Connect the cables to the antennas, and place the transmit antenna (for example, a mini-biconical antenna) on the turntable in one of the measurement locations defined in Table 2 (or Table 3). Place the receive antenna (for example, a double-ridged waveguide horn antenna) in the receive position at the distance indicated in Table 2 (or Table 3). . Adjustments may be made if a quiet zone of a different size is needed. The basic positions in Table 2 and Table 3 are similar to the ones in CISPR 16-1-4:2017-01.
d) First, set the VNA to perform an S21 sweep. Next, perform a time-domain transform of the S21, for example, by enabling the time-domain mode of the VNA. The resulting view is the impulse response of the antennas. The earliest impulse is the direct antenna-to-antenna response. The time of the direct response should correspond to the antenna-to-antenna free space propagation distance plus each of antenna internal cable lengths (i.e., cable within antenna mounting shaft or boom, between the“bulkhead” connector and the feedpoint connection of the radiating elements).
e) Set the gate such that only the direct antenna response and any ring down time (see Annex A for discussions on antenna ring-down time) associated with the antennas are within the gate span. The time gate is to be set to band-pass mode. The purpose of the gate is to retain the direct time response between the antennas and to suppress all other responses. For typical antennas, the antenna ring-down times are less than 2 ns. It is sufficient to set the time gate center at the peak of the main response, and the gate span symmetrically around the peak at a value of 4 ns (i.e., twice the antenna ring-down time).ANSI C63.25.1-2018 pdf download
