Guidance for Power Spectral Density (PSD) Regulatory Testing#

Deciding on the appropriate testing method and the data pattern to use is a crucial prerequisite of regulatory testing. Without a clear description of such, measurement results cannot be interpreted definitively and can bring up doubts regarding credibility. Therefore, a mandatory set of instructions must be proposed for regulatory testing that best represents the common use case of the Device Under Test (DUT).

Testing Method#

The FCC 15.247 testing procedure recommendation is to follow ANSI C631.0-2020 Chapter 11.10.3. This method (AVGPSD-1) allows the measurement of the average (rms) PSD instead of peak PSD, resulting in a more favorable margin to the 8 dBm / 3 kHz limit. The procedure may be used when the average (rms) conducted output power was used to determine compliance with the total output power limit. The method AVGPSD-1 uses trace averaging while the DUT is transmitting at full power throughout every sweep. If the instrument has a power averaging (rms) detector, then it must be used; otherwise, a sample detector must be used.

The following list of instructions is from the referenced chapter of ANSI C63.10-2020:

  1. Set instrument center frequency to DTS channel center frequency.

  2. Set span to > 1.5 times the OBW.

  3. Set RBW to: 3 kHz ≤ RBW ≤ 100 kHz.

  4. Set VBW ≥ [3 × RBW].

  5. Detector = power averaging (rms) or sample detector (when rms not available).

  6. Ensure that the number of measurement points in the sweep ≥ [2 × span / RBW].

  7. Sweep time = auto couple.

  8. Use trace averaging (rms) mode over a minimum of 100 traces.

  9. Use the peak marker function to determine the maximum amplitude level.

  10. If the measured value exceeds requirement, then reduce RBW (but no less than 3 kHz) and repeat. Note that this might require zooming in on the emission of interest and reducing the span to meet the minimum measurement point requirement as the RBW is reduced.

Selecting the Appropriate TX Packet Type#

The DUT must be configured to transmit continuously, which means a continuous modulated PN9 sequence (with no preamble) is sufficient and is the packet type recommended by Silicon Labs.

RAILtest Example#

A complete RAILtest example for the recommended continuous modulated PN9 stream is shown below.

'setzwavemode 1 3'      ;; setup for Z-Wave mode
'setzwaveregion 11'     ;; set Z-Wave region to 11 (region 12 could also be used)
'setchannel 3',         ;; set Z-Wave channel to 3
'setpower xx raw'       ;; set PA output power to setting number xx
'settxstream 1'         ;; enable modulated PN9 stream

Note: AVGPSD-1A (alternative) method can be used instead of AVGPSD-1. If a continuous (D > 98%) modulated PN9 stream is not possible, then sweep/gate triggering methods are required if the recommended AVGPSD-1 or its alternative versions are the chosen test methods. If the requirements are not met, AVGPSD-2, AVGPSD-3 or their alternative versions must be followed.