Zigbee Device Compliance and Manufacturing Tests

This article covers zigbee device compliance and manufacturing tests, along with the test setup typically followed by test and certification or pre-certification companies.

To perform compliance tests on the transmitter and receiver parts, an RF Signal generator/analyzer along with a baseband zigbee application is needed. This RFSG and RFSA generates and analyzes zigbee compliant RF signals.

An automation application is also needed to put the zigbee DUT (Device Under Test) in various modes so that all the transmitter and receiver specifications can be tested. The figure below depicts a typical setup.

To perform tests on the DUT’s transmitter part, it is interfaced with an RFSA (RF Signal Analyzer). To perform tests on the DUT’s receiver part, it is interfaced with an RFSG (RF Signal Generator). RS232 is needed to control the DUT through firmware so that various parameters as per tests can be set.

Following are typical transmitter and receiver tests performed during zigbee pre-compliance testing.

Zigbee Transmitter Tests

The Zigbee transmitter tests include:

• Instantaneous Transmit power
• Power Spectral Density (PSD)
• Error Vector Magnitude (EVM)
• PLL Frequency convergence
• Phase Noise
• Spurious Emission
• IQ Measurements (constellation diagram)
• LO Leakage (carrier suppression)
• Tx/Rx turn around time

We will see major tests in detail below.

Transmission Power

The aim of this test is to verify that the transmitter output power of the zigbee DUT conforms to the limit set by the standard.

• Set the DUT to the unmodulated continuous transmit mode with the appropriate output power level. This is done either using smartRF studio or using RS232 control.
• Measure the output power on the spectrum analyzer.
• Verify whether the measured power matches the set value using the program on the DUT.
• Repeat the above steps for different frequencies and tabulate the results.

Power Spectral Density (PSD) Mask

The aim of this test is to verify that the PSD of the zigbee DUT conforms to the limit set by the standard mask. For example, the standard specifies: For frequency |f-fc| > 3.5MHz, the relative limit is -20dB and the absolute limit is -30dBm.

• Set the DUT to continuous transmit mode.
• Verify that the output power spectrum is well within the developed standard mask on the spectrum analyzer.

Error Vector Magnitude (EVM)

This test helps us verify transmission modulation accuracy.

• Set the DUT to continuous Tx mode with random modulated data.
• Measure the EVM of the transmitted zigbee signal using appropriate application software. Rohde and Schwarz FSQ also provides EVM measurements of zigbee compliant signals.

Transmission Center Frequency Offset

The aim of this test is to verify that the center frequency offset is within the limits.

• Set the zigbee DUT to continuous Tx mode.
• Set the RF center frequency to the desired RF channel and make sure it is an unmodulated signal.
• Note down the actual frequency displayed on the spectrum analyzer.

The difference between this frequency and the RF center frequency is the frequency offset.

• Compare the difference obtained above with the ppm limit specified in the standard limit or conformance document.

Spurious Emissions

The aim of this test is to verify that conducted spurious emissions are under the limit.

• Set the zigbee DUT to continuous Tx mode with random modulated data. Also, set the RF center frequency to the desired channel frequency.
• Measure the spurious emissions across the entire span of the spectrum analyzer.

Zigbee Receiver Tests

Typical zigbee receiver tests include:

• Maximum Input Power
• Receiver sensitivity
• Adjacent Channel Rejection
• Alternate Channel Rejection
• RSSI (Received Signal Strength Indicator)

Let’s understand a few of these tests below.

Receiver Sensitivity

The aim of the test is to verify that receiver sensitivity is conforming to the standard limit.

• Put the Tx-Rx in loopback mode with a variable resistor in between.
• Start the receiver first and set the payload and pattern type appropriately.
• Start the transmitter and measure the RSSI on the receiver side.
• Calculate PER (Packet Error Rate) as per the equation or read out this value in the zigbee DUT’s registers if available.
• Increase the attenuation until PER reaches a value of 1%. This will define the receiver sensitivity threshold.

Maximum Input Power

The aim of the test is to verify that the DUT performs satisfactorily with the maximum input power level defined in the standard. Do this test and the measured PER should be less than 1%.

Adjacent Channel Rejection

• Set the DUT to the desired RF channel frequency.
• Note down the power at this RF channel as well as the adjacent channel.
• Calculate the difference between them.
• Compare the result with the rejection limit specified.

The pass criteria is 0 dB for adjacent channel rejection.

Alternate Channel Rejection

• Set the DUT to the desired RF channel frequency.
• Note down the power at this RF channel as well as the alternate channel frequency.
• Calculate the difference between them.
• Compare the result with the rejection limit specified for Alternate Channel Rejection.

The pass criteria is 30dB for alternate channel rejection.

Energy Detection/RSSI

The RSSI parameter can be measured using a spectrum analyzer. It can also be read out from the registers of the DUT.

Zigbee Manufacturing Tests

In manufacturing tests, a few of the tests mentioned above are performed. The idea of manufacturing tests is to check the functionality of the zigbee device (DUT).

In manufacturing tests, more than one DUT is being tested simultaneously. Special provisions and a zig need to be designed to perform parallel tests on the multiple zigbee devices.

All the results are collected in text or excel files in the application running on the computer. The tests per device need to be performed in a few seconds, and the results will be checked later.