Table of Contents

RF isolation

Extraneous RF signals add to the measurement as noise. RF isolation of the DUT from the outside interference and from ambient paths of the test signal is critical when measuring low signal levels. RF isolation is particularly important for measuring RX sensitivity, rate adaptation over a wide dynamic range or roaming behavior.

data rate vs rssi screen.

Data rate adaptation behavior (top) vs. RSSI (bottom) measured
in the octoBox. RSSI is measured all the way down to -96 dBm,
demonstrating the RF integrity of the octoBox environment.

Interference is difficult to control at Wi-Fi frequencies, since signals above 2.4 GHz propagate easily into the test chamber via copper lines, including Ethernet, USB or power lines connecting to the DUT through the chamber walls.

This plot of rate adaptation vs. RSSI demonstrates the effectiveness of the octoBox isolation. RSSI is measured down to -96 dBm, which means any noise coupling into the chamber is extremely well-controlled or this low signal level would have been buried in noise.

Without isolation, typical coupling loss between Wi-Fi devices (e.g. between AP master and client DUT) is only about 40 dB, even when antennas are removed and antenna ports terminated. At 5 GHz, with a shorter wavelength, Wi-Fi coupling is much worse.

Without shielding and filtering of power and data lines, metrics such as RX sensitivity, data rate or MCS (modulation coding scheme) adaptation, roaming behavior and other tests can only be performed down to about -60 dBm.

Design for optimum isolation

The octoBox is carefully designed for optimum isolation all the way up to 6 GHz. Isolation is a function of sealing the doors and seams of the box and filtering all the power and data lines entering the box.

Isolation is particularly difficult to achieve at higher frequencies in the GHz range because the shorter the wavelength of the radio transmission, the easier the signal propagates through seams in the box or over the copper cabling used to communicate with or power the test equipment and the DUT.

Shielding, gasketing and sealing are not enough to get superior isolation. Just as important is filtering of copper lines (power, Ethernet, USB) since these can act as antennas bringing in noise from the outside.

Ethernet and USB filter design can be challenging because it must maintain the integrity of the high frequency data signals while attenuating the RF frequencies that interfere with the wireless test signals. Ethernet filter poses the additional challenge of passing through the high-current PoE (Power over Ethernet).

OctoBox isolation

With its careful design, incorporating shielding, right-angle seals, gasketing and feed-through power and data filtering, octoBox achieves excellent isolation of:

Better than 90 dB at 2.4 GHz
Better than 80 dB at 6 GHz

The octoBox isolation is measured with a fully cabled test setup, which demonstrates the effectiveness of feed-through filters.

Figure 4: Measured isolation of the octoBox at 2.4 GHz right angle seals