P-Series and EPM-P Power Meters for Bluetooth Testing

Bluetooth® is a technology specification designed for low-cost short-range radio communications between devices such as PCs, mobile phones, and other portable devices. On top of that, Bluetooth enables your devices to access the Internet.

Originally developed in Scandinavia, this technology that unites products has a code name that is inspired by a 10th century Danish Viking king, Harald Bluetooth, who united and controlled Denmark and Norway during an era when Scandinavian Europe was torn apart by wars and feuding clans.

Today, Bluetooth is administered by the Bluetooth Special Interest Group (SIG), whose members are leaders in the telecommunications, computing, network­ing, industrial automation, and automotive industries.

The Technology

Bluetooth operates within the industrial, scientific, and medical (ISM) band at 2.4 GHz. It is a short-range wireless communication standard defined as a cable replace­ment for a personal area network (PAN), an individual’s own personal space.

A cable replacement standard has been defined because cables limit mobility. They are cumbersome to carry around and are easily lost or broken. It is frequently difficult to diagnose failure in the connectors, and they are often proprietary. Bluetooth counteracts these limitations by being light, portable, robust, and not limited to one manufacturer.

To serve effectively as a cable replacement, Bluetooth keeps its cost comparable to that of cable by operating in the license-free 2.4 GHz ISM band, while remaining backward compatible whenever possible to avoid upgrades. The relaxed radio specification also enables single-chip integrated circuit solutions.

Frequency Hopping

The ISM band used by Bluetooth® is available from 2.40 GHz to 2.4835 GHz in most countries, although there are restrictions in some countries. In this band, Bluetooth uses frequency-hopping spread spectrum (FHSS) techniques to mitigate interference.

In countries without restrictions, the radio signals hop in pseudorandom sequences around all 79 available channels with a channel spacing of 1 MHz. Starting at the base channel of 2402 MHz, the frequency of the channels can be expressed as below: f = 2402 + n MHz

where n is the channel number with an integer value in the range from 0 to 78.

In countries with restrictions, a limited frequency hopping scheme with just 23 channels is used and accounted for in the Bluetooth specification. Both hopping schemes have 1 MHz channel spacing. This allows a simple radio interface design, whereby the baseband only has to specify a channel number and the radio multiplies this up to the appropriate frequency offset.

In this FHSS scheme, there are 1,600 hops per second, which is a hop every 625 μs. Part of this hop timing is taken up by a guard time of 220 μs, allowing the synthesizer time to settle. The frequency hopping implements time division multiplexing graph is shown in Figure 1. During the first 625 μs slot, k, the master device transmits while the slave receives. In the following slot, the slave may transmit and the master listen.

The radio must be able to be retuned and stabilized to a new frequency within tight time constraints. This is pushed further when establishing a connection; the hop rate is can be shortened to every 312.5 μs. As the radio is constantly hopping to different radio channels, this ensures that packets affected by interference on one channel can be retransmitted on another channel. To further enhance resilience, both automatic repeat request (ARQ) and forward error correction (FEC) form part of the specification.

Protocol Stack

Figure 2 shows a configuration of the Bluetooth protocol stack with different basic layers. At the base of the Bluetooth protocol stack is the Bluetooth radio. It modulates data for transmitting and demodulating received data. This layer defines the physical characteristics a Bluetooth transceiver must have. The Baseband and Link Controller controls the physical links via the radio, as­sembling packets and controlling frequency hopping. Ascending the stack, the Link Manager converts host controller interface (HCI) com­mands into baseband-level opera­tions. The Host Controller Interface handles communication between host and the module.

Keysight Technologies, Inc. Power Meter and Sensor Solutions for Bluetooth Transmitter Test

The P-Series and EPM-P power meters fully meet the Bluetooth SIG specified Bluetooth certification test requirements for transmitter tests as listed in Table 1. These tests are relevant to a full functional Bluetooth devices, the transmitter, or even the RF components of the transmitter.

Output Power Test TRM/CA/01/C

Power meters can measure output power at a lower cost compared to spectrum analyzers and vector signal analyzers. Both the P-Series and EPM-P Series power meters have predefined Bluetooth setup stored in non-volatile memory. The gate setup and control function allow closer analysis of the Bluetooth signal.

Power Control TRM/CA/03/C

Power control test allows testing or calibration to be performed on the level control circuitry. This test is only needed for devices that support power control. It is performed in the same manner as the average power measurement, but at the three discrete channels — lowest, mid, and highest operating frequencies (2402, 2441, and 2480 GHz). The power control test verifies power levels and power control step sizes to ensure they are within specified ranges.