Receiver Sensitivity

When building a receiver, there is really only one question that matters: How good is it? This is quantified by measuring the receiver sensitivity: how sensitive the radio is to detecting wireless transmissions from another device. This is measured in dBm, and is typically a very small number. The required receiver sensitivity for Bluetooth low energy is –70dBm. In other words, it has to be able to pick up 0.0000001mW of electromagnetic energy to be able to work. However, noise will always be present. There is no point in being able to detect a signal if you can’t decode it. Therefore, in practice, the sensitivity threshold is set at the value where a signal can be decoded with an acceptable bit error rate (BER). For Bluetooth low energy, this has been chosen as 0.1 percent BER.

Most controllers supporting Bluetooth low energy will have a receiver sensitivity of about –90dBm, or 1pW. This is an incredibly small amount of energy that is able to be detected from the noise of the band, but this leads to impressive ranges, as explained in the following section.

5.10. Range

To calculate the range of a Bluetooth low energy radio, the link budget of the system needs to be determined. The link budget is made up of a number of elements that use the power from the transmitter in a silicon chip before it is received by a peer silicon chip. These

elements include the antenna and matching circuit gains and losses. However, assuming that the antenna and matching circuits make little difference,¹ the main contributor to the link budget is the path loss. Path loss is a measure of how much the radio signal has reduced in power between the antenna in the transmitter and the antenna in the receiver. Equation 5-3 determines the path loss required for a given distance. Table 5–1 presents the correlation between path loss and distance; Figures 5–8 and 5–9 show the relationship graphically. It should be noted that this equation is an approximation, valid only for an isotropic antenna, and ignores any losses in the transmit/receive systems.

Figure 5–8. A graphic representation of path loss

Figure 5–9. Path loss (log graph)

In the equation, d is the distance between the transmitter and the receiver.

Table 5–1. The Relationship of Path Loss to Distance

When the transmit power is –20dBm and the receiver sensitivity is –70dBm, a path loss of 50dB, the range is 2.5 meters. This is the distance possible when the minimum transmit power is used, with the minimum receiver sensitivity.

When the transmit power is 0dBm and the receiver sensitivity is –80dBm, a path loss of 80dB, the range is 40 meters. This is the distance possible when a moderate transmit power is used, with a moderate receiver sensitivity.

When the transmit power is 10dBm and the receiver sensitivity is –90dBm, a path loss of 100dB, the range is 250 meters. This is the distance possible when the maximum transmit power is used with the receiver sensitivity possible with modern chips.

Chapter 6. Direct Test Mode

Knowledge must come through action; you can have no test which is not fanciful, save by trial.

—Sophocles

6.1. Background

One of the biggest problems with wireless systems, especially those that are designed for the lowest possible cost of production, is how to calibrate them and perform qualification and product line tests of their performance. This is especially true after the device has been packaged into another module or product, and there is no way to move aside the host stack to perform a few seconds of testing at the start of the device’s life. Direct Test Mode solves all these problems by defining standard testing procedures and a hardware interface to drive this protocol even after a host stack and other parts of the device have been incorporated in the device.

For the direct test mode to work, three devices are required (see also Figure 6–1):

• A Device Under Test (DUT)

• An Upper Tester (UT)

• A Lower Tester (LT)

Figure 6–1. Test configuration

The DUT is the controller, module, or end product that is being tested. The device must have both an antenna and a Universal Asynchronous Receiver Transmitter (UART) or Host/Controller Interface (HCI) to the UT.

The UT is typically manufactured by a test-equipment manufacturer and includes software to drive the device under test through the UART or HCI interface as well as the ability to communicate and drive the LT.

The LT is a device that can transmit and receive packets, effectively communicating with the device under test through the device’s antenna.

The device under test is told what to do by the UT and transmits or receives packets. The UT at the same time informs the LT to do the opposite; that is, to receive or transmit packets, respectively. This means that the device under test will transmit packets to or receive packets from the LT. At the end of the test, the UT can use the information available from both

devices, a packet count from the device under test or more comprehensive information from the lower tester, to determine if the DUT passed the tests.

The UT can also do calibration of the controller on a production line by asking the device to transmit packets at a known frequency and measuring the actual frequency that the

controller is transmitting. Typically, this is required if the external crystals used for a timing reference are not exactly at their design frequency. This crystal trimming would be done while the controller is transmitting packets, allowing very fast calibration of parts.