Low-cost BER tester measures errors in low-data-rate applications

You don't need an expensive pattern generator to produce a PRBS (pseudorandom-bit-sequence) signal for making elementary BER (bit-error-rate) measurements in low-data-rate continuous-transmission systems (Reference 1). You also need not spend time programming on a computer to compare sent and received data patterns. Moreover, most professional BER-measurement equipment doesn't cover lower bit rates. This Design Idea offers a simple, low-cost alternative that can accommodate data rates as high as 20 kbps. The system tests a 10-kbps transceiver in low-power sensor networks. The pattern generator, a Hewlett-Packard (www.hp.com) E1401B, can produce PRBS streams of only 150 kbps and higher.

An Atmel (www.atmel.com) AVR microcontroller creates a PRBS signal and compares the generated output stream with received data bits (Figure 1). After sending 1 million bits, the system displays the number of badly received bits on a two-row-by-16-character LCD. You can program the unit to transmit longer sequences of bits; however, doing so significantly increases the measurement time. Many low-cost or free development tools are available for AVR microcontrollers. This Design Idea uses an assembler and a serial programmer (references 2 and 3).

The design uses an 8-bit Fibonacci-type LFSR (linear-feedback-shift register) to produce the PRBS stream. The basic design includes a serial-shift register with modulo-2 addition using XOR instructions (Figure 2). You select the feedback taps' position to obtain a maximal-length sequence that has a period of 2⁸–1 bits. Additional LFSR designs of different lengths and optimal feedback taps are also available (Reference 4). You can easily adapt the software in Listing 1  to produce PRBS signals with longer periods. A simplified flow chart of the assembler program is written for the AVR microcontroller (Figure 3).

The generated bit sequence appears at Pin Port A1, which you connect to a transmitter that's suitable for the system under test. Connect the digital output of a convenient receiver to Pin Port A4. The processor compares the received input with the output at Port A1 between two "send" bits. When the bits sent and received don't match, the number of displayed errors increases. If the system exhibits throughput delay, you need to modify the software to cope with the delay.