Antenna Systems & Technology - Fall 2011 - (Page 12)

FEATURE ARTICLE AISG OOK Modem Implementations By Clark Kinnaird, Systems Engineer • Texas Instruments The AISG On-Off Keying (OOK) interface was developed to provide a data channel for command and diagnostic information between a base station and its associated mast-mounted equipment. Semiconductor manufacturers have recently developed integrated transmitter/receivers to facilitate design of this standard interface. Most existing AISG OOK implementations use discrete designs, and these designs have proven performance in the field. This article will illustrate the tradeoffs of designing with the new integrated devices. Both board-level design tradeoffs and system-level changes will be discussed. Block Diagram In basic terms, the AISG OOK modem interface needs a transmitter to generate modulated signals based on logic inputs, and a receiver to demodulate RF signals into logic levels. Beyond this, the AISG and 3GPP TS 25.461 specifications give the spectrum, protocol and timing requirements that the interface must follow. The block diagram below shows the requisite building blocks. In this diagram, the blue blocks are common to both discrete and integrated implementations, while the yellow blocks indicate features that are typically found only in the integrated solutions. on intermodulation products. Therefore, careful analog design is needed to modulate the carrier without introducing significant high-frequency terms. The complexity of the filter is beyond the scope of this article, but as an example the mask requires approximately 40 dB of difference between the 2.176 MHz carrier frequency and 1 MHz, little more than an octave. The order and ‘Q’ of the necessary bandpass filter can be determined from these requirements. One discrete implementation is discussed below, as well as the inherent difficulties with component tolerance. • Output stage – The output stage must be sufficient to develop the necessary signal amplitude into a 50 Ohm load; this imposes requirements on both the circuit and the available power supply. The nominal +3 dBm signal means about 1 Vpp; this could theoretically be achieved with low voltage supplies, e.g. ±1 V, or +2.5 V. However, in order to preserve the linearity necessary to avoid violating the emissions mask, at least 3 V of supply is needed in practice. This allows sufficient bias on the line driver circuits to avoid non-linear saturation effects. The bandwidth of the output stage must also be sufficient that no frequency-dependent distortion is introduced on the carrier. Receiver (Demodulator) • Input filter – The incoming modulated signal should be filtered to remove energy outside frequency of the carrier. The same concerns regarding filter complexity and component tolerance will apply as for the output filter requirements. If a passive filter implementation is used, a single bi-directional bandpass can implement both input and output filter functions. • Demodulator – The demodulation function is typically implemented using an envelope detector. TS 25.461 specifies OFFlevels below -18 dBm (80 mVpp) and ON-levels from -12 dBm (160 mVpp) to +5 dBm (1120 mVpp). This wide dynamic range becomes a challenge due to the additional constraint that data duty cycle distortion must not exceed 10 percent. The figure below illustrates the impact of both small and large input signals on the comparator output, when compared to a fixed threshold. The threshold must be set low enough that small valid signals will be recognized. This leads to a long delay in recognizing the end of carrier energy after a large amplitude signal. Similarly the RC time constant must be long enough to hold the signal above the threshold for small signals, but this also contributes to the OFF recognition delay. Solutions for this issue can include adjusting the comparator threshold to match the specific received signal in each installation, or adding circuitry to adapt the demodulator based on the characteristics of the incoming signal. Figure 1. AISG OOK modem block diagram The AISG standard specifies the requirements for the OOK modem in terms of signal energy, frequency characteristics and data timing. These requirements then determine the circuits by which the modem function is implemented. We briefly discuss these circuit elements and the standard requirements which determine their implementation in the following paragraphs. Transmitter (modulator) • Oscillator – the carrier frequency must be 2.176 MHz with 100 ppm (±218 Hz) precision. This implies crystal control of the carrier frequency. Either an integer multiple of the carrier can be used, or a very solid PLL (phase-locked loop) circuit is needed. • Switch, preamp and bandpass filter – The key requirements for these is the conducted emissions mask, and the requirements Discrete implementations of AISG modems have certain advantages. The most significant for many designers is that these present solutions have been proven, and already exist in fielded hardware. Typically the individual components are not very ex- Discrete Implementation Advantages 12 ANTENNA SYSTEMS & TECHNOLOGY FALL 2011 WWW.ANTENNASONLINE.COM http://WWW.ANTENNASONLINE.COM

Table of Contents for the Digital Edition of Antenna Systems & Technology - Fall 2011

Planning of Advanced Wireless Systems
Selectable Multi-Beam Smart Antennas
AISG OOK Modem Implementations
Antenna Systems 2011: Conference Preview
Editor’s Choice
Antennas
Components
Test & Measurement
Manufacturing/Design
Base Stations
Marketplace, Advertising Index & Calendar of Events
Industry News

Antenna Systems & Technology - Fall 2011