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.
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.
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.
- 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 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.
- 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 OFF-levels 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.
Discrete Implementation Advantages
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 expensive, and most can be bought from several vendors, ensuring a reliable supply chain. In most cases, the use of passive filter design means the quiescent power consumption of the discrete implementation can be quite low. And if special characteristics are needed to personalize the modem beyond the requirements of the normal AISG standard, discrete designs are easily modified.
The disadvantages of a discrete implementation are board size, total tolerance effects, bill of materials logistics and lack of advanced features such as fault tolerance.
Integrated Circuit Implementation Advantages
One obvious advantage of an integrated circuit implementation is size. All of the highlighted blocks in Figure 1 can be integrated into a single 3 mm by 3 mm package. Even the simplest discrete implementation, lacking the advanced features discussed below, requires dozens of components to accomplish AISG-compliant modulation and demodulation. Board size for the discrete implementation is typically 400 mm2 or more. So the IC implementation represents a significant potential for board size reduction.
Another advantage of integrated circuits is the extremely low incremental cost for adding circuitry, compared to discrete circuit implementations. This facilitates features such as power-on-reset (POR), thermal shutdown (TSD) and electro-static discharge (ESD) protection, all of which are considered standard on most industrial interface devices. POR keeps all of the modem outputs in a safe state while the power supply is transitioning. This prevents unwanted glitches from corrupting the interfaces during power-up and power-down. The TSD shuts down the high-power output stages in the case of an overload or short-circuit fault, saving the circuits from damage. The ESD circuits provide a path to direct ESD energy away from the sensitive circuits, avoiding the most common cause of electronic circuit damage. Each of these features would require many additional components in a discrete implementation.
An advanced feature added to the integrated AISG modem is a Direction Control output, which facilitates RS-485 communication between tower-mounted equipment. Normally in a low logic state, this bit goes high when modulated signal is received on the coax line. The duration of the high logic state is configurable by setting input pins, and corresponds to the message length which varies with the selected network baud rate. While this feature would require several components if implemented discretely (one-shot timer plus resistors and capacitor), it is easily added to the integrated modem at the cost of three additional I/O pins.
In terms of parametric precision, integrated circuits benefit from the “all-in-one” effect. That is, the effects of manufacturing tolerance, supply variation and temperature influence all the internal components more predictably than a collection of discrete components, which are manufactured separately and distributed spatially on the board. Analysis of the critical bandpass filter, for example, shows that small variations in the values of the components can shift the characteristics such that unacceptable performance results. In the Figure 3, one such discrete design is illustrated; the gain-versus-frequency curves show significant variation around the 2.176 MHz center frequency. In this case, only the values for C1 and L2 were allowed to vary (by ± 7 percent) from the ideal values. These curves indicate that compliance to the conducted emissions template is not guaranteed. In practice, there will be variation for all the components, as well as the effects of temperature and supply variation. Integrated implementations benefit when the internal components tend to “track” during manufacturing, and see less temperature and supply variation across the circuit. Further, the entire bandpass filter is effectively tested for compliance during manufacturing, ensuring the circuit as a whole meets the necessary requirements. These same advantages for an IC implementation over a discrete implementation apply to the other critical parameters, such as modulated output amplitude and receiver threshold level.
One more feature offered in the integrated implementation is power supply flexibility. While discrete implementation must be tailored for one supply, the integrated version gives the designer options for single supply operation, either 3.3 V or 5 V, and further includes a separate logic supply input if lower levels (down to 1.6 V) are needed to interface with a low-voltage controller. This eliminates any need for external level-shifting, further reducing parts count.
Overall, designers should evaluate the tradeoffs between the existing discrete implementations and the new integrated solutions when looking at new AISG modem designs. In many cases the advantages of an IC will enable board size reduction and enhanced reliability.
About the Author
Clark Kinnaird is a systems engineer in the high performance analog department at Texas Instruments. He is responsible for defining new data transmission products including RS-485 and CAN transceivers. n addition, he supports network designers with systems analysis, electrical design, and detailed laboratory investigations. Clark Can be reached at firstname.lastname@example.org.
Published in Antenna Systems & Technology magazine’s Fall 2011 Issue