Detailed Description

The MAX11947 is an AISG v2.0 and v3.0-compliant, fully integrated modem with a 4:1 multiplexer facilitating connectivity between a single modem and up to four RF ports.

The MAX11947 receiver includes a narrow 200kHz bandwidth bandpass filter centered at the 2.176MHz carrier frequency. It also includes an OOK demodulator and a comparator that reconstructs the digital signal. The typical sensitivity threshold of the receiver is -15dBm (when set to the default value 0xB), in compliance with the AISG standard specifications. This threshold is adjustable through an internal register setting (RXSENS) from a -20.5dBm to -15dBm level.

The MAX11947 transmitter includes an OOK modulator, a bandpass filter that is compliant with the AISG spectral emission profile, and an output amplifier. The output power can be varied with an internal register setting (TXPWR) from under 0dBm to over +6dBm (measured at the MUX-selected RF port) to compensate for loss in the external circuitry and cabling. The OOK carrier is generated from an external crystal at 8.704MHz connected to the XTAL1 and XTAL2 pins. An external clock source at the same frequency can also be applied to XTAL1 from the SYNCOUT pin of another MAX11947.

The MAX11947 also features a DIR output to facilitate the RS-485 bus arbitration in tower-mounted equipment.

The integrated 4:1 multiplexer allows the AISG system designer to connect a single modem to as many as four RF ports, simplifying implementation of a multi-primary or ALD PING-capable design. The MUX uses a CMOS analog switch architecture to provide a bidirectional signal, switchable between one common internal modem port and the four RF ports.

The 4:1 MUX provides both isolation and minimal crosstalk, allowing all four ports to be connected to a single modem without impacting the connected RF channels. To address the MUX performance, the port separation is characterized by three forms of isolation or crosstalk, which are primarily related to the frequencies involved: RF isolation, AISG isolation, and modem crosstalk.

Figure 2 shows a simplified block diagram of a single MUX RF port. A 50Ω series resistor is integrated into each RF port and the MUX common node is internally biased to approximately 1.5V DC.

The 4:1 MUX can be configured to either have all of the P_ RF ports internally terminated and the common node not connected, or any single P_ RF port can be connected to the MUX common node. All switch changes occur in a break-before-make sequence. When an RF port is not connected to the MUX common node, that P_ port is internally terminated through the 50Ω resistor to ground. When a port is selected, any previous selected port is terminated to GND prior to the new port connection being made. The process of connecting a P_ RF port to the common port is measured by the t_SW time. This switching time is measured by monitoring the CSB and SCLK signals of the SPI interface and the DC level on the RF port. The time starts when the port change message is programmed into the RFPORT [1:0] bits in the MUX (0x1) register and is triggered by the 16th SCLK rising edge (last register bit is written). The switch time ends when a 1.5V DC bias level is measured on the selected RF port. It should be noted that this timing can vary due to delays with transitioning between the SPI clock domain and the internal operational clock domain. Figure 3 shows how the MUX switch time is measured from the 16th SCLK transition to the measured 1.5V DC bias on the RF port (t_SW), to the DIR output transition (t_SW-DIR), and to the first RXOUT bit (t_SW-RX).

RF Isolation

The first form of separation to address is radio frequency signal isolation between RF ports. The primary implementation of the MAX11947 in a cellular tower application would have multiple RF frequencies in the cellular bands connected to separate ports on the 4:1 MUX. RF isolation describes how much of a high-frequency (cellular) signal on one RF port is detectable on another RF port. In Figure 4, an aggressor signal is applied to the coax cable tap of the P2 channel and the coax cable tap of P1 is measured for any impact on the victim channel. A majority of this isolation is provided by the lowpass filters on the P2 and P1 connections (LPF2 and LPF1, respectively).

AISG Isolation

The next form of separation to address is AISG carrier signal isolation between RF ports. Implementation of AISG v3.0 allows for the possibility of each RF port to carry either a multi-primary AISG signal or a PING signal. AISG isolation describes how much a 2.176MHz carrier signal on one RF port is detectable on another RF port. In Figure 5, an aggressor AISG signal is applied to the coax cable tap of the P2 channel and the coax cable tap of P1 is measured for any impact on the victim channel. A majority of this isolation is provided by 4:1 MUX switch within the MAX11947.

Modem Crosstalk

The last form of separation to address is modem crosstalk between RF ports. Again, implementation of AISG v3.0 allows for the possibility of each RF port to carry either a multi-primary AISG signal or a PING signal. Modem crosstalk describes how much a modulated AISG signal on a nonselected RF port is detectable by the MAX11947's internal receiver while connected to a different, selected RF port. In Figure 6, an aggressor AISG signal is applied to the coax cable tap of the P1 channel and the 4:1 MUX is connected to the P0 RF port. The aggressor signal is set to the maximum allowed power of +5dBm and the receiver threshold is set to the minimum programmed level of 0x0 (about -20.5dBm) and is monitored for any indication of a detected signal on the DIR or RXOUT pins. A majority of this isolation is provided by 4:1 MUX switch within the MAX11947.

Register Programming

The multiplexer is programmed through two register settings. In the MUX register (address 0x01), the OPEN bit internally disconnects the modem from the common port of the MUX, and the default condition (0x0) connects. In the same MUX register, the two RFPORT bits are used to select which external RF port the common port is connected to.

The modem receiver consists of an LNA, a demodulator, and a sensitivity threshold adjustment system to convert the 2.176MHz OOK modulated signal into a digital output at RXOUT.

The maximum OOK input power at the internal LNA into the 50Ω internal termination is +5dBm. For a single-tone signal at 2.176MHz, +5dBm corresponds to 1.12V_P-P.

Input Range and Sensitivity Threshold Control

The MAX11947 internal threshold is set to -15dBm (112.5mV_P-P) by default (RXSENS set to 0xB). This centers the receive sensitivity threshold in the -12dBm to -18dBm undefined window specified by the AISG standard. This threshold sets the minimum input signal power recognized as an OOK ON carrier (asserting DIR level, logic-high and setting the RXOUT bit, logic-low).

Consider a corner case where the OOK signal at 2.176MHz present at the selected RF port, is at the minimum power of -15dBm ±3dB. To avoid saturation of the receiver input stage, any other adjacent carrier with power up-to +5dBm must be at a frequency below 1.25MHz or above 3.7MHz.

Referencing section 10.3.13 of the AISG v3.0 specification, ALD system designers are permitted to include splitters and/or combiners with an insertion loss of 4.5dB for a two-way split or as much as 6.3dB for a three-way split. With an outgoing transmission the MAX11947's adjustable transmitter power allows the designer to increase the modem output up to over +6dBm, which would result in an effective level of +1.7dBm or more after a two-way split (falling within the +3dBm ±2dB window). Similarly, an incoming transmission with a maximum signal power of +5dBm would have no issues being decoded at the modem receiver. However, the potentially minimum receive power of -12dBm may not be properly decoded after the 4.5dB splitter attenuation. The adjustable threshold of the MAX11947 allows the AISG designer to move the target from -15dBm to as low as -20.5dBm, effectively allowing a drop of 5.5dB which can accommodate the 4.5dB attenuation from a two-way splitter. See the In-Line Attenuation section for additional information.

The MAX11947 receiver sensitivity threshold can be varied with an internal register setting (RXSENS) from a default level of -15dBm down to -20.5dBm in about 0.5dB steps. To obtain the nominal sensitivity threshold of -15dBm at the RF port as the AISG standard requires, set RXSENS to 0xB (default value). For additional setting values and their equivalent threshold, see the RXTX register details.

The modem transmitter consists of a local oscillator (LO) circuit, an on-off-keyed (OOK) modulator, and an adjustable power amplifier (PA).

Output Power Control

The MAX11947 output power can be varied with an internal register setting (TXPWR) from < 0dBm to over +6dBm (measured at the MUX selected RF port: P0, P1, P2, or P3) to compensate for loss in the external circuitry and cabling. The maximum voltage available at the MUX selected RF port is about 1.29V_P-P. Assuming that the feeder cable is terminated into a 50Ω impedance, the external filter is lossless at 2.176MHz, where a series 50Ω termination is being used internally, the output level of 1.29V_P-P corresponds to +6.2dBm at the feeder cable. See the External Low-Pass Filter and AC-Coupling to Feeder Cable section for more details.

The P_ voltage level can be varied according to the following equation:

P_OUT (at P_ in dBm) = TXPWR (decimal) x 0.5 (dB) - 0.5dBm

The output power has a soft ceiling around +6.0dBm at code 0x0D and rolls off the 0.5dB per code rate with the last two settings of 0x0E and 0x0F. The maximum PA output power is governed by the V_CC supply and when using 5.0V the MAX11947 can likely achieve transmitter output power levels up to +7.0dBm.

To obtain the nominal power level of +3dBm at the feeder cable as the AISG standard requires, set TXPWR to 0x7 (default value), which provides about 0.89V_P-P at the MUX selected RF port P_.

The MAX11947 provides a direction output pin (DIR) that indicates the direction of the data flow. This feature is very useful in the tower that acts as a slave in the AISG protocol. In most applications the base is the master and it controls the flow of data by performing bus arbitration. The output DIR allows the equipment in the tower to avoid any involvement in the bus arbitration. The Typical Application Circuit shows how the MAX11947 can be used in the tower in conjunction with the RS-485 transceiver such as the MAX13485E or MAX13486E.

Typically, the DIR output drives the DE (driver output enable) and RE_ (receiver output enable) of the RS-485 transceiver.

Whenever the data flows from the selected RF port (P0, P1, P2, or P3) to RXOUT, the DIR output is asserted high. When the MAX11947 is located in the tower, the data flow is being sent from the base (master) to the tower (slave). On the other side, when the data flows in the opposite direction, from TXIN to the selected RF port, the DIR output is deasserted low. However, the MAX11947 internal state machine is sensing both the TXIN and the status of the internal LNA, it can recognize the correct flow of data and avoid asserting the DIR high.

Figure 9 and Figure 10 show the timing diagrams of the DIR functionality. When the data flows from the selected RF port to RXOUT, DIR remains high for 16 bit times after the last logic-level low bit within the 8-bit protocol data (DIR dwell time). This is compliant with the AISG specification saying that the RS-485 transmitter stops driving the bus within 20 bit-times after the last stop bit is sent.

The DATARATE register value defines the duration of the bit time, as shown in Table 2.

The MAX11947 utilizes a 4-wire SPI protocol for programming its registers, configuring and controlling the operation of

the modem. The register contents can be read back through the SDO pin. The digital I/Os in Table 3.

control the operation of the SPI.

Figure 11 shows a general SPI write transaction. Figure 12 show the format of a SPI read transaction. As shown in the diagrams, the R/W bit is set to 0 for a write transaction and the R/W bit is set to 1 for a read transaction.

Operational Modes

The MAX11947 has three modes of operation that include two power-saving modes. Each power-saving mode shuts down various internal blocks to save on current and reduce the overall power consumption of the modem. The SPI interface always remains operational, and the internal register values are always retained as long as the V_CC and V_L supplies remain within specified voltage limits.

Standby Mode

Setting the STANDBY bit in the CFG (0x00) register places the modem in standby mode. This configuration shuts down the transmitter signal chain including the TXIN interface, the modulator, the power amplifier, and all associated filtering blocks. While in the STANDBY state, the modem consumes less power and thus has a lower supply current. All other modem functions, including the receiver signal chain (RXOUT) and the DIR signal, continue to operate normally.

Power-Down Mode

Setting the PWRDN bit in the CFG (0x00) register places the MAX11947 modem into power-down mode. This configuration shuts down the transmitter signal chain as well as the receiver signal chain including the LNA, the demodulator, the RXOUT interface, and the DIR system.

Depending on the state of the XTALOSC bit in the CFG (0x00) register, the power-down mode maintains the crystal oscillator and SYNCOUT operation or shuts down those blocks as well. By default, the XTALOSC bit is set to 1 and the 8.704MHz system remains operational for cascaded master/slave clocking. See the Crystal and External Clock section. If the XTALOSC bit is set to 0 prior to or during the same write command as the PWRDN bit, the crystal oscillator and SYNCOUT blocks are also shut down while in power-down mode.

While in the PWRDN state, the modem consumes minimal power yet still retains values programmed in the registers and keeps the SPI interface operational for further programming and mode selection.

The SCAN (0x02) register bits are used to configure the port scanning feature of the MAX11947. This state machine allows the modem to automatically listen for a carrier tone and/or the ping message on any of the RF ports sequentially. By setting the MASK_Px bits to 1 or 0, the individual RF ports can be included or excluded from the scan sequence, respectively. The default port scan process includes all RF ports (PINGCFG = 0x0F by default).

The automatic scanning process is initiated by setting the START_SCAN bit to 1; this is a self-clearing bit and reverts back to a 0 if the register is read back. While the modem is actively scanning the RF ports, the SCANNING bit asserts (1) and the LSTN_PORT bits indicate which port is connected to the modem through the 4:1 MUX.