CMX971 Quadrature Modulator
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Quadrature Modulator COMMUNICA TION SEMICONDUCTORS D/971/4 February 2015 Provisional Issue Wireless Data Terminals HF/VHF and UHF Mobile Radio Avionics Radio Systems 20 – 1000MHz RF Output 0dBm Output 25MHz I/Q Bandwidth Serial Bus or Direct Control Operation Low Power 3.0V – 3.6V Operation Small 16-lead VQFN Package RFOUT CBUS logic SCLK/ RESETN RDATA/ DIVSET Brief Description The CMX971 is a high performance quadrature modulator featuring a wide operating frequency range and attractive power consumption. Control of the CMX971 may be either by serial bus or by direct control (with reduced functionality). Programmable features include the local oscillator divider ratio (2 or 4). The small, RF-optimised VQFN package and minimal external components make the device ideal for spaceconstrained applications.

Quadrature Modulator

Quadrature Modulator Added Smith Chart for LO input impedance, as section 8.1.1 Information added on modulator harmonics (section 8.2.4). Clarification of Front Page, sections 5.2 and 7.0. Additional information on FREQ bit settings, section 5.2.1 Updated parametric information, after further characterisation First Issue

Quadrature Modulator Block Diagram Control Logic TXIP TXIN RFOUT TXQP TXQN Figure 1 Block Diagram

Quadrature Modulator Main Functions Name TXIP RFOUT RFGND VCC LO LOGND CDATA / FREQ1* VDD SCLK / RESETN* RDATA / DIVSET* TXQN TXQP TXIN EXPOSED METAL PAD Signal Description Input for baseband transmit I signal (positive) RF output Analogue and RF ground Analogue and RF supply Input for local oscillator LO Buffer ground C-BUS data-in C-BUS and digital supply C-BUS clock input C-BUS data output C-BUS chip select C-BUS and digital ground Disable pin for C-BUS operation. If held low, direct control pin functions are selected as described in Table 2 and it enables modulator sections (equivalent...

Quadrature Modulator External Components Power Supply Decoupling The CMX971 has separate supply pins for the RF, analogue and digital circuitry; a 3.3V nominal supply is recommended for all circuits. It is recommended that the digital supply be decoupled from the RF and analogue supply; an example of such decoupling is shown in Figure 2. R2 LOGND RFGND Figure 2 Power Supply Connections and Decoupling C1 C2 Table 3 Power Supply Component Values Note: It is expected that low frequency interference on the 3.3V supply will be removed by active regulation; a large capacitor is an alternative but...

Quadrature Modulator Quadrature Modulator A typical circuit for the quadrature modulator output is shown in Figure 3 / Table 4. Figure 3 Modulator Output External Components (30MHz to 1GHz) C1 Table 4 Quadrature Modulator Output Components Local Oscillator (LO) Input The CMX971 has a single ended LO input. Users should be aware that the presence of high levels of harmonics in the signal applied to the LO Input might degrade quadrature accuracy. Figure 4 LO Input Configuration A separate ground pin (LOGND) is used for the LO buffer to allow routing of the ground feed to the device such that...

Quadrature Modulator General Description The CMX971 device is an RF integrated circuit providing a quadrature modulator. A detailed block diagram for the device is shown in Figure 1. The device can support a wide range of modulation formats and standards. The following sections describe its functionality. Programmable Data Interface The CMX971 may be controlled via its C-BUS serial interface (see section 6). Alternatively, the device can be used in direct control mode with reduced programmability; this can be advantageous in systems without a host controller (see section 7). Quadrature...

Quadrature Modulator C-BUS Interface and Register Description The C-BUS serial interface supports the transfer of control and status information between the CMX971’s internal registers and an external host. Each C-BUS transaction consists of the host sending a single Register Address byte, which may then be followed by zero or one data byte that is written into the corresponding CMX971 register, as illustrated in Figure 5. Data sent from the host on the Command Data (CDATA) line is clocked into the CMX971 on the rising edge of the Serial Clock (SCLK) input. The C-BUS interface is compatible...

Quadrature Modulator Single byte from µC (General Reset command) CSN SERIAL CLOCK (SCLK) COMMAND DATA (CDATA) REPLY DATA (RDATA) Note: The SERIAL CLOCK line may be high or low at the start and end of each transaction. One Address and one Data byte from µC CSN SERIAL CLOCK (SCLK) COMMAND DATA (CDATA) REPLY DATA (RDATA) One Address byte from µC and one Reply byte CSN SERIAL CLOCK (SCLK) 7 6 5 4 3 2 1 0 Address COMMAND DATA (CDATA) REPLY DATA (RDATA) Figure 5 C-BUS Transactions General Reset Command General Reset Command C-BUS address $1A (no data) This command resets the device and clears all...

Quadrature Modulator General Control Register 8-bit write-only General Control Register: C-BUS address $1B This register controls general features such as powersave. All bits of this register are cleared to ‘0’ during a General Reset command. Note: b0 (TXEN) and b4 (ENBIAS) are high if pin CBUSOFFN is low. 7 General Control Register b6, b4 and b0 Writing b6 = ’1’ the LO is divided by 2; writing b6 = ’0’ the LO is divided by 4. Writing b4 = ‘1’ Enables the internal bias current supplies. Writing b0 = ‘1’ Enables the quadrature modulator All other bits are reserved and must be set to ‘0’ for...

Quadrature Modulator best intermodulation intermediate value intermediate value intermediate value intermediate value intermediate value intermediate value intermediate value intermediate value intermediate value intermediate value intermediate value intermediate value intermediate value intermediate value best wideband noise Control Register C-BUS address $EE 8-bit read-only This read-only register mirrors the value in register $1E; see section 6.3.1 for details of bit functions.