Figure 2 shows a representation of how an IQ modulator generates and transmits a quadrature phase shift keyed (QPSK) carrier. The IQ modulator consists of two multipliers (mixers) whose outputs are combined and a signal splitter whose outputs are in quadrature (i.e., separated by 90°).

So, we can think of the IQ modulator as a pair of multipliers that are each driven by fixed vectors separated by 90°. Because the outputs of the two multipliers are combined, the signals applied to their second inputs (the I and Q inputs) give us the ability to generate arbitrary RF vectors and to control their instantaneous amplitude and phase.

We begin with a simple bitstream (in the context of IQ modulation, it is simpler to think of the bitstream consisting of -1 and +1 logic states instead of using the more conventional labels of 1 and 0). This bit-stream is split into two equivalent bit-streams, each with half of the data rate of the original. These bitstreams are oversampled and low-pass filtered (in the digital domain) to reduce the sidelobes and bandwidth of the final carrier. The two digital bitstreams are then applied to two digital-to-analog converters (DAC). The output signal from each DAC will again be low-pass filtered to remove DAC images (and possibly some of the DAC's broadband noise). Finally, the two baseband signals are applied to the in-phase (I) and quadrature (Q) inputs of the IQ modulator.

A phase locked loop (PLL) drives the local oscillator (LO) input of the IQ modulator. As previously noted, this signal is split into two equal components separated in phase by 90°. When these quadrature LOs are multiplied with the filtered baseband signals, the combined result is a modulated carrier with four phase states or symbols. Each symbol represents two databits from the original datastream (i.e., two bits per symbol).

In an ideal reconfigurable transmitter, a single IQ modulator would be used to cover all frequencies and air interfaces. However, in practice, most IQ modulators do not exhibit broadband performance. Consider the phase splitter that generates the quadrature signals that drive the two mixers. Polyphase filters, which are commonly used to generate precise quadrature in IQ modulators, have limited bandwidth. In practice, good quadrature balance (< 0.5°) is achievable over 1.5 to 2 octaves of frequency using a polyphase filter. Outside of this range the two outputs of the phase splitter will no longer be 90° out of phase with respect to each other. This will result in the symbols being modulated at the wrong phase angle. In addition, if there is any gain imbalance between the I and Q arms at the modulator input, the symbols will have slightly different power levels. These amplitude and phase errors in the modulator will combine to degrade the error vector magnitude (EVM) of the modulated carrier. Sideband suppression is a commonly used metric that expresses the combined effect of imprecise quadrature and imbalance between the I and Q channels of the modulator.

In general, the gain and output power of an IQ modulator will also vary with frequency. Since the noise floor of an IQ modulator tends to remain flat over a broad frequency range, this results in a dynamic range that will vary with frequency.

Figure 3 shows the nominal output power (approximately 6 dB below the 1 dB compression point) and sideband suppression for a family of five pin-compatible IQ modulators. Each device has been designed to provide optimum output power and sideband suppression over a relatively narrow frequency range. The output power and sideband suppression of the family remains relatively constant over a frequency range from 250 MHz to 4.5 GHz. Note that these devices each deliver a frequency-independent output noise floor of -158 dBm/Hz, resulting in a dynamic range that is relatively flat across a broad frequency range.

Figure 4 shows a simplified schematic of an alternative phase splitter design that is used in some IQ modulators. This is essentially a digital circuit that uses D-type flip-flops and an inverter to generate precise quadrature. Unlike a polyphase filter circuit where there is a natural frequency limitation, no such limitation exists here. As a result, excellent quadrature can be achieved over a multi-octave frequency range. However, the circuit does require an external LO operating at twice the frequency of the desired LO (commonly referred to as a 2XLO). In addition, the duty cycle of the externally applied LO is critical. Anything other than a 50% duty cycle at the input will result in quadrature errors at the output.

Figure 5 shows the output power and sideband suppression of an IQ modulator (ADL5385) that uses a 2XLO. The absolute frequency range over which this device operates is much smaller than the FMOD family. However, in terms of octaves, it is clearly a more broadband part with excellent operation (sideband suppression ≤ -40 dBc) from 50 MHz to beyond 1500 MHz (five octaves). Notice also that the output power vs. frequency is relatively flat. This does come at the cost of slightly lower output compression (approximately +10 dBm) compared to the narrowband devices (approximately +12 dBm) while still maintaining a broadband noise floor of -158 dBm/Hz.

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