Why iq modulation

Some tests require changing the order of the data being sent. For example, in WiGig, the digital signal may consist of a QAM modulated carriers, but a readable frame requires the transmission of each data packet with certain bytes before and after for example error protection bytes.

The same considerations apply to other wireless networks such as WiFi. Generating such signals requires storing a great number of symbols in the waveform memory and therefore requires large record lengths. To conclude, the requirement is for an arbitrary generation capability that can support as many realistic channel-coding requirements as possible. For supplying high precision baseband signals to feed a quadrature modulator Figure 2 or for direct creation of the modulated RF carrier, one would need to use a Two-channel Arbitrary waveform generator.

Using analog I-Q modulator — This can be accomplished in two ways. Using a dedicated filter, it is possible to capture the signal image in higher Nyquist bands, thus enable generation at higher carrier frequencies. Noise, distortion, spurious signals, and phase noise all degrade the quality of the modulation being generated. Tabor Electronics High-speed AWGs enables the user to generate I and Q components and even allows direct creation of multiple IF modulated carriers of any modulation scheme using its multi —Nyquist zones operation.

In an SSB modulation generated using a dual channel Tabor AWG as can be seen in figure 8, the output of each mixer is a dual side band signal. The two signals are summarized, which results in the cancellation of one of the sidebands. The amplitude of the quadrature waveform or the sine waveform is Q.

Modulation By appropriately varying the I and the Q as a function of time, we can generate the resulting sum of those wave forms as frequency modulation, or phase modulation. Demodulation All this works in the opposite direction just as well for demodulating signals. Fateme Talebi. Our Blog, delivered to your inbox. Subscribe Here! Ulrich Scholten, PhD. An error vector is a vector in the I-Q plane between the actual received or transmitted symbol and the ideal reference symbol.

EVM is the ratio of the average of the error vector power over the average ideal reference symbol vector power. It is frequently expressed in either dB or percentage. Figure 1 is a test setup example showing the modulation accuracy attainable with the LT C low power direct quadrature modulator. Figure 2 shows the results. A vector signal analyzer VSA examines the modulator output. Figure 2. This is indeed excellent performance, shown by a modulation error ratio MER of The LTC has internal trim registers that facilitate fine adjustments of I and Q DC offset, amplitude imbalance, and quadrature phase imbalance to further optimize modulation accuracy—results are even better if trim registers are adjusted.

In many ways, this test demonstrates the best-case capabilities of the modulator without optimization: baseband bandwidth is large, DAC accuracy and resolution are superb and digital filtering is nearly ideal. Many FPGAs and programmable devices support digital filter block DFB functionality, an essential building block for digital communications. Raw transmit data is readily IQ mapped and digitally filtered. This lowers the filter order requirement of the LC reconstruct filter, which serves to attenuate DAC images to acceptable levels, while minimizing phase error and wideband noise.

Figure 4 shows the complete circuit. The differential baseband drive to the modulator, as opposed to single-ended baseband drive, offers the highest RF output power and lowest EVM. The input amplifier U2 is also designed to supply the required input common mode voltage for the IQ modulator—important for maintaining proper modulator DC operating point and linearity. Figure 4. The passive Bessel filter attenuates DAC images and provides lowest RF output noise floor, while imposing negligible symbol error vector.

Some of the filter shunt capacitance is implemented as common mode capacitors to ground. This also reduces common mode noise, which can find its way to the modulator output. If active filters are used here, the final filter stage before the modulator should be a passive LC roofing filter for lowest broadband RF noise floor.

Table 2, Figure 5 and Figure 6 show the performance results. For this reason, EVM does not substantially improve when IQ modulator impairments are adjusted out, as shown in Table 2.

Figure 5. EVM measurement detail. Two IC devices replace the lab signal generator.