Quadrature phase shift keying modulator
Quadrature phase-shift-keying (QPSK) modulators are four-port devices with RF-in, RF-out, and two data input ports. A bipolar pulse signal is applied at data ports to produce an RF output signal phase-modulated by 0 (reference), 90, 180, or 270°, depending on the state of the modulating signal.
The insertion loss of a phase modulator is measured statically in one of the possible four states and represents the worst-case loss for any state. For QPSK modulators, the insertion loss is 6 dB typically.
VSWR is a measure of impedance match at input and output ports of the modulator. This parameter represents imperfect input and output impedances. Mini-Circuits’ modulators have a VSWR of 1.3:1 typically.
Amplitude unbalance is a measure of the worst-case variation in insertion loss between the four states in a QPSK modulator.
Phase unbalance is a measure of the offset from the desired carrier phase relative to the actual phase shift in any of the possible phase states. This is measured with respect to a reference or 0° phase state. In MCL catalog specifications, the reference is at the 0° phase state with + 20 mA for both control ports.
1 dB compression
As RF signal input level is increased, output should follow in a linear manner. However, after a certain point, output signal level increases at a lower rate until the modulator output level becomes fairly constant. The RF input level for which the output level deviates from linearity by 1 dB is termed the “1 dB compression point”.
I&Q: In-phase and quadrature-phase
The amount of carrier measured in dB below the desired output signal when a coherent signal of equal amplitude and 90° phase difference is applied to the I&Q ports. This is shown in figure 1. fc & fi, are carrier and IF (I-port/Q-port) frequency.
The amount of undesired sideband measured in dB below the desired sideband under the same conditions as the carrier rejection measurement.
In a modulator, besides the undesired sideband and carrier, harmonic products of the IF combine with the carrier to produce undesired signals at RF as below.fc ± nf
The most important products are the third and fifth, fc ± 3fi and fc ± 5fi since they are very close to the desired sideband and almost impossible to filter.
For a demodulator, the harmonic products of interest are:
n = 3 and 5 are the products of most interest.
For a modulator, conversion loss (C.L.), in dB, is defined as below C.L.(dB) = Input(I + Q) Power – Output RF Power of the dominant sideband.
For a demodulator, it is defined as C.L.(dB) = RF Input Power – (I + Q) Output Power.
The deviation from 90° of the phase angle difference of the I&Q port output signals.
For a demodulator, amplitude unbalance is the difference of the I and Q power.
Amplitude Unbalance = PI(dBm) – PQ(dBm)
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