Analysis of the characteristics of IQ modulators
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The analog IQ modulator includes a mixer. In the process of up-conversion, image frequency products are bound to be generated. When the output signal is frequency-free, that is, when the center frequency of the signal is the same as the LO signal frequency of the modulator, it is equivalent to using the Zero-IF mechanism. The image frequency product is inseparable from the signal itself, and the image frequency cannot be filtered out even through a filter. Fortunately, with the use of IQ modulation and demodulation, the original IQ signal can still be restored even if there are image frequency products. This is why an image frequency suppression filter is not required after the analog IQ modulator.
Due to this orthogonal architecture, the IQ modulator itself has a certain image frequency suppression capability, but the image frequency suppression characteristic can only be reflected when the output signal has a certain frequency deviation, that is, when the signal center frequency is different from the LO signal frequency. The following will analyze some special baseband IQ signals to explain the factors that affect the image frequency suppression characteristics and how to improve the image frequency suppression characteristics.
1. Impact of IQ signal amplitude balance on image suppression
The IQ signal amplitude is unbalanced (i.e. the amplitudes are different). Either the amplitudes of the I and Q signals input to the modulator are unbalanced, or the modulator has a certain gain imbalance (i.e. the gains of the I and Q paths are different). These will affect the ability to suppress image frequencies.
Let i(t)=Acoswbt, q(t)=sinwbt, then the RF signal s(t) output after IQ modulation is
s(t)=Acoswbt · coswct - sinwbt · sinwct
The sum of the sum
s(t)=0.5(A+1)cos(wc+wb)t + 0.5(A-1)cos(wc-wb)t
When A=1, there is only the upper sideband (wc+wb) component in the RF signal;
When A=-1, there is only the lower sideband (wc-wb) component in the RF signal;
When A≠±1, the RF signal contains both upper sideband (wc+wb) and lower sideband (wc-wb) components.
The above introduces the image frequency suppression characteristics of the IQ modulator by analytical method. In fact, it can also be explained clearly and simply by graphical method. Let's consider the case of A=1. Figure 1 shows the Fourier transform of the carrier signal, which is the double-sideband spectrum. The baseband signal is shifted by the IQ modulator. Figure 2 shows the spectrum transformation on the two branches of the modulator. Finally, after the combiner is combined, the lower sideband components cancel each other out, leaving only the upper sideband components.
Figure 1. Fourier transform of a carrier signal (double-sideband spectrum)
Figure 2. Schematic diagram of spectrum transformation during IQ modulation process
When A≠±1, the RF signal contains both upper and lower sidebands, and the sideband suppression ratio is defined as: 20lg│A+1│/│A-1│ (dB).
How to improve image rejection?
The gain imbalance characteristics of the two branches of the IQ modulator cannot be adjusted, but the image suppression can be improved by adjusting the amplitude of the I and Q waveforms on the baseband side. Both the vector signal generator VSG and the arbitrary waveform signal generator AWG provide IQ Gain Imbalance adjustment parameters, and fine-tuning them can improve image suppression.
2. Effect of IQ orthogonality on image suppression
Orthogonality includes two aspects: (1) orthogonality between baseband signals I and Q; (2) orthogonality between the LO signals of the two mixers of the IQ modulator. If the orthogonality is not good, it will cause modulation and demodulation errors (EVM, BER deterioration) when generating digital modulation signals without frequency deviation. On the other hand, it will deteriorate the image suppression characteristics when generating single-sideband signals.
Let i(t)=cos(wbt+), q(t)=sinwbt, then the RF signal output by the IQ modulator is
s(t)=cos(wbt+)· coswct - sinwbt · sinwct
The sum of the sum
s(t)=0.5(1+cos)·cos(wc+wb)t-0.5sin·sin(wc+wb)t-0.5(1-cos)·cos(wc-wb)t+0.5sin·sin (wc-wb)t
For the (wc+wb) component, let a=0.5(1+cos), b=0.5sin, then θ is taken to satisfy the following relationship:
cosθ=a/√( a2+b2), sinθ=b/√( a2+b2)
Similarly, for the (wc-wb) component, let c=0.5(1-cos), b=0.5sin, then θ1 is taken to satisfy the following relationship:
cosθ1=c/√(c2+b2), sinθ1=b/√(c2+b2)
Substituting the above formula into s(t), we can finally get
s(t)=0.707√(1+cos)·cos[(wc+wb)t+θ]+0.707√(1-cos)·cos[(wc-wb)t-θ1]
The image suppression caused by the orthogonal error is: 10lg(1+cos)/(1-cos) (dB).
The above is an analysis of the impact of the orthogonality of the baseband I and Q signals on the image suppression characteristics. If the baseband signals are ideally orthogonal, but the LO orthogonality of the two mixers of the IQ modulator is not good, the entire derivation process is similar and will not be repeated here. Of course, the characteristics of the IQ modulator are fixed, and the image suppression capability can only be improved by adjusting the orthogonality of the baseband signal.
3. Carrier suppression characteristics of IQ modulator
In addition to suppressing image frequencies, IQ modulators can also suppress carriers during digital modulation. Theoretically, as long as there is no DC component in the analog I and Q signals and the IQ modulator is ideal, there will be no carrier in the output RF broadband signal. However, the broadband signal actually generated always has a certain amount of carrier leakage, which comes from two parts: (1) a certain DC component in the IQ signal; (2) LO leakage of the mixer in the IQ modulator.
For digital modulation signals, carrier leakage is a kind of in-band interference. If the carrier component is strong, it will directly affect the communication quality of the entire system. Therefore, carrier leakage should be reduced as much as possible. Usually, I Offset or Q Offset is fine-tuned on the baseband side to improve the carrier suppression characteristics, which is equivalent to introducing a DC component. If the amount and polarity of the DC are set appropriately, the carrier leakage caused by the I and Q paths will offset each other, and even the influence of the LO leakage of the mixer can be offset.
The above introduces the image suppression and carrier suppression characteristics of the IQ modulator, which are inherent characteristics of the IQ modulator and must be tested in performance verification tests. In addition, the IQ modulator also has parameters such as amplitude-frequency response and third-order intermodulation, which also need to be tested.
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