Mixer Information

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Basic knowledge of mixer:

A mixer is a circuit that mixes signals and can achieve frequency up-conversion and frequency down-conversion functions.

1. working frequency

The mixer is a multi-frequency device. In addition to specifying the operating frequency of the RF signal, attention should also be paid to the application range of the local oscillator and intermediate frequency.

2. Noise Figure

The noise of the mixer is defined as: NF = Pno / Pso Pno is the total noise available power transmitted to the output port when the noise temperature of the input port is the standard temperature at all frequencies, that is, T0 = 290K . Pno mainly includes the thermal noise of the signal source, the thermal noise of the internal loss resistor, the current shot noise of the mixer device and the local oscillator phase noise. Pso is the noise available power generated at the output end by only the useful signal input.

3. Frequency conversion loss

The conversion loss of a mixer is defined as the ratio of the microwave signal power at the mixer's RF input port to the signal power at the IF output port. It is mainly caused by circuit mismatch loss, inherent junction loss of the diode, and net conversion loss of nonlinear conductance.

4. 1dB Compression Point

Under normal working conditions, the RF input level is much lower than the LO level. At this time, the IF output will change linearly with the RF input. When the RF level increases to a certain level, the speed at which the IF output increases with the RF input slows down, and the mixer becomes saturated. The RF input power when the IF output deviates from the linearity by 1dB is the 1dB compression point of the mixer . For mixers with the same structure, the 1dB compression point depends on the LO power and diode characteristics, and is generally 6dB lower than the LO power .

5. Dynamic Range

Dynamic range refers to the microwave input power range when the mixer works normally. Its lower limit varies depending on the application environment of the mixer, and its upper limit is limited by the saturation of the RF input power, which usually corresponds to the 1dB compression point of the mixer.

6. Two-tone third-order intermodulation

If two microwave signals fs1 and fs2 with similar frequencies are input to the mixer together with the local oscillator fLO , due to the nonlinear effect of the mixer, intermodulation will occur, of which the third-order intermodulation may appear near the output intermediate frequency and fall within the intermediate frequency passband, causing interference, which is usually described by the third-order intermodulation suppression ratio, that is, the ratio of the useful signal power to the third-order intermodulation signal power, often expressed as dBc . Because the intermediate frequency power is proportional to the input power, when the microwave input signal decreases by 1dB , the third-order intermodulation signal suppression ratio increases by 2dB .

7. Isolation

Mixer isolation refers to the mutual isolation between frequency ports, including the isolation between the local oscillator and the radio frequency, the local oscillator and the intermediate frequency, and the radio frequency and the intermediate frequency. Isolation is defined as the ratio of the power of the local oscillator or radio frequency signal leaking to other ports to the input power, in dB .

8. LO Power

The local oscillator power of the mixer refers to the local oscillator power required for optimal working conditions. In principle, the larger the local oscillator power, the larger the dynamic range and the better the linearity ( the 1dB compression point rises and the third-order intermodulation coefficient improves).

9. Port VSWR

The port standing wave directly affects the use of the mixer in the system. It is a parameter that changes with power and frequency.

10. IF residual DC deviation voltage

When the mixer is used as a phase detector, the output should be zero when there is only one input. However, due to reasons such as imperfect mixer tube pairing or balun imbalance, a DC voltage will be output at the intermediate frequency, that is, the residual DC deviation voltage of the intermediate frequency. This residual DC deviation voltage will affect the phase detection accuracy.

11. application

Frequency conversion: This is a well-known use of mixers. Commonly used ones are double-balanced mixers and triple-balanced mixers;

The triple balanced mixer uses two diode bridges. All three ports have transformers, so the bandwidth of its local oscillator, RF and IF can reach several octaves, and has a large dynamic range, low distortion and high isolation. However, its manufacturing cost is high and the process is complex, so the price is relatively high;

Phase detector: In theory, all mixers with DC coupling of intermediate frequency can be used as phase detectors. When two RF signals with the same frequency and amplitude are added to the local oscillator and RF ports of the mixer, the intermediate frequency end will output a DC voltage that varies with the phase difference between the two signals. When the two signals are sinusoidal, the phase detector output changes to a sinusoidal wave as the phase difference changes. When the two input signals are square waves, the phase detector output is a triangle wave. The recommended power is near the standard local oscillator power. If the input power is too large, the DC deviation voltage will increase. If it is too small, the output level will be too low.

Variable attenuator / switch: This type of mixer also requires intermediate frequency DC coupling. The transmission loss of the signal between the mixer's local oscillator port and the RF port is controlled by the intermediate frequency current. When the control current is zero, the transmission loss is the isolation from the local oscillator to the RF. When the control current is above 20mA , the transmission loss is the insertion loss of the mixer. In this way, a variable attenuator with a variation range of about 30dB can be formed by continuous control of positive or negative current , and the port standing wave changes very little within the entire variation range. Similarly, a switch can be formed by square wave control;

Phase modulator ( BPSK ): This type of mixer also requires IF DC coupling. The phase of the signal transmitted between the mixer's LO port and the RF port is controlled by the polarity of the IF current. By alternating the polarity of the control current at the IF port, the phase of the output RF signal will alternate between 0 ° and 180 ° .

Quadrature Phase Shift Keying Modulation: QPSK is composed of two BPSK , a 90 -degree bridge and a 0- degree power divider.

Modulation and demodulation are actually inverse processes and are reversible in the system. Here we mainly introduce the I/Q demodulator , which consists of two mixers, a 90 -degree bridge and a co-phase power divider.

Image suppression mixer: Filters that suppress image frequencies generally have fixed bandwidths. However, when the signal frequency changes, the image frequency also changes and may move out of the filter's suppression band. In a multi-channel receiving system or a frequency agile system, this filter will lose its effect. At this time, an image suppression mixer is used. When the local oscillator frequency changes, due to the phase matching relationship inside the mixer circuit, the suppressed image frequency range will also change accordingly, so that it can still play the role of image suppression. Since the circuit is not completely ideal, there is amplitude imbalance and phase imbalance, which may deteriorate the electrical performance of the image suppression mixer. The following figure illustrates the effect of amplitude imbalance and phase imbalance on electrical performance. </FONT> Single sideband modulator: In a multi-channel transmission system, since the baseband frequency is very low, if an ordinary mixer is used for spectrum shifting, there will be two sidebands within the channel bandwidth, which will affect the utilization of spectrum resources. At this time, a single sideband modulator can be used to suppress unnecessary sidebands. Its basic structure is two mixers, a 90- degree power divider, and a co-phase power divider. The baseband signal is decomposed into two orthogonal signals and mixed with the two orthogonal signals of the local oscillator. The phase cancellation technology is used to suppress the unwanted sidebands. The local oscillator is suppressed due to the isolation of the mixer itself.