Mixer principle

Mixer principle

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

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

Conversion loss The conversion loss of a mixer is defined as the ratio of the microwave signal power at the RF input port of the mixer to the signal power at the intermediate frequency output. Mainly caused by circuit mismatch loss, diode's inherent junction loss and non-linear conductance net frequency conversion loss.

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

Dynamic range Dynamic range refers to the microwave input power range during normal operation of the mixer. The lower limit varies depending on the application environment of the mixer, and the upper limit is limited by the saturation of the RF input power, which usually corresponds to the 1dB compression point of the mixer.

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, and third-order intermodulation may occur In the vicinity of the output intermediate frequency, it falls within the intermediate frequency passband and causes interference. It 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 is reduced by 1dB, the third-order intermodulation signal rejection ratio increases by 2dB.

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

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

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

Intermediate frequency residual DC offset 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 unmatched mixing tube or unbalanced balun, a DC voltage will be output at the intermediate frequency, that is, the residual DC deviation voltage of the intermediate frequency. This residual DC offset voltage will affect the accuracy of the phase detection.

Application frequency conversion: This is a well-known use of mixers. Commonly used are double-balanced mixers and triple-balanced mixers. Three-balanced mixers use two diode bridges. All three ports have transformers, so the local oscillator, radio frequency and intermediate frequency bandwidth can reach several octave bands, and the dynamic range is large, the distortion is small, and the isolation is high. But its manufacturing cost is high and the process is complicated, so the price is high.

Phase detector: In theory, all mixers whose intermediate frequency is DC-coupled can be used as a phase detector. Two RF signals with the same frequency and the same amplitude are added to the local oscillator and the RF port of the mixer, and the intermediate frequency terminal will output a DC voltage that varies with the difference between the two signals. When the two signals are sinusoidal, the phase-detection output changes to sine with the phase difference. When the two input signals are square waves, the phase-detection output is a triangular wave. It is recommended to use power near the standard local oscillator power. If the input power is too large, it will increase the DC offset voltage. 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 local oscillator port and the RF port of the mixer 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 radio frequency. When the control current is above 20mA, the transmission loss is the insertion loss of the mixer. In this way, the positive or negative current can be continuously controlled to form a variable attenuator with a change range of about 30dB, and the port standing wave changes little over the entire change range. Similarly, a square wave control can be used to form a switch.

Phase modulator (BPSK): This type of mixer also requires intermediate frequency DC coupling. The phase of the signal transmission between the local oscillator port and the RF port of the mixer is controlled by the polarity of the intermediate frequency current. Alternately changing the polarity of the control current at the IF port, the phase of the output RF signal will alternately change in two states at 0 ° and 180 °.

Quadrature phase shift keying modulation: QPSK is composed of two BPSK, a 90-degree bridge and a 0-degree power splitter.

The modulation and demodulation of I / Q modulator / demodulator are actually reciprocal processes, which are reversible in the system. The I / Q demodulator is mainly introduced here. The I / Q demodulator consists of two mixers, a 90-degree bridge, and an in-phase power divider.

Image suppression mixer: filters that suppress the image frequency are generally of fixed bandwidth. But when the signal frequency changes, the mirror frequency also changes, which may move out of the filter's suppression band. In multi-channel receiving systems or frequency agile systems, this filter will be useless. At this time, the image frequency suppression mixer is used. When the local oscillator frequency changes, the range of the image frequency to be suppressed will also change due to the internal phase matching relationship of the mixer circuit, so that it can still play the role of image frequency suppression. Because the circuit is not completely ideal, there are amplitude imbalance and phase imbalance, which may deteriorate the electrical performance of the image suppression mixer. The following figure illustrates the electrical performance of amplitude imbalance and phase imbalance.

Single-sideband modulator: In a multi-channel transmission system, since the baseband frequency is very low, if a common mixer is used for spectrum shift, there will be two sidebands within the channel bandwidth, which affects the utilization of spectrum resources. At this time, a single sideband modulator can be used to suppress unwanted sidebands. Its basic structure is two mixers, a 90-degree power divider, and an in-phase power divider. The baseband signal is decomposed into two quadrature signals mixed with the orthogonal two signals of the local oscillator. Phase cancellation is used to suppress unwanted sidebands. The local oscillator is suppressed due to the isolation of the mixer itself.

Music Headphone/Wired Headphone With Mic/Wired Headphone with mic

Introduction:

1. Super bass stereo sound quality .
2. Good for listening Music player, watching movie, online chatting and game playing, 
3. New model and best selling over ear headphone .
4. Headband is adjustable to fit different sized.
5. Factory price and high quality for mass purchase.
6. Retail package for your sale.

7.Color:depend on customer

8.Material:PVC


Music Headphones

Music Headphones

Music Headphones,Music Headphone,Audio Headphones,Children'S Over Ear Headphones

Shenzhen Greater Industry Co., Ltd. , https://www.szgreater.net