Barron Performance Index

Barron Performance Index :

There are many types of baluns. The type of balun used in microwave RF design depends on the required bandwidth, operating frequency, and the physical structure of the design. Most baluns usually contain two insulated copper wires twisted to each other and wound around a magnetic or non-magnetic core.

Key specifications when determining balun types for specific applications include:

– Frequency coverage

– Phase balance

– Amplitude balance

– Common mode rejection ratio

– Impedance ratio / turn ratio

– Insertion loss and return loss

– Balanced port isolation

– DC / earth isolation

– Group delay flatness

Phase balance

An important standard indicator of the balun is its balance, that is, the two balanced outputs (one is an inverted 180 ° output, and the other is a non-inverted output) and the ideal state of "the power level is equal and the phase difference is 180 °" How close it is. The degree of deviation between the phase angle difference between the two outputs and 180 ° is called the balun's phase imbalance.

Amplitude balance

This index is determined by the structure of the balun and the degree of line matching, usually in dB. Amplitude balance refers to the matching between the output power levels. The difference between the two output power levels is called the amplitude imbalance, and the unit is dB. In general, for every 0.1dB increase in amplitude balance or 1 ° increase in phase balance, the common mode rejection ratio (CMRR) will increase by 0.1dB.

Common Mode Rejection Ratio (CMRR)

When two identical signals with the same phase are injected into the balance port of the balun, two different results may be generated for transmission or reception. CMRR refers to the amount of attenuation that occurs when the signal is transmitted from the balanced port to the unbalanced port in dB. The CMRR is determined by the vector addition result of the two signals, and the vector addition result further depends on the amplitude balance and phase balance of the balun.

Impedance ratio / turn ratio

The ratio of unbalanced impedance to balanced impedance is usually expressed as 1: n. The differential impedance is the impedance between balanced signal lines, and is twice the impedance of the signal lines to ground. The turns ratio is a parameter of the flux-coupled balun transformer, which represents the ratio of the number of primary winding turns to the number of secondary winding turns of the transformer. The square of the turns ratio is equal to the impedance ratio. For example, when the turns ratio is 1: 2, the impedance ratio is 1: 4. Through the flux coupling transformer, a balun with a high impedance ratio can be designed.

Insertion loss and return loss

The lower the differential insertion loss and the higher the common-mode return loss, the greater the power of the inserted signal through the balun, the wider the dynamic range, and the lower the signal distortion. In an ideal balun without isolation, common-mode signals can be completely reflected with a return loss of 0 dB, while differential signals are completely passed with a return loss of -∞.

Balanced port isolation

Balanced port isolation refers to the insertion loss from one balanced port to another balanced port in dB. Since most baluns reflect even-mode reflections instead of properly terminating them with resistive loads, their balanced port isolation is not high. An exception is the 180 ° hybrid circuit, which outputs an even mode to a port that can be terminated in a resistive manner.

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