RF transformers are devices designed to transfer energy from one circuit to another by electromagnetic induction. RF transformers are typically used to increase or decrease the voltage from one side of the transformer to the other. The turn ratio, or the number of windings on the primary and secondary sides of the transformer, will determine the change in voltage. RF transformers are used in low-power circuits for impedance matching to achieve maximum power transfer, for voltage step-up or step-down, and for DC isolation of two circuits while maintaining AC continuity.

Specifications

Important performance specifications to consider when searching for RF transformers include:

• operating frequency range
• 3 dB bandwidth
• amplitude unbalance
• phase unbalance
• impedance ratio
• insertion loss
• rated DC current
• DCR
• operating temperature

The operating frequency is the frequency range for which the transformer will meet all guaranteed specifications. Three dB bandwidth is the frequency range for over which the insertion loss is less than 3 dB referenced to the mid-band insertion loss. Amplitude balance (also referred to as amplitude unbalance) is the absolute difference in signal amplitude between each output of a center-tapped RF transformer relative to the center tap. Amplitude balance is specified in dB. Phase balance (also referred to as phase unbalance) is the difference in signal phase between each output of a center-tapped transformer relative to the center tap. Phase balance is specified in degrees. Impedance ratio refers to the impedance ratio of a transformer and is determined by the square of the turn ratio. Insertion loss (in dB) is defined as the measured loss through the device excluding the power division factor. It is calculated as the ratio of power output to power input. The rated DC current (IDC) is the level of continuous direct current that can be passed through the transformer winding with no damage. The DC current level is based on a maximum temperature rise at the maximum rated ambient temperature. The rated current is related to the transformer's ability to minimize the power losses in the winding by having a low DC resistance. For low frequency currents, the RMS current can be substituted for the DC rated current. Direct current resistance (DCR) is the resistance of the transformer winding measured using DC current. The DCR is most often minimized in the design of a transformer and specified as a maximum rating.

Other important specifications to take into consideration when searching for RF transformers include:

• transformer type
• package type
• packing method
• center tap options
• connector type

Choices for transformer type include telecom, current sense, power, and Balun. Package choices include surface mount, flat pack, through hole or plug-in, connectorized, and waveguide assembly. Choices for packaging method are tape reel, tray, tube, and bulk pack. Center tap options for RF transformers are none, primary side only, secondary side only, and both sides.

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Image credit:

JFW Industries, Inc.