RF transformers are widely used in low-power electronic circuits for impedance matching to achieve maximum power transfer, for voltage step-up or step-down, and for isolating DC from two circuits while maintaining ac continuity. They are also used for common mode rejection and as baluns.
Essentially, an RF transformer consists of two windings linked by a mutual magnetic field. When one winding, the primary has an AC voltage applied to it, a varying flux is developed; the amplitude of the flux is dependent on the applied voltage and number of turns in the winding. Mutual flux linked to the secondary winding induces a voltage whose amplitude depends on the number of turns in the secondary winding. By designing the number of turns in the primary and secondary windings, any desired step-up or step-down voltage ratio can be realized. Mutual coupling is accomplished simply with an air core but considerably more effective flux linkage is obtained with the use of a core of iron or ferromagnetic material with higher permeability than air.
The relationship between voltage, current, and impedance between the primary and secondary windings of the transformer may be calculated using the following relationships. With reference to Figure 1:
As an example, if Z equals 50Ω and the turns ratio equals 2, the secondary impedance equals 200Ω. In this case, the secondary voltage is twice the primary voltage and the secondary current is one-half the primary current.
The basic phase relationship between the RF signals at the transformer input and output ports may be in – phase, 0°, or out-of-phase, 180°. Conventionally, the ports that are in-phase 1, and 3, are marked by dot notation as shown in Figure 2. Ports 1, 4 and 2, 3 are out-of-phase, 180°.
Mini-Circuits’ transformers are physically assembled by winding a pair of twisted wires around a ferrite toroidal core. The ends of the primary and secondary wires which leave the same side of the toroid are in the in-phase ports (Figure 3).
For transformers that have a secondary winding with a center-tap, the schematic representation and dot locations are shown in Figure 4.
In this case ports 1 and 3 are in-phase, 0°, and ports 1 and 6 are out-of-phase, 180°. These transformers may be operated as low as 12.5Ω at the primary with essentially the same impedance matching ratio and only a slight change in frequency response. For convenience, all Mini-Circuits transformers are specified as a step-up.
For each RF transformer model, the minimum and maximum frequency is given for the insertion loss at the 3 dB, 2 dB and 1 dB points, as shown in Figure 5 and listed in the data chart. For example, the T1-1 insertion loss is 1 dB from 2 to 50 MHz and 3 dB from 0.15 to 400 MHz.
Mini-Circuits offers various transformer package styles for assembly on a printed circuit board. These include:
(1) Plastic-case T series, style X65, plug-in leads.
(2) Plastic-case T series, style W38, flat-pack leads.
(3) Plastic-case T series, style KK81, surface-mount gull wing leads.
(4) Metal-case TMO series, plug-in leads.
(5) Open construction TC series, surface-mount leadless.
For these various types, the electrical configurations that are available are:
(A) DC isolated primary and secondary, center-tap secondary.
(B) DC isolated primary and secondary, center-tap primary and secondary.
(C) DC isolated primary and secondary.
(D) Unbalanced primary and secondary.
(E) DC isolated primary and secondary, balanced primary and unbalanced secondary.
(F) DC isolated, three open windings.
Coaxial connector models are available from 5 KHz to 500 MHZ and are offered with 50Ω and 75Ω impedances; 75Ω connectors are used at 75Ω ports. The FT1.5-1, with unbalanced input and output is especially useful for 50Ω to 75Ω matching applications. The FTB series with unbalanced output and balanced input as shown in Figure 6 (connector ground insulated from the case) helps eliminate ground loop problems, especially when long cable runs are involved.
In some applications there is a need to pass a relatively high DC current through the primary winding. In this case, the transformer core may saturate resulting in reduced transformer bandwidth and power handling capability. Mini-Circuits TH series of transformers are designed to handle up to 100 mA in the primary winding without appreciable saturation and change in RF characteristics.
Transformer core saturation is influenced by (1) DC current through the winding, (2) RF input power, and (3) frequency of operation. These three variables interact to affect the point at which saturation occurs. See Figures 7 and 8 in which conventional transformer saturation is compared to the TH series.
Mini-Circuits has developed many special transformer impedance ratios and configurations where high DC current is passed through one winding of the transformer. Many of these designs have been for open package surface-mount requirements. Consult our Applications Department for your particular needs.
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