In general, insertion loss is defined as the loss in signal power, when a device is inserted in the transmission path of a signal on a transmission line or circuit.Insertion loss is usually stated in decibels (dB).

So if the transmitted power is PT and the received power at the load, after the insertion of the device is PR , then the insertion loss is defined as

IL ( dB) = 10Log(PT/PR)

Where the Log is to the base of 10.

Insertion loss is also defined for filters as the ratio of the ouput signal level in a test configuration without the filter installed, to the signal level with the filter installed. So if the output signal level without the filter is V1 and the signal level with the filter installed is V2 then the insertion loss in dB is:

20Log(|V1|/|V2|) dB

If using scattering parameters use the following expression:

10Log(|s21|² /1 – |s11|² )

where the symbols have their usual meaning.

Transfomers also have an insertion loss specification. This is a figure of merit for a RF transformer.

The low end ( or low frequency) loss is determned by the primary inductance. The high frequency insertion loss is dependent on the losses in the inter – winding capacitance and the series inductance.

In addition, for transformers with metallic cores, the permeability is directly proportional to the temperature of operation. As the temperature decreases, the permeability decreases which causes an increase in the insertion loss.

The RF transformer is a very useful device for the design of many types of RF circuits. It can be used as a device for changing voltage and current levels in a circuit and matching impedances.

This very brief post is simply a reminder of the absolute basic design equations for the RF transformer (and indeed, any transformer).

1. n = N2/N1 , where n is the turns ratio of the two winding transformer. N2 and N1 are the number of turns of the secondary and the primary.
2. V2 = nV1, V2 is the secondary output voltage, V1 is the primary voltage.
3. I2 = I1/n. I2 is the current out of the secondary, I1 is the primary current.
4. Z2 = n*n*Z1. Z2 is the impedance seen looking into the secondary and Z1 is the primary impedance. These are very basic quantities of an ideal transformer. For second and third order effects please see the equivalent circuit model of the  transformer. Please visit the SPG website for  more interesting information.