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I am trying to understand how radio IF transformers are characterized and specified. In the image above from a Mouser catalog are three different IF transformers with a center frequency of 455 kHz. The inductance for all is listed as 680 uH, but the impedances for both the primary and secondary windings are all different, for the primaries 20K, 30K and 60 K; for the secondaries 6K, 500 and 600. All three have a capacitor of the same value (180 pF) across the outer leads of the primary winding.

As I understand it, the IF transformers resonant at their center frequencies. The calculation for resonance for 680 uH and 180 pF gives 455 kHz.

I am guessing that the listed impedances are actually resistances, i.e., no reactance, at the center frequency of 455 kHz, and are altered from one IF transformer to the next by the number of windings and the nature of the yellow, white or black tuning slug.

I am at the stage where I don’t know what I don’t know. I have looked for a white paper on IF transformers and so far have come up dry.

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  • \$\begingroup\$ My gut feeling is that the different transformers have different values of inductance. Can you check the part numbers against the data sheet? \$\endgroup\$ – Dwayne Reid May 10 '16 at 15:41
  • \$\begingroup\$ Dwayne, I am not sure I understand your comment. The 3 transformers listed in the datasheet excerpt have the same inductance, 680 uH, which is listed in a column towards the right. Are you suggesting that the datasheet has a typo? I do not have these transformers. I have one with no markings on it at all from an AM radio kit. It has a black center slug, which is all I know about it. \$\endgroup\$ – user34299 May 10 '16 at 15:49
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I am guessing that the listed impedances are actually resistances,

No, they are the impedances of the circuit they are used in. The DC coil resistances will be much lower (as they should be for low loss).

A transformer 'reflects' the circuit impedance on one side to the other, transformed by the turns ratio squared. So if a source with impedance of 60KΩ was connected to the primary side of a 10:1 transformer, it would appear as 600Ω (102 = 100 times less) on the secondary side.

The IF transformers in your table are typically used in AM broadcast receivers that use one bipolar transistor per stage. Each stage has different input and output impedances, which the transformers are designed to match. Here is an example:-

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Note the center tap on the primary side of each IFT (which improves Q by reducing loading on the tuned circuit). To determine the primary to secondary turns ratio and resulting impedance transformation, we must decide which primary coil the input will be connected to. For example, using coil 1-2 of the 1st (yellow) IFT we get a turns ratio of 70:7 = 10:1, which would transform 60kΩ to 600Ω (or 30KΩ to 300Ω, or...).

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  • \$\begingroup\$ Thanks, Bruce. I am used to electronic part datasheets detailing the characteristics of the part. If I understand you correctly, in the case of IF transformers (IFT) some of the specifications are to allow the designer to choose an IFT for a given circuit. (I assume that specifications such as the inductance and the tuning capacitance, as in my original question, are values for the standalone IFT.) It seems to me that this presents a chicken and egg problem in that the IFT choice depends on the circuit and the circuit design depends on the IFT. (see next comment.) \$\endgroup\$ – user34299 May 10 '16 at 21:57
  • \$\begingroup\$ Is it perhaps the case that the circuit defines itself, so to speak, insofar as having a certain resulting impedance as a result of the design requirements? Might you suggest any books, design guides, white papers, etc. which details the design effort for a superheterodyne AM receiver? The kit I have breaks down the various receiver sections but the theoretical explanations are kind of thin. \$\endgroup\$ – user34299 May 10 '16 at 21:58

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