10 March 2016

455 KHz IF Transformer Characterization

I found an unmarked IF Transformer in my junkbox which seems to have reasonable characteristics for use in my 455KHz IF stage for my shortwave broadcast band radio project. This one had a yellow inductor adjustment screw and a number of 29501 printed on one side. What attracted me to this item was the fact that it had a reasonably high inductance of 91uH on the primary. Thus I could resonate it with a capacitor of approximately 1200pF on 455KHz.  The secondary winding has a very low inductance of 0.34uH. Mysteriously the measured inductances between pin 1-2 and 2-3 of the primary do not add up to the 91uH.

Refer to the attached Mystery IF Transformer Characterization. Using the method outlined by W7ZOI on his webpage The two faces of Q . A Bandwidth of approximately 4.6KHz was measured which pertains to an Unloaded Q = 100.

I next connected the transformer to a 455KHz source, using a 10pF coupling capacitor (Xc = 35K ohms) IF Transformer loaded with Detector circuit. This lightly loaded tank circuit exhibited an open circuit voltage ratio of 15 which increased to 17 when the secondary was loaded with the detector. The 3dB bandwidth was measured as 5.3KHz with the 6dB points at 10KHz. Loaded Q was 86. Thus the Equivalent Loaded Parallel resistance on the tank circuit is approximately 25K ohms of which 35K is contributed from the signal source load (via the 10pF coupling capacitor. Thus the IF Amp collector load can be expected to be in the 20K ohm range.

The small signal voltage swing of 1.86 Vp-p on the tank coil gave a secondary voltage into the detector of 100mV. This is enough to generate an acceptably loud signal into the connected speaker.

Clearly this is not going to be a highly selective Receiver although the first IF transformer should improve the selectivity further.

Next task is to build the cascode IF Amplifier. Lets see how much gain can be achieved from that stage?

Test Fixture showing the IF transformer connected to the Detector stage.

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