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T-R-F Receivers: Sensitivity Variation over Tuning Band

Ernst Erb Otmar Jung 
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Papers » Basic principles of radio technique » T-R-F Receivers: Sensitivity Variation over Tuning Band
Dietmar Rudolph † 6.1.22
Dietmar Rudolph † 6.1.22
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18.Aug.10 20:25
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R-F (radio frequency) transformers which were used in T-R-F (tuned radio frequency) receivers sometimes have "gimmick" loops like the 2nd R-F transformer of this German set from 1932.

At the anode of the 2nd RENS1234 a coil (of 2 turns in the schematic) is connected, and the other end of this coil is open. So, what is that?

An explanation can be found in "Ghirardi, A.: Receiver Circuitry and Operation, Rinehart, 1951 / 55"

3-13. Sensitivity Variation over Tuning Band

A difficulty which arises in connection with transformer-coupled t-r-f amplifiers is variation in sensitivity which occurs over the tuning range of the receiver. This variation results from either of the following two causes:

  1. The mutual inductance between the primary and secondary windings of an r-f coupling transformer does not transfer energy equally at different radio frequencies within the tuning range of the receiver.
  2. The loosely coupled medium-impedance primary winding of the antenna coil (and of some interstage coils) is large enough to resonate with its own distributed capacitances and circuit capacitance to a frequency just below the low end of the tuning range of the receiver.

Both of these factors tend to increase the gain of the transformer-coupled amplifier at the low end of the frequency range compared to the gain at the high end.

To overcome this tendency, capacitive coupling between the primary and secondary windings is introduced into the circuit. This is usually achieved by using an open-ended loop of wire connected at one end to the top (antenna or plate end) of the primary winding and wrapped one or more times around the top (grid end) of the secondary winding. Capacitive coupling is more effective at the higher frequencies because of the decrease in reactance of the capacitance, allowing the flow of more r-f current. It therefore compensates for the two effects mentioned above.

Figure 3-9 (A) shows the schematic diagram of a transformer with this capacitive-coupling link, which is usually called a "gimmick" loop. Figure 3-9 (B) shows a typical r-f transformer used in t-r-f AM broadcast receivers, with the shield partly cut away to reveal the interior. The gimmick loop is illustrated in its position over the grid end of the secondary winding.

Another contribution to increased gain at the high-frequency portion of the tuning range is the increased efficiency of the tuned circuit at lower capacitance values of the tuning capacitor.