3.9-a Reaction - type Receiver
In the previous project, in the context of Pic.3.26, we have seen how important it its for an oscillatory circuit of the receiver to have as big Q- factor as possible. The bigger it is, the receiver gets more selective, i.e. capable to separate the signal of the station the oscillatory circuit is tuned at from the multitude of signals in the reception antenna, at the same time suppressing other signals. There is yet another important thing to be emphasized: the voltage in the oscillatory circuit is Q times bigger than the voltage that is being led to it from the antenna. If Q=95 and the voltage in the antenna is, say, 1 mV, the voltage exiting the circuit is then 95 mV. It is then clear that the Q- factor should be as big as possible. Some improvements can be made by using the silver - coated wire instead of the plain copper one, of using ceramic material for the coil body instead of carton etc. These improvements, however, are not significant. The solution for this was, eventually, found: It was the process known as the “regeneration”, many scientists had been working on it simultaneously, A. Meissner being one of the more important ones. The complete process was patented in 1913 by the American radio - pioneer Edwin H. Armstrong, two months before his 23rd. Birthday. He discovered that the Q- factor could be extremely increased (even by couple of tens of times), if the signal from the inlet circuit would be amplified with the triode and then the small part of this amplified voltage adequately returned into the inlet circuit. The radio station signal could therefore be amplified to much greater extent, and the reception of very far stations became possible, the thing that wasn’t even thought of, until then. From an average listener’s point of view, the main mishap of this regenerative, or, here better known as reactive, receiver was the fact that optimal tuning required somewhat more skill and basic knowledge of the working principles. Much easier tuning was that of the superheterodyne receiver (invented also by Armstrong), which was reduced to turning only one reel, and it contributed, to a great extent, to its final victory. The battle between the two concepts lasted for almost half a century, but the reactive receiver did not retreat itself completely. Even nowadays an electrical diagram of some younger and more modern brother, or rather a grand-grandson of once famous reactive receiver, appears in some popular electronics magazine. One such diagram, where the triode is replaced by a MOSFET, is given on Pic.3.29-a.
Radio engineering enthusiasts know that, as soon as the signal from the input circuit is led on the gate (G1) and that the source (S) isn’t connected to the ground but to the coil leg (point No.3), they have a diagram of the Hartley’s oscillator in front of them and that connecting the source to the leg No.3, the so-called positive feedback (positive reaction)is achieved. But, the abovementioned labourers also know that the oscillator shall oscillate only if the leg No.3 is made on the right spot, and the P1, TP1 and TP2 variable resistors are set on the right values. Let us imagine that everything is OK: the coil leg is right where it should be, the slider of the variable resistor P1 is in the middle of the range, and trimmers TP1 and TP2 are set in such a manner that the oscillator is really oscillating. It behaves then as the generator, that creates the sinusoidally - shaped AC voltage, whose amplitude is a couple of volts. Theoretically speaking, the Q- factor is now infinitely big . All this shall produce a very strong whistling sound in the loudspeaker. The slider of P1 should now be carefully moved downwards. The transistor amplification is hereby reduced and the oscillating stops. The Q-factor is being reduced too, it is no longer infinite but is still very big. The receiver is now tuned on the desired station. If the station signal is weak, everything should be OK and the programme should be heard. The reception can then be made better, by carefully operating with P1’s slider. If the whistling emerges again, the slider should be moved backwards until it stops. If we come upon some stronger station the whistling will start immediately, in which case the slider P2 should be carefully moved downwards until it stops and the station programme is heard, loud and clear. As one may notice, every time this receiver is being attuned to optimal receipt the whistling is being heard for a short while. This is why it has been named “The Whistler” in some countries .
The amount of the reaction (feedback, regeneration) is controlled with P1 potentiometer, which sets the magnitude of DC voltage on the gate G2 of the MOSFET, changing therewith the amount of its amplification. The range of this control is being determined with TP1 trimmer, G2 is connected to the AC signal’s ground over C2 simultaneously eliminating the noise coming from the potentiometer, and the FET is de-coupled from the supply line (and therefore all other stages of the device by the LF filter made of C3, C4 and R2.
The receiver is being tuned as follows: Put the slider of P2 in mid position and later, after tuning, you can set the volume as you wish. Set the P2 at minimum resistance (slider full down), and P2 on maximum (slider full right), connect the antenna and close the switch S. Start moving the slider P1 upwards, the reaction gets stronger and stronger, and you can hear the typical hiss or some radio programme from the loudspeaker. Move the variable capacitor C and tune the receiver to various stations. If the whistling starts, put the slider P1 back down.
Set the capacitor is minimum capacitance position (see Pic.3.7), put the slider P1 fully upwards and start carefully increasing the resistance TP2 until the whistling stops. Measure the TP2 and insert in the device the ordinary resistor of similar resistance. The TP1 trimmer should be set in such a way to have as big resistance as possible, keeping at the same time the reaction effective throughout the entire reception bandwidth of the receiver.
* During every station change (with C), a maximum amount of reaction should be set (with P1). Move the slider upwards until the oscillating occurs, then put it back down a little.
* While receiving very strong signal (local transmitter), an overload can occur. If that happens, you should insert a 1 MOhm potentiometer between the antenna (A) and the upper end of C1 capacitor; it should be connected as the rheostat (like TP1 and TP2), then you can set the optimum reception with slider.
* SW - band stations can also be received with this device, with a different coil. In this case it would be very useful to add a trimmer capacitor in parallel to the variable capacitor, being marked as Ct on Pic.3.29-a. With it the so-called “range yielding” can be done (the initial, approximate setting is done with C, and fine tuning between closely placed stations with Ct). It should be mounted on the front panel, as close to C as possible. Another type of capacitor can be used as Ct, see more about it in the Appendix. The SSB (Single Side Band) technique transmissions are also being placed in the SW band area. These signals cannot be received with the earlier described receivers, but they can with this one. In that case the slider P1 should be moved a bit more upwards, so that oscillating can occur. The reception becomes clear, before that it was unrecognizable.
* If the local radio station still corrupts the reception of other stations, you should insert the circuit that will suppress its signal. You can read more about it also in the Appendix.
