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”, here in Yugoslavia .
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.
3.9-b Direct SW Receiver for AM, AM-SSB & CW Signals
SSB stands for Single Side Band, which signifies the amplitude - modulated signal which gets its signal carrier and one sideband suppressed in the transmitter. Carrier suppressing gives huge savings in transmission power (the power necessary to accomplish the desired reach of the signal is significantly smaller than in the conventional - type transmitters), and cancellation of one sideband makes the signal have its spectrum two times narrower, allowing twice as many transmitters as usual to be placed into the same bandwidth.
CW is for Continuous Wave, which determines the radio link where the Morse Code is being transmitted by cutting the oscillator work in the transmitter.
SSB and CW signals are impossible to accomplish with the receivers that use the ordinary diode - type detector (earlier described AM receivers). The receipt can be done only by bringing another signal into the detector, the HF signal from the oscillator, known as the BFO (Beat Frequency Oscillator). Simpler solutions, however, do exist. These are the reaction - type receivers, i.e. receivers with positive feedback.
The coil L and capacitors C and C1 form a parallel oscillatory circuit whose role is to separate and amplify the signal of the tuned station, and to suppress all others. It doesn’t entirely succeed in that, however. The reason for this is small Q- factor of the oscillatory circuit, being such because of big energy losses in the circuitry. There are many kinds (reasons) of these losses, but we can imagine in first approximation that there’s a resistor RG in the circuit which represents these losses, its resistance being such that the oscillating current transforms itself into heat dissipation energy on it, its amount being the sum of all the (actual) losses in the circuit. We could, furthermore, solve the problem of these losses if connecting serially to RG a resistor RG’, whose resistance would be negative and equal to the value of RG by its absolute value. The overall resistance would then be zero, there would be no energy losses and the Q- factor would become infinite. The oscillatory circuit would, together with the components that create this negative resistance, become an oscillator capable of receiving SSB and CW signals.
We don’t really need an infinite Q- factor while receiving usual (conventional) AM RG by its absolute value. The resistances would not cancel each other completely, but the losses would be made very small, the Q- factor therefore becoming very big therefor increasing both the selectivity and sensitivity of the oscillatory circuit.
Transistors T1 and T2 constitute, together with resistor R3, a two-stage amplifier with strong positive feedback that has a negative dynamic input resistance. This negative resistance is connected between the leg No.3 on the coil and the ground, therefore superimposing itself with the resistance representing losses of the circuit. The quantity of this negative resistance depends on the amount of the DC current flowing through the transistors, which is being regulated by altering the DC voltage on the right end of the R3 resistor (by moving the slider of the P1 potentiometer).
The red LED D and the resistor R2 comprise a simple voltage stabilizer, obtaining 1.8 V of stabilized voltage on the P1. That means that the voltage on the right end of R3 shifts between 0 and 1.8 V while moving the slider of P1. The current flowing through the transistors thereat also changes, causing the voltage on the left end of R3 to vary between 0 and 0.6 V.
The signal of the station is being led from the leg No.3 of the coil into the collector-type detector made of T3, R3, R4 and C4. That is an AM signal detector that performs both signal detection and its amplification. Its name is the Audion. The LF signal is then, from the collector of T3, over the coupling capacitor C5, being led onto the sound volume potentiometer P2 and the audio amplifier. For the latter any of the earlier described devices can be used.
Tuning this receiver on the desired station requires both some knowledge and patience (that’s what finally “buried” this kind of receivers). Put the slider P1 in the upmost position. If strong whistling is heard that means the oscillating began. Move the slider carefully downwards until the oscillating stops. Then start slowly turning the rotor of the capacitor C until you come upon some station. If the whistling re-appears, move the slider of the potentiometer very little downwards, the whistling should stop and you should be able to hear the radio - station programme from the loudspeaker (loud and clear). For the next station tune yourself with C, then move the slider P1 upwards until the whistling appears, then put the slider back until it stops etc. All this may seem rather complex at first, but with a little practice and with two hands all will go quick and smooth.
The abovementioned method is for the signal reception of ordinary, broadcast stations. If you wish to receive the SSB and CW signals you should move the P1 slider upwards until the oscillating is achieved, so that articulate speech (SSB) or Morse code signs (CW) can be heard from the loudspeaker.
* The coil L is being made on the cylindrically - shaped body 6 mm in diameter, about 25 mm long. The plastic - made body taken from an old device is the best, like the one shown on Pic.5.14-b. The screw-shaped core allows the setup of the inductance, adjusting therewith the reception bandwidth of the device. If you cannot find such coil body, any plastic- or carton- made cylinder can be used instead. If you don’t have even that, then make yourself one. Cut the paper band to be 25 mm wide and about 150 mm long and reel it around the flat part of the 4 mm drill, adding every now and then some glue (UHU or similar). When the glue gets dry, remove the coil body off the drill.
* The feedback may happen to be not big enough, causing that there’s no oscillating even when the P1 slider is in the rightmost position. In that case, leg No.2 of the coil should be used instead of No.3. Switching between the legs can be done in many ways, the nicest (?) one given on Pic.3.29-d, made with factory-made contact pins and jumpers. On Pic.3.29-d-c you can see a detail of the PCB for the receiver shown on Pic.3.29-b. In the contacts marked as x, y and z (distance between them is 1/10 inch) the contact pins are soldered. The jumper is in position marked with dashed line, therefore making contacts x and y short-circuited. When it is moved in vertical position, the x and z contacts are in junction. In former case the coil leg No.3 is used, and in the latter it is No.2. In factory-made devices, these
* Setting the collector-type detector circuit to optimum operation is done by changing the R3 resistance, until voltage on the collector of BC549C is 1.2 V.
* The antenna can be a piece of copper wire no longer than 50 cm, but with longer (few metres), external antenna, the results will be much better.
* This receiver is scheduled for the reception of SW stations from 6 MHz till 9 MHz, which is accomplished with C1 value of about 400 pF. The exact value for C1 is being determined experimentally and can be significantly different. Going down to the amateur range (about 3.75 MHz) is performed with bigger C1 capacitance.
