Norcim rc electronics club page 17……
SUPER-REGENERATIVE RECEIVER HISTORY By David Caudrey
(Further details on Super-regenerative receivers can be found on page 19 of this website.)
THE SUPER-REGENERATIVE RECEIVER
In the years immediately following the Second World War radio control of model aircraft became a practical proposition, probably as a result of the development of radio navigation devices during the war.
What was required was a small and light detection device, capable of responding to a weak and wandering 27MHz carrier wave signal and the super- regenerative receiver was the answer.
The idea was probably born of the domestic tuned radio frequency (trf) radios of the 1930s. As the name suggests these radios had amplifiers tuned to the incoming frequency over a wave band.
Because of the wide frequency range covered and amplifier bandwidth limitations, the response to different stations could be extremely variable; a problem which was largely overcome by the introduction of the superhet receiver and automatic gain control.
However it was well known that the gain of a given rf. amplifier could be greatly enhanced by the application of positive feedback if the feedback was insufficient to cause oscillation.
Positive feedback was introduced by via a variable potentiometer called the Reaction Control which would be used in the following manner: - With a weak signal tuned in, the reaction control would be very carefully advanced. The signal would increase in volume until the RF amplifier burst into oscillation; the idea then being to back off the control until oscillation stopped. However, in the usual way of things, there was considerable hysteresis between position and effect and enhanced performance could only be attained if the advance could be stopped just before oscillation commenced; needing skill and patience. Pre superhet radios were therefore quite tricky beasts but it is the author’s contention that a good trf. receiver produces a crisper sound than a superhet receiver in the medium and long wavebands.
It is thought that the super-regenerative receiver grew out of the wartime need for a very high gain simple single valve receiver for which gain-enhancing positive feedback could be automatically controlled. The solution to this was to allow oscillations of the tuned frequency to alternatively build up and be suppressed under the control of a second (quench) oscillator operating at a lower radio frequency.
In this device the tuned amplifier is allowed to start oscillating during one half cycle of the quench oscillator waveform and its oscillations are suppressed during the following half cycle. During the ‘on’ part of the quench cycle, oscillations of the tuned amplifier build up exponentially from circuit noise. The time for these oscillations to reach their full amplitude is proportional to the Q value of the tuned circuit (number of cycles equals Q is a rule of thumb) which is why crystal oscillators take an age to build up. Therefore, depending upon the frequency of the quench oscillator, signal frequency oscillations can be allowed to reach full amplitude (logarithmic mode) or be curtailed before doing so (linear mode). The logarithmic mode is the more appropriate for model control because sensitivity is more important than preservation of the shape of a modulating waveform such as speech but it is important to note that the mean current drawn by the amplifying device (valve or transistor) is dependent upon what is going on in the oscillatory circuit.
Three basic types of supper-regenerative receiver were used for radio control of models:-
The hard valve receiver, the soft valve receiver and the transistor based receiver with circuits appropriate to single channel, galloping ghost and reed operation. For simplicity, circuits shown in the following illustrations are of the single channel variety.
The more complex valve receiver is the hard valve version but it is easier to understand than its soft valve counterpart. A typical hard valve receiver circuit is shown in Figure 2.
In the figure the 27MHz tuned and feedback windings areL2 and L1 respectively and similarly the tuned and feedback windings for the quench oscillator are L4 and L3. Voltages of the sine wave quench oscillator are dominant in the circuit and are such as to auto bias the valve into class C operation, whereby the action of the CR coupling to the grid biases it to a negative potential; causing the valve to conduct only for part of positive going half cycles of the waveform. It is during these positive half cycles that the 27MHz oscillator can start and build up. Perhaps counter-intuitively, the additional 27MHz oscillation causes a reduction of the mean current drawn by the valve and, if sufficiently strong due to the presence of signal from the aerial, the reduction causes the sensitive relay in the anode circuit to de-energise, breaking the circuit to the sequential escapement.
A typical soft valve receiver circuit is shown in Figure 3.
It may be seen from figure 3 that the soft valve receiver has fewer components that its hard valve counterpart. This is a direct result of the special properties of the gas filled triode (Thyratron) valve and it is necessary to have a basic appreciation of these in order to understand how the circuit works.
Mains voltage indicators are small glass bulbs filled with an inert gas (neon) at low pressure and having two electrodes spaced within the gas. In use they are connected across the mains supply via a series resistor of 470kW which limits the current through the indicator to about 300uA. With an applied emf. of around mains voltage or greater applied present current passes through the gas and causes it to glow the familiar orange colour.
The emission of light is the result of ionisation and excitation of the gas and it is called glow discharge. If the voltage directly across the bulb is measured it will be found to be of the order of 80 volts and this voltage will be found to be essentially constant irrespective of the applied emf.
In the valve era this property was employed to produce voltage references. In fact the glow discharge has a small positive resistance so the reference voltage increases very slightly for large increases of applied voltage and the concomitant increases of current through the gas.
It is apparent from the foregoing that the indicator will not pass significant current and operate in glow discharge unless the applied emf. exceeds 80V. Above this voltage the mains indicator works equally well for alternating or unidirectional applied emfs. because the electrodes are identical and it is unidirectional operation which is the subject of further discussion.
With an applied unidirectional emf; if the current limiting resistance is reduced progressively the light output increases and voltage across the bulb increases slightly until a point where it suddenly collapses to about 14 volts. The indicator has now entered the state known as arc discharge.
What has happened is that the large number of gas ions reaching the negative electrode (cathode) have heated it and caused it to emit electrons which in turn cause further conduction through the gas causing the voltage drop across the gas to collapse to a voltage close to the ionisation potential for the gas, called the burning voltage The symmetrical mains indicator would not do this at all efficiently but special diode valves were made with cathodes designed to emit t electrons. These were known as cold cathode rectifiers because the cathode was not heated separately as in a thermionic diode rectifier.
What is apparent from the foregoing is that it is not possible to achieve arc discharge without first going into glow discharge with a cold cathode. However if the cathode is heated and designed to emit electrons, the arc discharge state may be entered directly with a positive voltage of more than 14V, called the striking voltage, applied to the other electrode or anode. It stressed however that even, with a source of electrons, the arc discharge will only be sustained provided the voltage is above the ionisation potential and sufficient current is flowing.
Consider now the circuit of Figure 4A which embodies a gas filled thermionic diode connected to a positive supply via a series resistor and shunted by a capacitor. The value of the resistor is such that the current which could flow through it if the diode is short circuited is less than that necessary to sustain arc discharge. With the diode in situ, current through the resistor will charge the capacitor causing the voltage across it to rise exponentially to the striking voltage and an arc discharge is initiated. Once struck, the voltage across the diode drops rapidly to the burning voltage and remains until the capacitor can no longer sustain sufficient current, at which point the discharge is extinguished and the capacitor starts to charge again. The circuit is therefore oscillatory with voltage excursions between striking voltage and burning voltage determined by the CR time constant.
Figure 4B shows the effect of introducing a negatively biased grid to produce a gas filled triode. As for a vacuum triode, the grid controls the proportion of the electrons emitted from the cathode which reach the anode. Negative bias therefore ‘delays’ striking and concomitantly increases the amplitude and reduces the frequency of the oscillations. Once struck, however, the grid exerts no influence whatsoever.
It can be seen from figure 4B that the thyratron valve can be used in a relaxation oscillator circuit which is capable of fulfilling the function of the quench oscillator for a super-regenerative receiver. However ionization and deionization takes tens of micro seconds and there is no possibility of such an oscillator functioning at 27Mhz. Paradoxically the thyratron valve can sustain oscillations at 27MHz and beyond due to the phenomenon of negative resistance.
It will be remembered that the step from glow discharge to arc discharge involved a dramatic reduction in voltage for an increase of current – effectively a negative resistance step.
This behavior continues on a very much reduced scale when the arc discharge is burning ie. the current reduces slightly if the voltage across the discharge rises and vice versa. Arc discharge is therefore characterized by a small negative resistance. If a tuned circuit is connected in series with a valve in arc discharge, it will oscillate if the negative resistance of the arc exceeds the resistive loss of the tuned circuit.
Referring to Figure 3 It can be seen that the 27MHz tuned circuit is connected in series with the thyratron valve and that feedback to the grid of the valve from the circuit tap, in conjunction with the resistor and capacitor, will bias the grid to a negative voltage. The tuned signal oscillations and the quench oscillations are therefore interdependent.
In service, the performance of gas filled valves (thyratrons) gradually degenerated due to a phenomenon known as ‘gas clean up’ whereby the inert filling gas apparently disappeared; presumably due to adsorption by the electrode structure. This effect was quite rapid for the sub miniature valves (XFG1 etc.) used for model control and for this reason the aerial connection to the input tuned circuit of some receivers was via a selection of taps which could be changed in an attempt to maintain operation with an aging valve.
Super-regenerative receivers continued for a limited time after the advent of transistors, primarily in modulated quasi proportional control systems such as ‘galloping ghost’. The super-regenerative front end used in Terry Tippett’s Gallatrol receiver is shown in Figure 5. and which is based on Doug Boltons’s Flexitone receiver.
Referring to Figure 5 it is apparent that, like the soft valve receiver, there is no separate quench oscillator circuit. However a transistor does not have the properties peculiar to a thyratron valve which enable the latter to function as a relaxation oscillator. As the author has never built a transistor based super-regenerative receiver the lack of a quench oscillator caused a bit of head scratching.
It can be seen from the figure that the transistor is used in the common base configuration to function as an oscillator; having positive feedback from collector to emitter via a 25pF capacitor. Calculation of the quiescent voltages in the circuit places the collector at -3.6V and the emitter at -1.2V approximately with the transistor passing around 255uA. Thus, with just sufficient feedback, the oscillatory voltage at the collector would be expected to approach 4.8V peak to peak. However, with a 25pF capacitor, the feedback is excessive; resulting in the emitter base junction of the transistor being driven into heavy forward conduction by the positive going peaks of the oscillatory waveform. This diminishes the negative bias applied to the base, causing the transistor to cut off until the base bias network recharges the 2uF capacitor; permitting the transistor to turn on and for oscillations to build up again. The circuit squeggs. Ie. it is self quenching. However a signal introduced via the aerial will still influence how the oscillations build up and signal modulation can be separated by the low pass RC filter in the collector circuit.
Used in communications the super-regenerative receiver had some useful properties:-
It was very sensitive and would lock on to the strongest signal, suppressing interfering signals. (possibly of doubtful advantage for radio control)
It acted both as a receiver and a transmitter, permitting full duplex telephony communication between receivers. This capability made it extremely useful as a transponder during wartime, for example early IFF. equipment employed super-regeneration receivers to receive a coded signal and return a coded reply.
EXAMPLES OF SUPER-REGENERATIVE RECEIVERS :-
THE MINI 4 SUPER-REGENERATIVE RECEIVER
The circuit employs the common base oscillator which is employed for all of the transistorized super-regenerative receivers examined. The purpose of the 2n and 2n2 capacitors is a mystery because the 10F capacitor would need to be pretty inductive at 27MHz for them to have an effect.
Like other circuits published in RCM&E in the early 1960s, a builder would need a pretty good knowledge of electronics to get it working. For example diodes D1and D2 do not appear in the components list and were drawn with the wrong orientation in the published circuit (correct orientation in this circuit).
THE ‘NEW’ 305 SUPER-REGENERATIVE RECEIVER
I still have difficulty figuring out exactly what goes on in transistorized super-regenerative receivers but it appears that the New 305 has a separate quench oscillator incorporating the tuned transformer T1 and feedback to the transistor emitter via a 470pF capacitor. However bootstrapping the emitter signal to the base via the 30mF capacitor tends to counteract this supposition? Whatever actually occurs in the circuit, the voltage developed across the primary and secondary windings of T1 will be subject to the perturbations of the collector current due to 100% low frequency modulation of the 27MHz carrier.
I admire those who built such receivers and got them to work despite typographical errors and inconsistencies in the information provided. For example four capacitors in the component list have locations on the circuit board but do not appear in the circuit schematic.
BELOW IS ANOTHER TONE RECEIVER. SIMILAR TO THE CIRCUIT ABOVE.
THE RCM+E MINIATURE SUPER-REGENERATIVE RECEIVER
ONE OF THE FIRST RADIO CONTROL SUPER-REGENERATIVE MODEL CONTROL RECEIVERS IS SHOWN BELOW.
IT WAS CALLED THE ‘IVY CARRIER WAVE RECEIVER’ AND WE THINK, DESIGNED BY TOMMY IVES.
The receiver may have been related the ECC 951B Commander Rx or perhaps ED Boomerang Rx and possibly designed by Sommerhoff or Sallis
but that is purely conjecture. There also appears to have been some involvement with Macgregor Industries UK at the time.
THE TYPICAL CONSTRUCTION METHODS of radio control receivers of the 1940s early 1950s are clearly shown above with the Epic ‘Ivy’ receiver.
The current drop of the receiver would typically fall by around 1.5 milliamps with a 27MHz carrier signal from the transmitter. The polarized
relay used in this example was more capable of detecting this small current drop and clicking out onto its back contact.
JUST A LITTLE LATER IN THE 1950s…..the copper/paxolin laminate called Printed Circuit Board became available and R/C receiver construction
became more compact. The photo above is shows an early RCM+E super-regenerative receiver using the ‘new’ printed circuit construction.
Printed circuit boards offered compactness and neatness of the completed circuit. Much in those days, relied on the ‘ART’ of
the designer. The ‘pattern’ of the copper under the plastic board was manually drawn depending on the component sizes used.
The preparation of the printed circuit board involved painting the ‘pattern’ on the copper side of the board using cellulose
Model aircraft paint. When dry, the board was dumped into a bath of sulfuric acid which etched all of the copper film
Except the bits that were covered with paint. Cellulose thinners removed the paint and Holes were then drilled in
the copper areas for the component legs to be inserted and soldered. A typical printed circuit ‘pattern’ of the
late 1950s is shown above. This one is of the Gallatrol Galloping Ghost receiver.
THE HILL TWO VALVE SUPER-REGENERATIVE RECEIVER
The operation of the Hill two valve super-regenerative receiver is perhaps a little easier to understand than its single valve contemporaries. The circuit incorporating V1 constitutes both the RF and Quench oscillators. These are both of the Hartley (tapped coil) oscillator type; shunt fed for the RF oscillator and series fed for the Quench oscillator.
An unspecified wave wound component is specified for the quench coil – fabrication of which would be well beyond the capabilities of most enthusiasts. The component list contains no sourcing information.
The out put from this circuit will be short bursts of 27MHz repeating at quenching frequency. The presence of a signal at the aerial will increase the duration and possibly the amplitude of the 27MHz component.
The output signal is capacitively coupled to a voltage doubling rectifier D1 and D2 and the 5n capacitor between the grid and cathode of V2. The output of the rectifier is negative going and thus an increase of signal will reduce the current passed by V2.
A negative grid bias of something less than 1.5volts is achieved by connecting the cathode of D2 to the +1.5volts supply but this is counteracted by the forward volt drop across the rectifier due to current via the 1M resistor and it is likely that the resulting grid bias is close to zero. Thus V2 is at maximum conduction in the absence of a drive from V1 and, in the absence of a signal at the aerial, this reduction must not be sufficient to cause the relay to de-energize.
A signal at the aerial increases the activity of the RF component of the signal from V1, increasing the negative going output from the rectifier such that the relay de-energizes.
Correctly set up, the Hill receiver is capable of being much more reliable than the single valve variety, producing a larger change of relay current than can be obtained with the latter. Operation is thus far less affected by engine vibration and electrical noise. It is believed that a later version used a DCC90 double triode valve reducing weight and bulk. It is probable that the circuit schematic would not have been dramatically changed by this departure because both DL96 valves are triode strapped.
It is interesting to note that the valve cathode filaments are paralleled for operation at 1.5volts. series connected they may be used with a 3volt supply at half the current consumption. However, because the cathodes are directly heated, the filament voltage constitutes a cathode bias with an effective value of approximately half the filament voltage. Therefore the characteristics of valves operated from a three volt filament supply differ slightly from those of valves operated from a 1.5volt supply.
In an e-mail, Derek Round points out that the operation of V2 is in fact the reverse of that deduced by examination of the circuit schematic and that receipt of a signal results in an increase of relay current. Apparently the increased 27Mhz oscillation on receipt of a signal causes a concomitant decrease of the amplitude of the quench frequency and that this reduces the negative bias applied to the grid of V2 via the Selenium
rectifier. Although both the 27MHz and quench frequency oscillations are applied to the rectifier it appears that it is not responsive to the former, which possibly explains the odd choice of a contact cooled Selenium rectifier instead of Germanium diodes; the Selenium device being far too capacitive to be effective at 27MHz. This all goes to show what tricky beasts super-regenerative receivers can be. Derek sent a photograph (right) of his vintage Hill Mk 2 receiver.
PRESENT DAY SUPER REGENERATIVE RECEIVERS now look like the picture to the left. They are used in Remote locking systems for cars, garage doors and other applications. It seems that they are commonly used on the 433 and 868 MHz bands. The example shown is just 4cm maximum and wafer thin. Shape and component layout can be made to suit the application (key-fob etc.). It is interesting that both Super-Regenerative receiver and also Spread-Spectrum technology began during the WW2 years yet both are thriving today.
Further details on Super-regenerative receivers can be found on page 19 of this website.
When the author drafted this page he imagined that he was recording a long forgotten technology which afforded the only practical means for the radio control of models between the late 1940s and early 1960s. Consequently he was surprised by the apparent level of interest. He has learned subsequently that super regenerative receivers are widely used for consumer applications! (Dec 2016).
Thanks for reading !……David Caudrey and Terry Tippett.