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Practical Radio Circuits Part 3

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Practical Radio Circuits Part 3 Constructional Project PRACTICAL RADIO CIRCUITS RAYMOND HAIGH Part 3: Regenerative Receivers – A modern-day version of Armstrong’s 1913 circuit. Dispelling the mysteries of Radio. Circuits for the set builder and experimenter. N Part Two we reviewed the history and Further, because tuned circuit Q is raised, AUDIO SIGNAL theory of regeneration and used the there is also a dramatic improvement in Itechnique to improve the performance selectivity. The amplified audio signal is devel- of two, portable, radio receivers. This Source bias is developed across resis- oped across drain load resistor R2 and month a regenerative receiver designed for tor R3, which is bypassed by capacitor coupled to the audio amplifier stages via serious listening on the long, medium and C2, and signal detection occurs by way d.c. blocking capacitor C3. Residual short wave bands will be described. of rectification at the gate/source junc- radio frequencies are bypassed by tion of transistor TR1. (In the original capacitor C4. BASICS valve versions, diode rectification In some versions of the circuit, a transis- For a regenerative receiver to perform between grid and cathode resulted in tor type audio transformer (e.g., an LT44) well, three basic requirements have to be detection or demodulation). Low value is substituted for the resistive load in order met. (1) Its regeneration control must be blocking capacitor C1 prevents the to maximize signal transfer. However, even smooth, completely free from backlash, audio voltage developed across gate with this arrangement, the audio output is and have a minimal effect on tuning. resistor, R1, being shorted by the tuning very low, and at least one additional stage (2) The tuned circuit to which the Q coil. of pre-amplification will be required ahead enhancing positive feedback is applied must be isolated from the aerial: failure to do this will result in reaction dead spots on frequencies (or harmonics) at which the VR1 +9V aerial is resonant. 47k (3) Provision must be made, at the receiver input, for the attenuation of pow- R2 4k7 erful signals, otherwise the regenerative circuit will lock onto strong carriers and it + AERIAL C5 will not be possible to receive weak trans- 47µ L4 missions on adjacent channels. 1mH R.F. ARMSTRONG CHOKE C4 UPDATED 1n TR1 A modern, transistor version of C1 d 22p 2N3819 Armstrong’s circuit, patented in 1913, is g shown in Fig.3.1. In the original, the feed- s + back coil (tickler coil in the USA), L3, was C3 R4 µ connected in place of the radio frequency 47 22k VC2 choke, L4, and regeneration controlled by L2 200p adjusting its proximity to tuning coil L2. L3 TUNING L1 This was eventually superceded by the AUDIO variable capacitor system shown here, REGEN. OUTPUT R1 C2 R3 where increasing the capacitance of VC1 VC1 1M 100n 2k7 increases feedback through L3. 50p Choke L4, in the drain (d) circuit of 0V TR1, acts as a load for the radio frequency component of the amplified signal, con- EARTH trolled amounts of which are fed back through L3 to overcome losses in the tuned circuit (L2/VC2) and increase its efficien- cy or Q factor. Fig.3.1. Circuit diagram for a modern transistor version of Armstrong’s Armstrong discovered that the tech- Regenerative Detector. In Armstrong’s circuit, patented in 1913, feedback coil L3 is nique permits the amplification of weak connected in place of r.f. choke L4 and its proximity to L2 is adjusted to control signals by a factor of more than 1000. regeneration. VR1 presets regeneration control range. 568 Everyday Practical Electronics, August 2003 of the headphone or speaker amplifiers described in Parts 1 and 2. The gain of TR1, and hence its willing- ness to regenerate, is determined by preset potentiometer VR1, which adjusts the drain voltage. By this means the circuit can be optimized for different transistors and coil feedback winding ratios. POPULARITY Using gradually improving versions of Lee de Forest’s triode valve as the amplify- ing device, Armstrong’s circuit, followed by a one or two valve audio amplifier, remained popular as a domestic receiver until the end of the 1920s in the USA and well into the 1930s in the UK. Regenerative receivers were still being manufactured by Ever-Ready (their Model H) as late as the 1950s, and they were con- structed by amateurs up to the close of the valve era. It is not easy to use low impedance, cur- rent amplifying bi-polar transistors in this circuit. However, it saw something of a revival in amateur circles following the introduction of the field effect transistor Fig.3.2. Circuit diagram for an improved Regenerative Detector based on a Hartley (f.e.t.), with its more valve-like characteris- oscillator. VR2 presets regeneration control range. This circuit forms an excellent tics, in the late 1960s. Q-Multiplier for use with a separate detector, in which case increase capacitor C4 value to 100nF and delete C6, C7 and R4. IMPROVED CIRCUITS Smooth regeneration can be obtained characteristic curve, forms an excellent more readily, and with simpler coils, by detector stage. The “drain bend” version +9V configuring the Q multiplier, or regenera- (the transistor equivalent of the valve R7 10k tive detector, as a Hartley oscillator. A “anode bend” detector) is included as typical circuit is given in Fig.3.2, where TR3 in the Regenerative Receiver design TO 'HOT' END OF TUNING d feedback from the source(s) of the dual- illustrated in Fig.3.4. This arrangement COIL g TR3 gate MOSFET, TR1, is connected to a is discussed later. 2N3819 C14 47µ tapping on the tuning coil L2. Alternatively, the audio output can be s + The level of feedback is controlled by taken from the source of the f.e.t. We then + C13 VR1, which varies the gain of the transistor have the transistor equivalent of the valve 47µ by adjusting the voltage on its gate g2. “infinite impedance” detector. The modi- R9 C10 AUDIO Preset potentiometer VR2 (wired as a vari- fied circuit, using the component number- 22k 100n OUTPUT able resistor) determines the source bias ing of Fig.3.4 for ease of comparison, is and optimizes the action of the regenera- shown in Fig.3.3. 0V tion control for individual tuning coils and High-value source bias resistor R9 is transistors. bypassed only at radio frequencies by Audio output is developed across drain capacitor C10 (C9 is omitted), and C13 is Fig.3.3. Source-follower detector cir- load resistor R3. The stage is decoupled increased to 47mF to decouple the stage cuit. Transistor equivalent of the valve from the supply rail by resistor R2 and which is now in the “anode-bend” detector. capacitor C5, and the filter network formed common drain mode. by C4, R4 and C6 removes radio frequen- The r.f. filter compo- cies from the output. nents, R8 and C15, and the original decouplers, SEPARATION R6 and C11, are not During the valve era, the functions of required. signal detection and Q multiplication or There is little to regeneration were invariably carried out choose between the two by a single device. This combining of detectors: both work functions can make it more difficult to well, imposing very lit- obtain the smooth, backlash-free control tle damping on the tuned of regeneration which is crucial to the circuit. In theory there is efficient operation of a receiver of this some gain with the drain kind. Best modern practice uses separate bend version whilst the transistors. gain of the source fol- The dual-gate MOSFET circuit illus- lower is slightly less trated in Fig.3.2 can be used just as a Q- than unity. multiplier by increasing the value of In practice, the need capacitor C4 to 100nF. Filter components, to ensure non-linearity R4, C6 and coupling capacitor C7, can be over a range of f.e.t. omitted when the stage is configured in characteristics results in this way. the drain bend circuit The “hot” end of the tuned circuit must, providing very little of course, be connected to gate g1 of the gain. If the value of r.f. transistor, and resistor R1 is best retained bypass capacitor C10 in to hold gate g1 at 0V during coil changing. Fig.3.3 is reduced, the source-follower detector DETECTORS may become unstable A field effect transistor (f.e.t.), biased when the regeneration into the non-linear region of its control is critically set. Everyday Practical Electronics, August 2003 569 HIGH PERFORMANCE REGENERATIVE RADIO A modern-day update of a 1913 circuit The full circuit diagram for a High and control of regeneration Performance Regenerative Radio incorpo- will become erratic, especially rating the essential features described here when tuning capacitor VC1 is set at is given in Fig.3.4. It is easy to set up and a low value. performs well. It could be argued that using a field Grounded base stage, TR1, isolates the effect transistor, with its near square law wound, short wave coil L2 is illustrated in tuned circuit L2/VC1 from the aerial and characteristics, in the TR1 position, would Fig.3.6. TR2 functions as the Q-Multiplier. Field reduce the receiver’s susceptibility to The tuning capacitor VC1 is a 10pF to effect transistor TR3 is a drain bend detector cross-modulation. 260pF unit formed by connecting both a.m. and transistor TR4 an audio preamplifier. Cross-modulation occurs when a powerful gangs of a polyvaricon (polythene dielec- Although excellent Q multipliers can signal drives the input stage into non-lineari- tric) capacitor in parallel.
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