Encyclopedia of radio electronics and electrical engineering / radio reception

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The economical receivers that drive loudspeakers described in amateur radio literature have power. consumed in silent mode, from a few to tens of milliwatts. The maximum output power of economical receivers is usually not lower than 50 mW. However, it is possible to design receivers that consume power on the order of 100 μW. These receivers can sound small rooms (<20m2) with low noise levels. Despite the fact that the design of ultra-economical receivers has not been developed, examples of such receivers can be found. These are primarily loud-speaking receivers powered by the field energy of a nearby radio station. A good example of an ultra-efficient receiver powered by a telephone capsule is the small-sized receiver described in (1).

How much power is needed to listen to radio broadcasts in a small room? It was experimentally found that, depending on the listening conditions, in a room with an area of ​​about 16 m2, radio transmissions can be listened to with a minimum power supplied to the loudspeaker, of the order of 10 ... 1000 μW. Two heads of type 1 GD-40 were installed in the loudspeaker. The signal level was evaluated visually using an oscilloscope connected in parallel to the loudspeaker. The power required for listening depends mainly on the presence of noise and air currents, as well as on the distance between the listener and the loudspeaker. Naturally, these estimates are subjective, but they give an idea of ​​what kind of power we are dealing with when listening to programs in a small room quietly.

When developing the described receiver, the goal was to make a direct amplification receiver with minimal power consumption. Two options have been developed. The first one is with a ferrite antenna and a resonant UHF (3 transistors). The disadvantage of this option is the narrow UHF bandwidth at long wavelengths. The second option is a receiver without UHF. but with a loop antenna with an area of ​​about 1 m, the reception quality in this case has improved.

The diagram of the second version of the receiver is shown in fig. 1.

Rice. 1 (click to enlarge)

The receiver's quiescent current is 20 μA; at an average volume, the current consumed is in the range of 35 ... 60 μA. With an average current consumption of 50 μA, the peak power at the loudspeaker reaches 100 ... 120 μW. The supply voltage is 2.5 ... 3 V. During the tests, the receiver provided reception of three stations in the LW range, the nearest of which was at a distance of 120 km.

The bandpass filter formed by the elements WA1, C1, C2, L1, C3 provides the receiver with good selectivity and sufficient bandwidth. At the input of the transistor detector, the RF signal level reaches 10...15 mV, the AM detector based on the transistor VT1 according to the scheme of V. Polyakov [2] works quite well at currents of several microamperes.

Pre-amplification of the low-frequency signal occurs in a cascade on transistors VT2, VT3, VT4, VT5. The circuit used with a counter dynamic load allows you to adjust the current consumption with just one resistor R7. Capacitors C9 and C11 are designed to boost the upper frequencies of the low-frequency signal. The output stage on transistors VT6, VT7, VT8, VT9 operates in class AB mode. With the correct setting of the quiescent current VT8, VT9, such a cascade provides a fairly good sound quality. The voltage gain of the cascade is 4...6. Transformer T1 is necessary to match the output stage of the ultrasonic frequency converter and the loudspeaker heads BA1, BA2. The load resistance of the ultrasonic ultra-efficient receiver can be in the range from hundreds of ohms to tens of kilo-ohms. The maximum output power of the UZCH is about 120 μW.

Transistors VT2..VT5, VT8, VT9 are selected with a current transfer ratio of 120..200. The loop antenna has 15 turns with an area of ​​\u1b\u0,35babout 1 m. The wire is PEV 160. Coil L200 is wound on a standard ferrite rod 60 mm long. contains 1 turns with a tap from the 31th turn. As a transformer T9, a rewound transformer TV2200-600 (from tube TVs) was used. In the primary winding - 130 + 0,4 turns, in the secondary - 1 turns (PEV 40). The XNUMXGD-XNUMXR loudspeaker heads are installed in a small box without a back wall. Diffusers are open.

It is advisable to start setting up the receiver by tuning the circuits WA1, C2 and L1, C3 to the frequency of the radio station. This can be done with an oscilloscope or millivoltmeter. The signal voltage at the output of the coil L1 should be 5..20 mV. At high values, distortions in the detector are possible. Adjusted resistors R1, R7 set the optimal current consumption of the detector and the preliminary stage of the ultrasonic frequency converter. After tuning, they can be replaced with fixed resistors. Adjustment of the output stage is reduced to setting the quiescent current of transistors VT8, VT9 with a trimming resistor R10. To achieve the lowest values ​​of the consumed current, the quiescent current is set equal to 5..10 μA. When the supply voltage changes, the quiescent current will have to be adjusted, but very rarely, perhaps once every few months. If the adjustment is undesirable, we can recommend setting the quiescent current in the range of 100 ... 150 μA.

Experiments with the described receiver showed that a battery of old, used galvanic cells can be used as a power source. A battery was made up of four old 316-type cells with a total voltage of 3 V. The receiver worked from such a source for about two months without turning off the power: approximately 8 hours a day - at medium volume, and the rest of the time - in silence mode. From a ionistor charged to 3 V with a capacity of 1 F, the receiver worked for more than 6 hours. According to calculations, from two fresh cells of type 316, the receiver should work for about 10000 hours, i.e. batteries can be changed every few years.

It is necessary to remind once again that the described receiver is intended for operation in a small room in the absence of noise. It is not necessary to think that the receiver produces sounds at the threshold of hearing. The volume is such that good intelligibility of transmissions is maintained at a distance of 5..6 m from the loudspeaker. If necessary, the output power can be increased. As a transformer T1, the output transformer from the Alpinist 405 receiver was tested.

Its connection diagram is shown in Fig.2. In this case, the quiescent current of the output transistors had to be increased to 80..100 µA. The average current consumption of the receiver was in the range of 300..600 μA, depending on the volume. The output power increased to 1,8..2 mW.

Fig. 2

In the described receiver, the economy limit has not yet been reached. What is this limit? Who will take on the development of a receiver with record efficiency?

Literature

1. Malshievsky I. Small-sized broadcasting receiver. - Radio, 1989. No. 1.
2. Polyakov V. Sensitive amplitude detector - Radio, 1994, No. 7.

Author: I. Gilmanov, Sterlitamak, Bashkortostan

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