Field indicator on two chips
Encyclopedia of radio electronics and electrical engineering / Field strength detectors
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Brief characteristics:
- Operating frequency ............................................20-1300 MHz
- Sensitivity.................................1 mV
- Limits of localization ............................... 0,05-7 m
- Supply voltage ............................... 4,5-9 V
- Consumption current .................................. 8 mA
Antenna type - telescopic
The scheme of a simple field indicator (1) has a very big drawback, namely, low filtering at the input, as a result of which the device reacts even to electrical wiring in the room, besides, it has a very low sensitivity - about 50 mV, so it is difficult to find low-power transmitters .
Slightly more complex in design, but much more convenient to use, the search device shown in this figure is widespread among organizations and individuals.
It is convenient to use the device for monitoring the operation and tuning of low-power transmitters operating in a wide frequency range.
This device is designed for local search of radio bookmarks. Its distinctive features are ease of repetition, reliability, small dimensions. The disadvantage is that it reacts a little to extraneous radio emissions from television and radio transmitting stations, radio telephones. But this disadvantage is more than compensated by the simplicity and cheapness of the indicator.
The input signal induced by the telescopic antenna is fed to the RF input amplifier, built on the transistor VT1, and then, through the filter C1, L1, C3 to the detector-comparator DA1. The threshold for turning on the comparator is set by resistor R5. The comparator signal from output 6 through the inverter DD1.3 and the key VT2 controls the generator of rectangular pulses on the elements DD1.4, DD1.5 with a frequency of 1 Hz, which, in turn, turns on the audio frequency generator on DD1.1, DD1.2.
LED VD1-two-color. VD1.1 signals the power on with green light, VD2.2 indicates the detection of a source of radio emission in red.
Setting up the device is to select the op-amp DA1 with the highest possible gain. The distance at which the indicator must respond stably with an antenna 30 cm long to a radio transmitter with a power of 1 mW must be at least 50 cm.
The KT3101 transistor can be replaced with KT371, KT368 with a gain of at least 150. Operational amplifier - K140UD608. K140UD708. The ALS331 LED can be replaced with conventional ones, such as AL307, by turning them on instead of VD1.1 and VD1.2. The inductor has 19 turns wound in a row on any MLT 0,125 resistor with PEL-0,1 wire.
Author: tolik777 (aka Viper); Publication: cxem.net
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