Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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When creating field strength indicators, the dynamic range problem arises - it is difficult to manufacture a device that would allow monitoring both strong and weak signals. The task is simplified if a logarithmic amplifier is used, as the author of the proposed article did.

Field strength indicators are used to search for sources of radio emission or interference, as well as when tuning and testing antenna equipment. The usual requirements for these devices are large ranges of operating frequencies and indicated signal level, efficiency and small dimensions. All these requirements can be realized if specialized microcircuits are used. An example is the microcircuits of logarithmic amplifier-detectors from ANALOG DEVICES - AD606, AD8306, AD8307, etc.

The following is a brief description of the AD8307 chip and the design based on it. This microcircuit includes a six-stage (14,3 dB gain per stage) limiting amplifier with detecting circuits and other auxiliary nodes.

The main parameters

• Operating frequency range, MHz......0...500
• Input signal voltage range, dB......92
• Output voltage slope (with non-linearity no more than 1 dB), mV/dB......25
• Supply voltage (single-polar), V......2,7...5,5
• Current consumption, mA......7...8
• Noise power spectral density, nV/vHz......1,5
• Input impedance, kOhm......1,1
• Input capacitance, pf ...... 1,4

The circuit of the field strength indicator on this microcircuit is shown in fig. one.

Pins 1 and 8 of DA1 are a differential input, when using only one of them, the second one must be connected to a common wire through a capacitor. Pin 4 is an output, in the absence of an input signal, a voltage of about 0,2 ... 0,25 V is present at this output, and the output resistance is about 12 kOhm. When an input signal is applied, the output voltage increases by 25 mV for every 1 dB increase in the input signal.

The operating frequency range is limited from below by the capacitances of capacitors C1 and C2, from above - by the frequency properties of the DA1 chip and is approximately 500 MHz, and with a decrease in sensitivity by 20 dB - approximately 900 MHz. A pointer device, a microammeter RA1, was used as an indicator. Diodes VD1-VD4 are installed at the input, which protect the microcircuit from powerful signals and interference. The positive output of the pointer device is connected to the output of the microcircuit through the trimmer resistor R1, and the voltage from the trimmer resistor R3 is applied to the negative terminal. This is done so that in the absence of a signal, the instrument needle is set to zero.

The DA1 chip is powered by an integrated voltage regulator on the DA2 chip. The device is switched on by switch SA1. The current consumption is 11 ... 12 mA.

As an antenna, it is convenient to use a telescopic antenna several tens of centimeters long. The input impedance of the device is several hundred ohms, so to match the indicator with 50- or 75-ohm lines or antennas, a resistor with a resistance of 51 or 82 ohms, respectively, must be installed at the input.

Capacitor C1 is selected depending on the required sensitivity and operating frequency range. Its capacity can range from a few to several hundred picofarads.

Most of the parts are placed on a printed circuit board made of double-sided foil fiberglass with a thickness of 1 ... 1,5 mm, a sketch of which is shown in fig. 2.

The second metallized side is used as a shield and is connected to the common wire of the first side in several places. The board, together with the microammeter, is placed in a metal case with a lid. In the upper part there is an XW1 coaxial socket and a printed circuit board, which must be soldered along the edge to the case. Microammeter RA1 is installed on the side wall.

If the device is planned to be powered by a rechargeable battery, then any small-sized socket must be provided for charging it. In this case, the charger must be galvanically isolated from the mains.

The following parts can be used in the device: DA2 chip - KR1157EN502A, KR1157EN502B, microammeter RA1 - M4247 with a total deviation current of 100 μA. Jack XW1 - high-frequency small-sized any type, for example, SMA. Adjusted resistors - SPZ-19, constant - MLT, S2-33, R1-4. It is desirable to use capacitors C2-C6 - K10-17, C1 with an operating voltage of 300 V or more (K73), this will increase the safety of using the indicator. The fact is that when searching for sources of radio emission, there is a possibility that the antenna will touch the conductors connected to the network.

Setting up the device is easy. Resistor R3 sets the microammeter needle to zero in the absence of a signal. Then, an RF signal with a frequency of about 100 MHz and a voltage of 1 V is fed to the input. With a trimmer resistor R1, the microammeter needle is set to the mark "100".

The experimentally taken characteristics of the indicator are shown in fig. 3. They show that at frequencies less than 100 MHz, the indicator begins to respond to signals with a voltage of 20 ... 30 μV, and the dynamic range of the indicated voltage is 92 ... 95 dB. At a frequency of 500 MHz, the sensitivity drops to 80...100 µV, and at a frequency of 900 MHz it decreases to 500...600 µV. After the adjustment, it is necessary to remove such a dependence, build it in the form of a graph and place it on the indicator body.

If this device is used in conjunction with replaceable band-pass filters, then it can be used to tune or orient television antennas for the maximum received signal.

Author: I. Nechaev (UA3WIA), Kursk

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