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RF decibel meter circuit

An DIY RF decibel (or power) meter is an indispensable instrument in any radio workshop. Unfortunately, accurate, wideband models are fairly expensive, and home-constructed ones are generally not sufficiently sensitive and/or are very temperature-dependent. These drawbacks are overcome by a device from Analog Devices which has recently become available: a low-cost DC–500 MHz, 92 dB logarithmic amplifier that enables an accurate, not too expensive RF decibel meter to be constructed.
A few small modifications make the meter also suitable for low frequency measurements.
Decibel meter parameters:
Frequency range:
100 kHz – 110 MHz with an error <1 dB
100 kHz – 200 MHz with an error ≤ 2 dB
Decibel range:
32 – 117 dBμ with an error at 10 MHz ≤ 1 dB
Scaling: 10 mV dB–1
Input impedance: 50 Ω

DIY RF signal meter circuit schematic


Decibel meter PCB layout


Parts list
Resistors:
R1, R2 = 100 Ω, SMD
R3, R4 = 10.0 Ω
R5, R7 = see text
R6 = 5.62 kΩ
P1 = 5 kΩ (4.7 kΩ) multiturn upright preset potentiometer
P2 = 25 kΩ multiturn upright preset potentiometer
Capacitors:
C1, C4 = 0.01 μF, SMD
C2 = 0.1 μF, SMD
C3 = 2.2 μF, 10 V, tantalum
C5 = 0.1 μF. metallized polyester
C6 = 10 μF, 63 V, tantalum capacitor
Integrated circuits:
IC1 = AD8307AN (Analog Devices)
IC2 = 78LO5
IC3 = CA3140E
Miscellaneous:
K1 = 50 Ω BNC socket for board mounting
Enclosure
RF decibel meter description
The circuit diagram of the decibel meter in Figure 1 stands out by its simplicity, which is due to the Type AD8307 monolithic demodulating logarithmic amplifier, IC1, from Analog Devices.
The measurand (quantity to be measured) is applied to pin 8 (INP) of IC1 via input socket K1 and capacitor C1. The capacitor ensures that no direct voltage can reach the IC. The second input of the IC, pin 1 (INM) is linked to the earth line via capacitor C4. The values of C1 and C4 are chosen to give a lower limit of the frequency range below 100 kHz.
Resistors R1 and R2 ensure that the input impedance of the meter is the usual value in RF equipment of 50 Ω. A parallel network is used to minimize any parasitic properties of the resistors. It is recommended to use SMT (surface mount technology) resistors.
The output of IC1 is essentially a current that causes a potential dropacross a 12.5 kΩ internal resistor which is available at output pin 4. Resistors series network R6-P1 is in parallel with the internal resistance to modify the scale factor, which is 25 mV dB–1 in the absence of an external circuit.
Capacitor C5 averages the output signal to ensure a stable display. Its value depends on the application: a larger capacitance gives a more stable, but slow, display; a smaller value is recommended for fast sweeping.
Preset P2 permits parallel shifting of the characteristic to give an attenuation of up to 14 dB or an amplification of up to 26 dB between the input socket and pin 8 of IC1, provided that R5 = 0. Resistor R5 provides a narrowing of this preset range.
RF decibel meter description
The display may be a digital multimeter, but, although this is accurate, it is not easily calibrated.
A moving coil metering network with series resistor R7 facilitates recognizing any drift such as encountered, for instance, during calibration, but does not make reading it easy.
Measurements with sweep frequencies can, of course, be displayed on an oscilloscope. The decibel meter outputs a direct voltage that is directly proportional to the input signal. The display is calibrated in dBμ (decibel referred to 1 microvolt). The scale factor is 100mV dB–1, so that an input signal of 100 dBμ results in an output voltage of 1 V.
Radio decibel meter construction
The meter circuit is best built on the printed circuit board shown in Figure 2, but this is not available ready made. As mentioned earlier, some of the components should be SMDs (surface mount devices) as specified in the components list. If the circuit is constructed on prototyping board, standard components may, of course, be used. Keep all wiring as short as possible, however.
If operation up to 30 MHz only is needed, IC1 may be inserted in a socket, but for use at higher frequencies the circuit should be soldered directly on to the board. This is best done after all other components have been fitted and the board has been checked thoroughly. This measure is to protect the AD8307, since this is not a cheap component.
Since the meter is an RF unit, it is clear that it should be fitted in an earthed metal enclosure. The power supply should, of course, not be fitted in the same enclosure. Another important aspect is that the 9–15 V supply voltage should be ‘clean’. It is advisable to use feedthrough capacitors at the power line inputs and measurement output.
Calibration
The meter circuit should be calibrated with a suitable RF signal generator or, in an emergency, an AF signal generator with calibrated attenuator. Apply a signal at a frequency of 10 MHz and a level of 60 dBμ (1 mV r.m.s.) to the input of the meter circuit. Using a digital multimeter, measure the voltage at pin 3 of IC3, increase or reduce the output of the signal generator by exactly 10 dB and turn P1 to cause a change in the multimeter reading of 100 mV. The absolute value of the output voltage is not significant.
Next, apply a signal at a level of exactly 60 dBμ to pin 8 of IC1 and turn P2 until the meter indicates 600 mV. If the requisite equipment is available, the calibration process can be repeated at a number of frequencies for greater versatility of operation.
If a signal generator is not to hand, adjust P1 until the resistance between its wiper and earth is 1383 Ω measured with a digital multimeter. Finally, adjust P2 to obtain a voltage of 1.627 V at pin 5 of IC1, again measured with a digital multimeter.

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