Linear RMS-Sensing Voltmeter and AM Detector
By David Knight.
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Conventional rectifier type voltmeter arrangements require high
voltage inputs in order to overcome the effect of diode forward
threshold voltage on linearity. A bridge formed with germanium
diodes will provide a reasonably linear display with 2V RMS input,
but it will barely register at all until the input exceeds 0.4V
P-P. Thus, using a conventional diode bridge to measure an input
signal of 1mV requires a voltage gain
of 126dB in order to achieve modest linearity. Partial biasing
of the diodes may be used to improve linearity, but such arrangements
are prone to thermal drift. In this design, the signal is fed to a complementary symmetrical push-pull amplifier, with a feedback factor of 1, which delivers power into a load resistor. See fig. 1 below. |
The amplifier is biased for zero quiescent current (class B/C).
The large feedback ratio forces the system into linearity, and
reduces cross-over distortion to a negligible value for the purposes
of this application. Thus the voltage appearing across the load
resistor is the same as the input voltage, and hence: TR1 charges C1, TR2 discharges C1; hence the average current flowing in TR1 collector is half the RMS load current. The meter reading is therefore:  The meter sensitivity is thus defined by the choice of load resistor, and the scale is perfectly linear. Note that the meter can be placed in the collector circuit of either TR1 or TR2. By placing a resistor in the collector of TR2 and shunting with a capacitor to provide a suitable time-constant, the circuit also behaves as a linear AM detector. The audible noise due to the circuit is negligible, but good power supply smoothing is necessary. A working example of the voltmeter / detector is shown in fig 2. |
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In this case, the load resistor is 100W,
and the meter movement is 100mA FSD,
giving a sensitivity of 20mV RMS FSD at the wiper of the input
calibration potentiometer. Thus a 1mV
signal only requires 86dB overall voltage amplification in order
to give a full-scale reading. The circuit also features a ×2
(+6dB) scale expansion switch (giving 10mV RMS FSD), implemented
by reducing the load resistor to 50W.
The LM386 audio amplifier provides 26dB (×20) voltage gain.
The inductor in the LM386 supply feed prevents RF distortion
products from the audio amplification process from appearing
on the supply rail. Note that in this battery powered circuit,
the audio is taken from the top of the totem pole and the meter
from the bottom. This arrangement was chosen to give the lowest
possible level of spurious radiation from the meter wiring, the
detector being part of a high gain ultrasonic receiver with the
meter mounted close to the input socket. In a mains powered instrument,
the audio should be taken from the bottom of the totem pole,
since this is the arrangement giving best supply (mains hum)
rejection in the audio output. The example in fig. 2 is part
of a system operating at 40KHz. The detector input response however,
with the 68pF capacitor at the input removed (and assuming a
sensible layout), was found to be flat within 0.5dB up to 1.1MHz,
and -3dB at 1.6MHz. A further example of this detector operating
at 100KHz is given in the author's RA17
adapter article. The meter movement can, if desired, be calibrated in dB. The following table gives dB markings against a 0 - 100 logging scale, assuming +3dB FSD. |
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D.W. Knight. 1981,1987,2000