tune the FM receiver to locate the transmitter frequency. If
necessary, trim the slug to tune the transmitter to a clear
spot in the FM band. RV1 should then be trimmed to set
the modulation at a clean level.
TRANSISTOR AC VOLTMETERS
An ordinary moving-coil meter can be made to read
AC voltages by feeding them to it via a rectifier and suitable multiplier resistor, but produces grossly non-linear
scale readings if used to give FSD values below a few volts.
This non-linearity problem can be overcome by connecting
the meter circuitry into the feedback loop of a transistor
common-emitter amplifier, as shown in the circuits of
Figures 13 to 15, which (with the Rm values shown) each
read 1 V FSD.
The Figure 13 circuit uses a bridge rectifier type of
meter network, and draws a quiescent current of 0.3 mA,
has an FSD frequency response that is flat from below 15
Hz to above 150 kHz, and has superb linearity up to 100
kHz when using IN4148 silicon diodes or to above 150 kHz
when using BAT85 Schottky types. R1 sets Q1's quiescent
current at about treble the meter's FSD value, and thus
gives the meter automatic overload protection.
Figures 14 and 15 show pseudo full-wave and ghosted
half-wave versions of the above circuit. These have a performance similar to that of Figure 13, but with better linearity and lower sensitivity. D3 is
sometimes used in these circuits to apply slight forward bias
to D1 and D2 and thus enhance
linearity, but this makes the
meter pass a standing current
when no AC input is applied.
The diodes used in these and
all other electronic AC meter
circuits shown in this article
should be either silicon
(IN4148, etc.) or (for exceptionally good performance)
Schottky types; germanium
types should not be used.
In the circuits in Figures 13
to 15, the FSD sensitivity is set at 1 V
via Rm, which can
not be reduced
below the values
shown without
incurring a loss of
meter linearity.
The Rm value
can, however, safely be increased, to
give higher FSD
values, e.g., by a
factor of 10 for 10
V FSD, etc.
If greater FSD
FEBRUARY 2004
Figure 18. This x10
wideband pre-amplifier
is used to boost an AC
millivoltmeter's sensitivity.
Figure 15. Ghosted half-wave version
of the 1 V AC meter.
sensitivity is wanted from the above circuits, it can be
obtained by applying the input signal via a suitable pre-amplifier, i.e., via a + 60 dB amplifier for 1 mV sensitivity,
etc. Figure 16 shows this technique applied to the Figure
13 circuit, to give an FSD sensitivity variable between 20
mV and 200 mV via RV1. With the sensitivity set at 100
mV FSD, this circuit has an input impedance of 25 K and
a bandwidth that is flat within 0.5 dB to 150 kHz.
AC MILLIVOLTMETER CIRCUITS
A one-transistor AC meter cannot be given an FSD
sensitivity greater than 1 V without loss of linearity. If
greater sensitivity is needed, two or more stages of transistor amplification must be used. The highest useful FSD
sensitivity that can be obtained (with good linearity and
gain stability) from a two-transistor circuit is 10 mV, and
Figure 17 shows an excellent example that gives FSD sensitivities in the range 10 mV to 100 mV (set via Rx). It uses
D1 and D2 in the "ghosted half-wave" configuration, and its
response is flat within 0.5 dB to above 150 kHz; the circuit's input impedance is about 120 K when set to give 100
Figure 16. This AC voltmeter can be set to give FSD
sensitivities in the range 20 mV to 200 mV.
Figure 17. Wideband AC millivoltmeter with FSD sensitivity
variable from 10 mV to 100 mV via Rx.
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