This document describes an audio output power meter. The power meter can measure power from 0.1 mW to 50 W across fifteen impedance ranges from 4 to 10k ohms. It uses a current transformer and fifteen switch-selected resistors to indirectly measure the power dissipated in a load. The power meter provides a simple and accurate way to measure the power delivered by an audio frequency circuit such as an amplifier to different terminating loads.

2. INTRODUCTION:
It measures directly the power
delivered by an audio frequency
circuit such as an amplifier to a
terminating load of known
impedances.
It is absorption type of audio
output power meter indicating the
power indicating the power
dissipation in the load in watt or
dB . The power meter has a wide
dynamic range i.e. from .1 mw to
50 watt, the dB indication is from -
13dB to +47 dB . It has fifteen
impedance ranges-
4,8,15,25,50,75,10,150,400,600,8
00,1k,2k,5k,10k ohms.
The instrument is simple in
design and the performance is
excellent. I consists of fifteen
switch selected resistors
corresponding to and very close
in tollerence to the impedance
ranges, which are connected
fifteen corresponding taps on a
current transformer of CRGO
laminations.

3. BLOCK DIAGRAM OF OUTPUT POWER METER

4. CIRCUIT DESCRIPTION
The power meter indirectly measures microwave frequency
power by using two bridge circuits—the detection bridge and
the compensation and metering bridge. The detection bridge
incorporates a 10-kilohertz (kHz) oscillator in which the
amplitude is determined by the amount of heating of the
thermistors(thermally variable resistor) in that bridge caused
by microwave power. The compensation and metering bridge
contains thermistors that are affected by the same microwave
power heating as those of the detection bridge.

5. CIRCUIT DESCRIPTION
An unbalance in the metering bridge produces a 10-kHz error
signal. This error signal, plus the 10-kHz bias that is taken
directly from the 10-kHz OSCILLATOR-AMPLIFIER, is mixed
in the SYNCHRONOUS DETECTOR. The synchronous
detector produces a dc current that is proportional to the 10-
kHz error signal. The error signal is fed back to the
compensation and metering bridge, where it substitutes for
the 10-kHz power in heating the thermistor and drives the
bridge toward a state of balance. The dc output of the
synchronous detector also operates the meter circuit for a
visual indication of power.

6. SPECIFICATIONS:
Output Power :
5mW, 50mW, 0.5W,
5W, 15W, Meter
50W and 150W full
scale (50W and
150W full scale
ranges are only for
4, 8 and 16 ohms
impedances).
Type of
Measurement
Unbalanced
dB Scale : -13dB to
+7dB on all ranges.
Impedance Ranges
: Twelve switch
selected (Load)
impedances viz. 4,
8, 16, 50, 75, 125,
150, 600, 1K, 2K,
5K & 10K in ohms.
Impedance : ±3% at
1KHz. Accuracy
±5% from 20Hz to
20KHz.
Power Accuracy :
For I/P 1/2 FSD to
FSD : 6% of reading
when measured at
1KHz at normal
ambient
temperature (15OC
to 35OC). For I/P
1/10 of FSD to 1/2
FSD : 6% of reading
+3% of FSD when
measured at 1KHz
at normal ambient
temperature (15OC
to 35OC).

7. SPECIFICATIONS:
Frequency
Characteristics :
Referred to 1KHz,
20Hz to 20KHz,
±1dB.
Power : 6V DC
(from 4 x 1.5V pencil
cell) or Mains 230V
AC, 50Hz
(Optional).
Operating
Temperature:
0-40OC.
Dimensions : 235
(W) x 150 (H) x 245
(D) mm approx.
Weight : 3.5 Kg.
approx.
Standard
Accessories:
Instruction Manual -
1 No. SP1 to
Alligator Clip - 1 Pair

8. APPLICATIONS:
Output power meters, although fairly new in the industry, have a
widespread application.
They are capable of measuring the power output of a generator
source such as an amplifier or oscillator by providing a large
selection of terminating impedances for such source and measuring
the voltage which is developed across such termination. With the
indicating meter calibrated to read in milli watts, it is possible to use
this instrument as a very rapid means of determining the power
delivered by any given generator source to any desired load, of
determining the impedance value of an unknown load, of determining
the proper impedance value of the load for maximum transfer of
energy, of determining the impedance versus frequency
characteristics of the generator source and/or with the connecting
transmission line, of determining the impedance versus frequency
characteristics of the existing load and of determining the proper
impedance value for a mismatched load when such circumstances
are desirable.