head
phones for listening. I have received AM broadcast stations in Canada, Texas, and a low power
AM station in Boise Idaho (from my northern California location). The 310 easily tunes around
hetrodynes on crowded AM channels. The only deficiency it has as a r
eceiver is relatively flat audio
quality. I intend to improve that, with the inclusion of a 20 transistor audio amplifier IC (well, that is
progress, I guess). HP's first wave analyzer was the 302 designed in 1959
, also a transistorized design.
Then came
the 310. The 312 came along next and featured a maximum frequency of 18 MHz.
The 312 also sported a digital frequency display. The 312 looks very much like an upgraded 310
(same chassis). Next came the digital 3581 family. This analyzer dropped back to 50
kHz maximum
frequency, but was much smaller. The last in the line was the 3586 produced in 1980. The 3586 is
fully synthesized and covered 50 Hz to 32 MHz. Digital techniques applied to Spectrum Analyzers
ended the need for Wave Analyzers and now cover the
market that was once held by these
instruments.
6. Draw and explain the working of Digital multimeter
Multimeter
A digital multimeter
A multimeter or a multitester, also known as a or VOM(Volt
-
Ohm meter), is an
electronic
measuring instrument
that combines several measurement functions in one unit. A typical multimeter
may include features such as the abilit
y to measure
voltage
,
current
and
resistance
. Multimeters may
use
analog
or
digital circuits
—
analog multim
eters(AMM) and digital multimeters (often abbreviated
DMM or DVOM.) Analog instruments are usually based on a
microammeter
whose pointer moves
over a scale
calibration
for all the different measurements that can be made; digital instruments usually
display digits, but may display a bar of a length proportional to the quantity measured.
A multimeter can be a han
d
-
held device useful for basic
fault
finding and field service work or
a
bench instrument
which can measure to a very high degree of accuracy. They can be used to
troubleshoot electrical problems in a wide array of industrial and household devices such as
electronic
equipment
, motor controls,
domestic appliances
,
power supplies
, and wiring systems.
Multimeters are available in a wide ranges of features and prices. Cheap multimeters can cost
less than US$10, while the top of the line multimeters can cost more than US$5,000.
The first moving
-
pointer current
-
d
etecting device was the
galvanometer
in 1820. These were
used to measure resistance and voltage by using a
Wheatstone bridge
, and comparing the unknown
quantity to a
reference voltage
or resistance. While us
eful in the lab, the devices were very slow and
impractical in the field. These galvanometers were bulky and delicate.
The
D'Arsonval/Weston
meter movement used a fine metal spring to give proportional
measurement rather than just detection, and built
-
in permanent
field magnets
made
deflection
independent of the 3D orientation of the meter. These features enabled dispensing with Wheatstone
bridges, and made meas
urement quick and easy. By adding a series or
shunt resistor
, more than one
range of voltage or current could be measured with one movement.
Multimeters were invented in the e
arly 1920s as
radio receivers
and other
vacuum tube
electronic devices
became more common. The
invention
of the first multimeter is attributed to British
Post Office engineer,
Donald Macadie
, who became dissatisfied with having to carry many separate
instruments required for the maintenance of the
telecommunications
circuits.
[1]
Macadie invented an
instrument which could measure
amperes
(aka amps), volts and
ohms
, so the multifunctional meter
was then named
Avometer
.
[2]
The meter comprised a moving coil meter, voltage and precision
resistors, and switches and sockets to select the range.
Macadie took his idea to the
Automatic Coil Winder and Electrical Equipment Company
(ACWEEC, found
ed in ~1923).
[2]
The first AVO was put on sale in 1923, and although it was initially
a
DC
. M
any of its features remained almost unaltered through to the last Model 8.
Pocket watch style meters were in widespread use in the 1920s, at much lower cost than
Avometers
. The metal case
was normally connected to the negative connection, an arrangement that
caused numerous electric shocks. The technical specifications of these devices were often crude, for
example the one illustrated has a
resistance
of just 33 ohms per volt, a
non
-
linear
scale and no zero
adjustment.
The usual analog multimeter when used for voltage measurements loads the circuit
under test
to some extent (a microammeter with full
-
scale current of 50
μA
, the highest sensitivity commonly
available, must draw at least 50 microamps from the circuit under test to deflect
fully). This may load
a high
-
impedance circuit so much as to affect the circuit, and to give a low reading.
Vacuum Tube Voltmeters or valve voltmeters (
VTVM
, VVM) were used for voltage
measuremen
ts in electronic circuits where high
impedance
was necessary. The VTVM had a fixed
input impedance of typically 1
megohm or more, usually through use of a
cathode follower
input
circuit, and thus did not significantly load the circuit being tested. Before the introduction of digital
electronic high
-
impedance
analog
transistor
and
field effect transistor
(FETs) voltmeters were used.
Modern digital meters and some modern analog meters use electronic input circuitry to achieve high
-
input impedance
—
their voltage ranges are
functionally equivalent
to VTVMs.
Additional
scales
such as
decibels
, and
functions
such as
capacitance
,
transistor gain
,
frequency
,
duty cycle
, display hold, and buzzers which sound when the measured resistance is small
have been included on many multimeters. While multimeters may be supplemented by more
specialized equipment in a technician's toolkit, some modern multimeters include even more
additional functions for specialized applications (e.g., temperature with a
thermocouple
probe,
inductance, connectivity to a
computer
, speaking measured value, etc.).
Contemporary multimeters can measure many quantities. The common ones are:
Voltage
,
alternating
and
direct
, in
volts
.
Current
, alternating and direct, in
amperes
.The
frequency
range for which AC
measurements
are accurate must be specified.
Resistance
in
ohms
.
Additionally, some multimeters meas
ure:
Capacitance
in
farads
.
Conductance
in
siemens
.
Decibels
.
Duty cycle
as a
percentage
.
Frequency
in
hert
z
.
Inductance
in
henrys
.
Temper
ature
in degrees
Celsius
or
Fahrenheit
, with an appropriate temperature
test probe
,
often a
thermocouple
.
Digital multimeters may also include circuits for:
Continuity
;
beeps
when a circuit
conducts
.
Diodes
(measuring forward drop of diode junctions, i.e., diodes and transistor junctions) and
transistors
(measuring
current gain
and other
parameters
).
Battery checking for simple 1.5 volt and 9 volt batteries. This is a current loaded voltage
scale.
Battery checking (ignoring
internal resistance
, which increases as the battery is depleted), is less
accurate when using a DC voltage scale.
Various
sensors
can be attached to multimeters to take measurements such as:
Light level
Acidity/Alkalinity(pH)
Wind speed
Relative humidity
Resolution and accuracy in a
multimeter are not equal. The resolution of a multimeter is the
smallest part of the scale which can be shown. The resolution is scale dependent and in high end
digital multimeters it can be configured, with higher resolution measurements taking longer to
complete. For example, a multimeter that has a 1mV resolution on a 10V scale can show changes in
measurements in 1mV increments. Absolute accuracy is the error of the measurement compared to a
perfect measurement. Relative accuracy is the error of the meas
urement compared to the device used
to calibrate the multimeter. Most multimeter datasheets provide relative accuracy. To compute the
absolute accuracy from the relative accuracy of a multimeter add the absolute accuracy of the device
used to calibrate the
multimeter to the relative accuracy of the multimeter.
[3]
The resolution of a multimeter is often specified in "
digits
" of
resolution
. For example, the
term 5½ digits refers to the number of digits displayed on the display of a multimeter.
By convention, a half digit can display either a zero or a o
ne, while a three
-
quarters digit can
display a numeral higher than a one but not nine. Commonly, a three
-
quarters digit refers to a
maximum value of 3 or 5. The fractional digit is always the most significant digit in the displayed
value. A 5½ digit multim
eter would have five full digits that display values from 0 to 9 and one half
digit that could only display 0 or 1.
[4]
Such a meter could show positive or negative values from 0 to
19
9,999. A 3¾ digit meter can display a quantity from 0 to 3,999 or 5,999, depending on the
manufacturer.
While a digital display can easily be extended in precision, the extra digits are of no value if
not accompanied by care in the design and calibration
of the analog portions of the multimeter.
Meanin