Difference between coaxial line and waveguide

	Coaxial Cable	Wave-guide
1.	It consists of center conductor and outer conductor.	Waveguide consists of single metallic walls acting as conductor. There is no center conductor in the waveguide.
2.	The fundamental or dominant mode wave in a coaxial line is TEM.	TEM wave can not propagate through it. Wave transmission takes place using TE or TM modes.
3.	There is no concept of cutoff frequency.	Each waveguide has a finite cut-off frequency.
4.	The coaxial lines have PTFE as di-electric and have relatively higher loss.	As waveguide is air filled there will be less loss compare to coaxial line.
5.	The power handling ability of coaxial-cable is relatively lesser.	Waveguide can handle higher power compared to coaxial cable because waveguide is filled with air as dielectric and air has higher break down voltage.
6.	The bandwidth of coaxial line is broad.	The bandwidth of waveguide is smaller.

Analog Voltage Divider or voltage division or voltage ratio circuit using Op-amp

Operational amplifier is a powerful tool to perform different mathematical operations on analog signals. In this post we will discuss about circuit to implement voltage ratio or voltage division using operational amplifiers.

Let us design a circuit to implement the following expression

Vout = (V1-Vref)/ (V2-Vref)

We will use the property of "logarithm" to implement division. And voltage difference circuit to offset Vref.
We know that log(A) - log(B) = log(A/B).
In our problem,

A = V1 - Vref

B = V2 - Vref

First stage is a difference amplifier which subtract Vref from V1 and V2 respectively. This gives us voltage Va and Vb.

The second stage performs logarithm of input voltage to obtain Va' and Vb'.

The final output voltage will be Va'-Vb'.

By setting suitable value of gain factor (-kT/q) and resistance ratio in previous stages, we can get the required expression of voltage division at the out.

I to V conversion. Convert 4-20mA to 1-5V, 0-5V, 1-10V, 0-10V

Current to Voltage converters or I to V converters as they are generally known are popular in process control applications where we need to interface output from transmitters to a data-acquisition system. Transmitter output are generally in 4 to 20 mA format which has to be converted to voltage form before feeding to a DAQ device.

In this post I will discuss about different circuits which convert 4-20 mA signals to 1-5V, 0-5V, 1-10 V and 0-10V form.

• 4-20mA to 1-5VDC

This conversion can be simply implemented by using a precision 250 Ohm resistor.

The first circuit is a simple implementation of I-V converter. The value of resistor R is 250Ohm.

For 4 mA current, the voltage drop is 4*250 = 1V,

For 20 mA current, the voltage drop is 20*250 = 5V.

This circuit can load the current source (transmitter output DAC) when DAQ device or measuring meter is connected across the resistor causing error in measurement. By adding a buffer/ voltage follower, we are adding very input impedance across the resistor. This protects the source from getting loaded and provide accurate measurement of voltage drop.

One very common configuration of I-V converter using Op-amp is as below. The load is floated in this case.

At the output stage, inverting op-amp is used to get 1-5Volt.

• 4-20 mA to 0-5 VDC

Just by changing the resistor value to 312.5 Ohm and adding an adder circuit with -1.25 V offset voltage, we can convert 4-20 mA to 0-5 VDC. The schematic is shown below.

• 4-20 mA to 0-10 VDC

The circuit is same as above, only the value of resistor is changed to 625 Ohm and offset voltage is set to -2.5 VDC.

There is one more way of getting 0-10V. Here we will use 0-5VDC circuit with slight modifications. The op-amp is used in non-inverting configuration to provide a gain of 2.

• 4-20 mA to 1-10 VDC

Must have tools for an Electronics and Instrumentation Engineer

Hardwares

(1) Digital Multimeter: A 3-1/2  DMM is sufficient for most of the electrical and electronic measurements.

(2) Wires: 0.5, 0.75, 1 sq.mm wires spool are must. One can also use jumper wires.

(3) Screw-driver set

(4) Discrete electrical components (Inductors, Resistors, Capacitors)

(5) Power supplies (variable AC, DC)

(6) Wire cutter

(7) Wire stripper

(8) Insulation tapes

(7) ICs-

• Operational amplifiers (LM741, OP07, LF356)
• Timer IC (NE555)
• Comparator (LM311)
• Instrumentation amplifier (INA101, AD623)
• Monoshot (74LS121/74LS123)
• Digital IC -(AND, OR, NOR, NAND, NOT)

Softwares

(1) Proteus- For PCB designing
https://www.labcenter.com/downloads/

(2) LabVIEW - For data acquisition, processing, User interface design, machine vision,control-system design
https://www.ni.com/en-in/support/downloads/software-products/download.labview.html#329059

(3) Multisim- For circuit simulation
https://www.ni.com/en-in/support/downloads/software-products/download.multisim.html#312060

(4) Matlab- For mathematical analysis,image processing,control system design
https://www.mathworks.com/campaigns/products/trials.html

NB: Images copied from Google Images

Measuring temperature using Thermistor

• Thermistor is a semiconductor based (metallic compounds including oxides such as manganese, copper, cobalt and nickel as well as single-crystal semiconductors silicon silicon and germanium) temperature sensor.
• They are commonly packaged in a thermally conductive glass bead or disk with two metal leads.
  

• It is a NTC (negative temperature coefficient) device i.e. its resistance decreases with rise in temperature. Although PTC Thermistors are also available, it is the NTC type which has gained wide attention.
• When the doping is very heavy, the semiconductor achieves metallic properties and shows a positive temperature coefficient over a limited temperature range.
  

• Thermistor is highly non-linear and one of the most sensitive temperature sensor. The high sensitivity of thermistor allows it to detect very small change in temperature which otherwise would not be possible with Themocouple and RTD.

Listed below are the features of Thermistor one need to consider:
(1) Very high sensitivity: It yields higher resolution in temperature measurement

(2) High sensitivity allows use of small mas and thus fast response and low measurement error
(3) Non-linear response
(4) Small geometry allows them to be easily integrate with electronic circuitry

The Resistance-temperature relationship of a Thermistor is governed by the equation

Rt = Ro{exp[B(1/T - 1/To)]}

Rt = Thermistor Resistance at T Kelvin

Ro = Thermistor resistance at To Kelvin

B= Temperature coefficient, ~3000-4000K

The above curve shows characteristics of a Thermistor with resistance 10k at 25 degC.

 

Relative sensitivity of Thermistor is:

 

B can be calculated from the NTC thermistor resistance at two reference temperature T1 and T2

Because of its high sensitivity and large resistance, it is not required to use 4-wire method (Kelvin's Bridge configuration) for Thermistor resistance measurement. A lead wire of 20 Ohm resistance would only cause less than 0.05 deg C error in measurement at 25 deg C which is insignificant for most practical purposes.

Lets do some math to validate above statement.

1/T - 1/To = (1/B)log(Rt/Ro)

dT/dRt = (T*T)/(B*Rt)

dT = 298*298*20/(3500*10000) = 0.05deg C

 

There is also a 3-parameter model of Thermistor given by the following equation known as Steinhart and Hart equation:

 

Linearizing Thermistor response

One disadvantage of using Thermistor is its non-linear behaviour. Its response can be linearized by adding a fixed shunt resistor but linearity comes at the cost of reduced sensitivity. 

The value of the fixed resistor can be calculated from the following equation.

where RT1 is the thermistor resistance at T1, the lowest temperature in the measurement range, RT3 is the thermistor resistance at T3, the highest temperature in the range, and RT2 is the thermistor resistance at T2, the midpoint, T2 = (T1 + T3)/2.

 

Interfacing with Data acquisition system:

 

As most data acquisition systems accept voltage input, it is recommended to convert change in voltage to corresponding change in voltage. A voltage divider network consisting of Thermistor and precision resistor is generally used.The accuracy of reference voltage also affects the accuracy of measurement.

Usually the value of R is taken to be equal to Thermistor resistance @25 degC. To avoid loading, the output voltage is fed to a voltage buffer.

Questions

GATE-2007

GATE-2008

GATE-2009

GATE-2014

A thermistor has a resistance of 1 kΩ at temperature 298 Kand 465 Ω at temperature 316 K. The temperature sensitivity in /K[i.e. (1/R)(dR/dT), where R is the resistance at the temperature T(in K)], of this thermistor at 316 K is ___________.