Copyright Pakistan International Airlines Training Center Karachi

Pulse Circuits & Radar Receivers

In the name of ALLAH, Most Gracious, Most Merciful

Objectives• To explain the analysis of transient and non sinusoidal

voltages.– Square.– Saw tooth.– Trapezoidal.– Rectangular.

• To explain the transient voltages applied to CR/LR circuits and applications (differentiators/integrators).

• Limiters.• Square wave generators

RC Circuits• Making integrators and differentiators with RC circuits. • Coupling Circuits-Design and applications. • Clamping circuits.

– Positive clamping.– Negative clamping.

• Clipping circuits.– Series Limiting.– Parallel limiting.

• Applications of clippers & clampers. • Pulse Shaping circuits

– Diode multiplier.– Over driven transistor amplifiers.– Schmitt trigger.

Block Diagram-Primary Radar

Analysis of Waveforms

(For animation connect to the Internet & click on Link above to start the Java Applet)

Application Of Pulse Voltage to an RC Circuit

• What will be the effect of the application of the following waveform on the given circuits?

R

C R

C

+V Volts

Application Of Pulse Voltage to an RC Circuit-Short RC

R

C+V Volts

I/P

Vc

VrThe Output

Application Of Pulse Train to an RC Circuit-Short RC

R

C+V Volts I/P

Vc

Vr

The shorter the time constant the more well defined will be the “pips” which are then utilized (after getting rid of the negative going pips for synchronization).

(For animation connect to the Internet & click on Link above to start the Java Applet)

Application Of Pulse Train to an RC Circuit-Long RC

R

C

+V VoltsI/P

Vc

Vr

The Output

Application Of Pulse Train to an RC Circuit-Long RC; The Integrator

+V Volts

I/P

Vc

Vr

R

C

May be used to count the cycles of the given square train.

The eventual output of this will be the average value of the square wave input signal.

Coupling CircuitApplication Of Voltage to a RC Circuit-Long RC

+V VoltsI/P

Vc

Vr

The OutputR

C

Application Of Pulse Train to an RC Circuit-Long RC; The Coupling Circuit

+V Volts

I/P

Vc

Vr

The entire DC voltage will be dropped across the capacitor and only the AC signal will pass through-This will ensure effective isolation between two amplifier stages.

The eventual output of this will be the average value of the square wave input signal.

R

C

Output voltage of the Coupler

I/P

O/P

This “sag” may be reduced by either increasing R or C or both.

Capacitor Charging

Capacitor Discharging

SummaryR

C

R

C

R

C

O/P

O/P

O/P

Integrator:1. Long CR.2. Output across capacitor.3. Applications-Analog Pulse counter.

Differentiator:1. Short CR.2. Output across resistor.3. Applications-Pip generator.

Coupling Circuit:1. Long CR.2. Output across resistor.3. Applications-DC isolator.

DC Restoring Circuit-Clamper!

R

C

O/PI/P

R

CO/PI/P C

O/PI/P

The R may be eliminated & the diode reverse res

maybe used

(For animation connect to the Internet & click on Link above to start the Java Applet)

Low level clamping to 0 Volts

R

CO/PI/P

Exercise

This is a High Level Clamping circuit to Zero Volts;

Draw and label the input and output signals.

Solution

Clamping to Voltages other than 0 V

Exercise

• Sketch a circuit with all inputs and output waveforms which will provide low level clamping to -7 volts.

Solution

Limiting Circuits• Applications:

– Prevent waveforms from exceeding given values.

– Limit amplitudes of waveforms. – Preventing supply voltages from exceeding

thresholds. • Principle:

– Forward biased diode – low resistance.– Reverse biased diode - Very high resistance.

Series Limiting

R

I/P

Positive Limiting to Zero Volts

R

I/P

Positive Limiting to Zero Volts

+ 10 V

Parallel LimitingRI/P

Positive Limiting to Zero Volts

I/P

Positive Limiting to Zero Volts

+ 10 V

R

(For animation connect to the Internet & click on Link above to start the Java Applet)

Limiters-The rule of the thumb

R

I/P RI/P

R

I/P

R

Reference Voltage Reference Voltage

Diode

Diode

R

R

K A

K

K

K

A

A

A

Positive Limiter

Negative Limiter

Positive Limiter

Negative Limiter

(For animation connect to the Internet & click on Link above to start the Java Applet)

Major Applications in Radar

• Radar positive going “PIP” generator.• Diode Squarer.

Exercise-

• Sketch circuits & waveforms for a:1. A positive going “PIP” generator.2. A diode squarer.

PIP Generator

R

100 V

30 V

Out put

Differentiator Limitor

Diode Squarer

K

KA

A

Input

Positive limiting

Negative limiting

Creation of Square Waves

• Used in multiple applications in Radars.• The waves so created need to have very steep

edges.• There are three major waves to create these

square waves:– Use of limiters.– Overdriving transistors. – Schmitt trigger.

Diode Squarer

K

KA

A

Input

Positive limiting

Negative limiting

The lower these values the better is the smaller is the rise and fall times and thus the steeper the waveform edges

Overdriven TransistorVcc

R

Rb RL

TR 1

Input

Output

Bias point

Limiting is affected by the Base Emitter Junction

+ 5 V

- 5 V

Exercise

• What will be the result if a diode is included in this circuit between base and emitter?

Squarer with 0 volts bottom voltageVcc

R

Rb RL

TR 1

Input

Output

Bias point

K

A+ 5 V

+ 5 V

+ 0 V

The Schmitt Trigger• Schmitt trigger is a generic name of threshold circuits with

positive feedback having a loop gain > 1. • The circuit is named "trigger" because the output retains its

value until the input changes sufficiently to trigger a change:– In the non-inverting configuration, when the input is higher than

a certain chosen threshold, the output is high; – When the input is below a different (lower) chosen threshold, the

output is low; when the input is between the two, the output retains its value.

• This dual threshold action is called hysteresis and implies that the Schmitt trigger possess memory and can act as a bistable circuit (latch).

• There is a close relation between the two kinds of circuits that actually are the same: a Schmitt trigger can be converted into a latch and v.v., a latch can be converted into a Schmitt trigger.

• Schmitt trigger devices are typically used in open loop configurations for noise immunity and closed loop negative feedback configurations to implement bistable regulators, triangle/square wave generators, etc.

Working• Circuits with hysteresis are based on the fundamental

positive feedback idea: – any active circuit can be made behave as Schmitt trigger by

applying a positive feedback so that the loop gain is more than one.

• The positive feedback is introduced by adding a part of the output voltage to the input voltage; so, these circuits contain:

– An attenuator (the B box in the figure on the right)– A summer (the circle with "+" inside) – An amplifier acting as a comparator.

• There are three specific techniques for implementing this general idea.

– The first two of them are dual versions (series and parallel) of the general positive feedback system. In these configurations, the output voltage increases the effective difference input voltage of the comparator by decreasing the threshold or by increasing the circuit input voltage; the threshold and memory properties are incorporated in one element.

– In the third technique, the threshold and memory properties are separated.

Circuit Operation• The original Schmitt trigger is based on the basic dynamic threshold

idea that is implemented by a voltage divider with a switchable upper leg (the collector resistors Rc1 and Rc2) and a steady lower leg (RE).

• T1 acts as a comparator with a differential input (T1 base-emitter junction) consisting of an inverting (T1 base) and a non-inverting (T1 emitter) inputs.

• The input voltage is applied to the inverting input; the output voltage of the voltage divider is applied to the non-inverting input thus determining its threshold. The comparator output drives the second common collector stage T2 (an emitter follower) through the voltage follower R1-R2.

• The emitter-coupled transistors T1 and T2 actually compose an electronic double throw switch that switches over the upper legs of the voltage divider and changes the threshold in a different (to the input voltage) direction.

• This configuration can be considered as a differential amplifier with series positive feedback between its non-inverting input (T2 base) and output (T1 collector) that forces the transition process. There is also a smaller negative feedback introduced by the emitter resistor RE.

• To make the positive feedback dominate over the negative one and to obtain a hysteresis, the proportion between the two collector resistors is chosen Rc1 > Rc2. Thus less current flows through and less voltage drop is across RE when T1 is switched on than in the case when T2 is switched on. As a result, the circuit has two different thresholds in regard to ground (V- in the picture).

The Schmitt Trigger CircuitVcc

R1

RL1

TR 1TR 2

RL2

R3

R2

Re

cx

I/P

O/PInput

Switch on voltage TR1 Switch off voltage

TR1

Output

(For animation connect to the Internet & click on Link above to start the Java Applet)

Q & A

• List applications of triggering pulses in Radars.• What are the applications of pulse shaping

circuits in radars?• Explain the working of the Schmitt trigger.

Multivibrators• The Multivibrator is an electronic circuit that switches rapidly by means of

positive feedback between two or more states. The circuit is so-called because its output is rich in harmonics.

• There are three types of multivibrator circuit depending on the circuit operation:

– Astable, in which the circuit is not stable in either state —it continually switches from one state to the other.

• It does not require an input such as a clock pulse.

– Monostable, in which one of the states is stable, but the other state is unstable (transient). • A trigger causes the circuit to enter the unstable state. • After entering the unstable state, the circuit will return to the stable state after a set time. • Such a circuit is useful for creating a timing period of fixed duration in response to some external

event. • This circuit is also known as a one shot.

– Bistable, in which the circuit is stable in either state. • The circuit can be flipped from one state to the other by an external event or trigger. • The bistable multivibrator is simply a latch (flip-flop); it is added to this classification only for

completeness.

The types

• A multivibrator consists of two main components – two passive networks and a bistable circuit,

connected in a common feedback loop. – The networks can be both resistive-capacitive (in the

case of an astable circuit):• A resistive-capacitive and a resistive (monostable), and

both resistive (bistable).• There are two versions of this basic arrangement

(considered below) in the case of astable and monostable multivibrator (bistable multivibrators are implemented only according to the first version).

Astable Multivibrator

• Basic Mode Of Operation:– State-1.– State-2

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Waveforms

Monostable Multivibrator • In the monostable

multivibrator, the one resistive-capacitive network (C2-R3 in figure 1) is replaced by a resistive network (just a resistor).

• The circuit can be thought as a 1/2 astable multivibrator. Q2 collector voltage is the output of the circuit (in contrast to the astable circuit, it has a perfect square waveform since the output is not loaded by the capacitor).

(For animation connect to the Internet & click on Link above to start the Java Applet)

Waveforms

Bi-Stable Multivibrator• In the bistable multivibrator, both the resistive-

capacitive network are replaced by resistive networks (just resistors or direct coupling).

• This latch circuit is similar to an astable multivibrator, except that there is no charge or discharge time, due to the absence of capacitors. Hence, when the circuit is switched on, if Q1 is on, its collector is at 0 V. As a result, Q2 gets switched off. This results in more than half +V volts being applied to R4 causing current into the base of Q1, thus keeping it on. Thus, the circuit remains stable in a single state continuously. Similarly, Q2 remains on continuously, if it happens to get switched on first.

• Switching of state can be done via Set and Reset terminals connected to the bases. For example, if Q2 is on and Set is grounded momentarily, this switches Q2 off, and makes Q1 on. Thus, Set is used to "set" Q1 on, and Reset is used to "reset" it to off state.

(For animation connect to the Internet & click on Link above to start the Java Applet)

Ringing or Blocking oscillators

• The blocking oscillator is closely related to the two-transistor or two-tube astable circuit, except that it uses only one amplifying device.

• The other is replaced by a pulse transformer, which provides strong positive feedback at all frequencies.

• As a monostable, it was useful in the 1950's for producing what were then short pulses, in the microsecond range.

• It was much faster than the Abraham-Bloch monostable.

• It the transistor era, however, it fell from grace because it could not be miniaturized, since it requires a transformer, and was also rather hard on transistors

Radar Receiver’s Design Objectives

• High gain-so that very weak echoes can be amplified.

• Low noise from the receiver itself. • High Bandwidth-Due to the pulse shape.

Pulsed Radar Receiver

Factors in Radar Receiver Design

• Gain Requirements.• Noise.• Waveguide Mixing.

Automatic Frequency Control

Signal To Noise Ratio (SNR)

• Sources of Noise Inside a receiver:– Local oscillator/crystal mixer noise.– Thermal Noise.– Shot Noise.

(For animation connect to the Internet & click on Link above to start the Java Applet)

The Cascode Amplifier• High gain.• High Input impedence.• Low Miller effect. • Used at the input of the radar

receiver.

Reduces miller effect by grounding TR 2 base. Is a normal common emitter amplifier but

gives almost no gain due to connection with TR2 CB configuration. Thus:

No miller effect.High input impedence.

Positive Going half cycle of iF

How to Reduce Break Through

Gain Control

Radar Receiver Considerations

• To detect the radar pulses.• How video limits can be controlled.• How are video signals applied to the CRT.

Typical Detector Circuit

What should be the value of R1C1 and why?

The C1R1

C1 charges to peak value

Diode cut offC1 discharges through R1

Positive going half cycles removed by diode

Analog Radars-Video Limit Control

• Weak and strong signals both applied to the CRT.– Too strong a signal-Saturates the screen

(blooming!)– Too weak a signal-Can be missed.

• Sensitivity should not be modified since this will lead to the weaker signals getting dropped.

• Remedy-Apply a video limiting signal.

Limit Control Circuit

•R-3 provides bias current to T1.•Reducing R 3 increases the no signal bias on TR1.• R 3 is adjusted to set the limiting level.

RV 1 adjusted to just cut off.

Vcc – Vb is the limiting voltage.

• In absence of input the TR 1 is biased to take the maximum collector current with D 1 cutoff.

• Negative going pulses will reduce the value of TR 1 collector current & thus provide positive going pulses at the circuit output.

Circuit Operation

• Why are blanking pulses required ?

Video signal to Grid

Blanking Pulse to cathode. Coupling

circuit elements

Develops a clamping arrangement to ensure that the low level signals are not lost due to coupling circuit arrangements.

Bandwidth RequirementsV

Time Frequency

V

The Effects

The Bandwidth

IF Amplifier Compensation Techniques

• Damping.• Over Coupling.• Negative Feedback.• Staggered tuning.

Damping

• Can be used to broaden the response of the tuned circuit.

• Not a favored method since power drops across the resistance lowering the output power.

Damping resistor

Over-coupling

• Seen more in transistorized circuits.

• Presents alignment issues if multiple stages are employed.

Over-coupling gives rise to the double humped response.

Negative Feedback

• Gain is reduced but bandwidth is increased with negative feedback.

Feedback path.

Staggered Tuning

• Each stage is tuned to a separate band but together they cover a very large range.

Frequency (MHz)45 50 55

Effective bandwidth

Pulse Shapes & Types of Distortion

Video Amplifier Bandwidth Requirements

Normally occurs due to the reactance of the inter-stage capacitance. May be reduced in amplifiers where direct coupling is employed.

Normally occurs due to the shunting of the anode/collector load by the stray capacitances.

Video Amplifier Response CorrectionAt the high frequency end the capacitor is shorted and R2 is out and the only load is R1.

At the low freq end the load increases to R1 +R2 which increases the gain and compensates for the loss due to the inter-stage capacitance.

L & C resonate together at a freq higher than the video band to eliminate the high freq fall off.

The resonance is done at the higher freq portion of the video band where the high freq fall off starts to occur.

Video Amplifier Response Correction•L & C resonate together at a freq higher than the video band and presents a low resistance path.•The Emitter resistance falls. •The gain increases. •The higher frequency gain drop is compensated.

The same result is obtained by shunting Re with a capacitor having a small value.

Summary1. List at least two applications of each of the circuits that you studied in this

course plan.2. Explain where in a Radar system are the following used:

– Clippers.– Clampers.– Video signals.– Coupling circuits.– Video amplifiers.

3. Why are square wave generators used instead of crystal oscillators?4. What is a Schmitt trigger and what is its’ principle of operation? 5. Explain the differences between the following types of Multivibrators:

1. Astable2. Monostable.3. Bi stable.