Pulse radars transmit a burst of energy and listen for echoes between transmissions.

Leakage from the transmitterVery strong echoes from close-range clutter

Radar Receiver

The received signal is usually heavily corrupted by:

• Environmental noise• Interference• Noise from the radar system itself

Not occur at the receiving time

InputOutput

External Noise

Signal Receiver Noise

Receiver

Low

Output S/N

High

Input S/N

General pulsed Radar System Block Diagram

Receiver Components

Antenna: Some radar antennas include low-noise amplifiers prior to forming the receive beams

DuplexerPermits a single antenna to be shared between transmitter and

receiver.RF Filters

The receiver filters the signal to separate desired echoes from interference

Low Noise Amplifiers

Amplify the weak echo with minimum added noiseMixer

Down conversion

Oscillator

The radar front end consists of:Bandpass filter or bandpass amplifierLNADownconverter

Receiver Front End

Protection components

Protect the receiver from high power transmitter

The mixer itself and the preceding circuits are generally relatively broadband.

Tuning of the receiver, between the limits set by the preselector or mixer bandwidth, is accomplished by changing the LO frequency

LNAMixer

LO

Filter

IF

At IF:Amplification less costlyMore stable than at microwave frequencySimple filter operation

Superheterodyne principle is commonly used in radar receiver

More than one conversion step may be necessary to reach the final IF

IF range ( 0.1 and 100 MHz) without encountering serious image- or spurious-frequency problems in the mixing process.

Mixer

LO1

IF1RF

These advantages have been sufficiently powerful that competitive forms of receivers have virtually disappeared; only the superheterodyne receiver will be discussed in any detail.

IFIF-AmpMixer

LO2

Filter

IF2

The function of a radar receiver is to amplify the echoes of the radar transmission and to filter them in a manner that will provide the maximum discrimination between desired echoes and undesired interference.

The interference comprises:

• Energy received from galactic sources• Energy received from neighboring radars• Energy received from neighboring communication systems• Energy received from possibly jammers• Noise generated in the radar receiver • The portion of the radar's own radiated energy that is scattered

by undesired targets (such as rain, snow, birds, and atmospheric perturbations)

Receiver Function

In airborne radars are used for altimeters or mapping:

• Ground is the desired target

• Other aircraft are undesired targets

More commonly, radars are intended for detection of• Aircraft• Ships• Surface vehicles

Effect of Characteristics on Performance

Noncoherent pulse radar performance is affected by front-end characteristics in three ways:

1. Noise introduced by the front end restricts the maximum range. 2. Front-end saturation on strong signals may limit the minimum

range of the system or the ability to handle strong interference. 3. Finally, the front-end spurious characteristic affects the

susceptibility of off-frequency interference.

Coherent radar performance: Affected by spurious mixer characteristics:1- Range and velocity accuracy is degraded in the pulse doppler

radar2- Stationary-target cancellation is impaired in MTI (moving-target

indication) radar3- Range sidelobes are raised in high-resolution pulse

compression systems.

Vital receiver parameters:

1. Noise Figure or Noise temperature2. Dynamic range 3. Instantaneous bandwidth and tuning range4. Phase and amplitude stability5. Cooling requirements

Generally, the critical response is determined in the IF portion of the receiver

However, one cannot ignore the RF portion of the receiver merely by making it have wide bandwidth.

1- Paralyzing the receiver from reception (during transmission) 2- Protected from high power (burn out the input stage)

In low-power radars PIN diode switch can be used

More powerful modern radars may have twin PIN diodes, tuned respectively to magnetron frequency and magnetron principal spurious output frequency

Components of the radar receiver can cause degradation of the radiated transmitter spectrum?

Receiver protection

Magnetron

Circulator

Gas protection section

To antenna

High power Receiver protection

Waveguide a centimeter or so long, sealed by quartz windows at its ends and containing a 'gas' of particularly short de-ionisation time (~2 s), such as water vapour at absolute pressure ~ 0.O1 bar. After the transmission, the gas takes a few microseconds to de-ionise,

Components of the radar receiver can cause degradation of theradiated transmitter spectrum, generating harmonics of the carrier frequency or spurious doppler spectra, both of which are often required to be 50 dB or more below the carrier. Harmonics can create interference in other electronic equipment, and their maximum levels are specified by the National Telecommunications and Information Administration (NTIA) and MILSTD-469. Spurious doppler spectra levels are dictated by requirements to suppress clutter interference through doppler filtering.

Harmonics are generated by any component which is nonlinear at the power level created by the transmitter and which passes those harmonics to the antenna. Gaseous or diode receiver-protectors are designed to be nonlinear during the transmitted pulse and reflect the incident energy back toward the antenna.Isolators or circulators are often employed to absorb most of the reflected fundamental, but they are generally much less effective at the harmonics. Moreover, these ferrite devices are nonlinear in themselves and can generate harmonics.

Noise and Dynamic Range Considerations

The receiver is often the most critical component

The purpose of a receiver is reliably recovering the desired echoes from a wide spectrum of transmitting sources, interference and noise

Noise is added into an RF or IF passband and degrades system sensitivity

Receiver should not be overloaded by strong signals

Receivers generate internal noise which masks weak echoes being received from the radar transmissions. This noise is one of the fundamental limitations on the radar range

Noise

(So/No)min

ReceiverTo demodulator Desired echo + Noise

Internal NoiseDesired echo

Noise ultimately determines the threshold for the minimum echo level that can be reliably detected by a receiver.

The receiving system does not register the difference between signal power and noise power. The external source, an antenna, will deliver both signal power and noise power to receiver. The system will add noise of its own to the input signal, then amplify the total package by the power gain

Noise behaves just like any other signal a system processes

Filters: will filter noise

Attenuators: will attenuate noise

Noise Effects

Thermal NoiseThe most basic type of noise being caused by thermal vibration of bound charges. Also known as Johnson or Nyquist noise.

R

T Vn = 4KTBR

R

Available noise power Pn = KTB

Where, K = Boltzmann’s constant (1.3810-23J/K)

T Absolute temperature in degrees Kelvin

B IF Band width in Hz

At room temperature 290 K:

For 1 Hz band width, Pn = -174 dBm

For 1 MHz Bandwidth Pn = -114 dBm

Shot Noise:

Source: random motion of charge carriers in electron tubes or solid state devices.

Noise in this case will be properly analyzed on based on noise figure or equivalent noise temperature

Generation-recombination noise:

Recombination noise is the random generation and recombination of holes and electrons inside the active devices due to thermal effects. When a hole and electron combine, they create a small current spike.

Antenna Noise

In a receiving system, antenna positioned to collect electromagnetic waves. Some of these waves will be the signals we are interested and some will be noise at the same frequency of the received signal. So filters could not be used to remove such noise.Antenna noise comes from the environment into which the antenna is looking. The noise power at the output of the antenna is equal to KTaB. Ta is the antenna temperature. The physical temperature of the antenna does not influence the value of Ta.

The noise temperature of the antenna can be reduced by repositioning it with respect to sources of external noise

Assumptions

■ Antenna has no earth-looking sidelobes or a backtobe (zero ground noise)

■ Antenna is lossless ■ h is antenna elevation angle (degrees)

■ Sun not considered ■ Cool. temperate-zone troposphere

The background noise temperature increases as the antenna is pointed toward the horizon because of the greater thickness of the atmosphere. Pointing the antenna toward the ground further increas the effective loss, and hence the noise temperature.

At 22 GHz Resonance of molecular water At 60 GHz

Resonance of molecular oxygen

22 GHz 60 GHz

Equivalent Noise Temperature and Noise Figure

F = (S/N)i/(S/N)o

Ni = Noise power from a matched load at To = 290 K;

Ni = KTo B.

F is usually expressed in dB

F(dB)=10 log F.

Noise Figure (F)

Two-portNetworkSi + Ni So + No

Te = No/KB,

B is generally the bandwidth of the component or system

R No

white noise

source R R

Te No

Equivalent Noise Temperature (Te)

If an arbitrary noise source is white, so that its power spectral density is not a function of frequency, it can be modeled as equivalent thermal noise source and characterized by Te.

R

No

G

Noisy amplifier

T = 0

R R

No-int= GKTeBG

Noiseless amplifier

R

No-int

GKBTe =

Te ≥ 0

If To is the actual temperature at the input port, usually 290 K

Te may be greater or less than 290 K

Te = To( F – 1) F = 1 + TeTo

Output Noise due to the internal noise of the receiver

No = No-int + No-in

FGKToB = GKTeB + KToBG

Examples:

(1) the noise power of a bipolar transistor at 3 GHz is 0.001 pW for a 1-MHz bandwidth. What is the noise temperature?

Solution WN = KTB, T = WN/KB = 72.5 K ( ~ -217o

C)

F of the transistor is 0.97 dB

(2) the noise power of a mixer at 20 GHz is 0.01 pW for a 1 MHz bandwidth. what is the noise temperature ?

  Solution WN = KTB, T = WN/KB = 725 K (435oC)

F = 5.44 dB

Noise Figure of Cascaded Components

Te = To (F - 1)

Ts = Ta + Te Pn = KTsBG,

where, G is the overall gain of the system = G1×G2×G3……×Gn

F2 – 1

G1

F3 – 1

G1 G2

Fn – 1

G1 G2 ….. Gn-1

FT = F1 + + + …… +

F1

G1

F2

G2

FN-1

GN-1

FN

GN

Noise Figure of Passive and Active Circuits

Passive Components:

For Matching component:

F = L (L Insertion Loss)

Te = To (L-1)

F Increases if the component is mismatched.

Active Devices:

It is generally easier and more accurate to find the noise characteristics by direct measurement

Conversion Noise

Noise Free signal and Local Oscillator:

-10 dBm

-130 dBm

-17.5 dBm

-130 dBm

IL=7.5 dBF=7.5 dB

LO

RFIF

KTB = -130 dBm

17 dBm

-130 dBm

Noise Figure = conversion loss

Noisy received signal & Noise free local oscillator

17 dBm

-130 dBm

-17.5 dBm

-97.5 dBm

IL=7.5 dBF=7.5 dB

-10 dBm

-90 dBm

-130 dBmLO

RF IF

Noisy Local Oscillator & Noise free input signal

-10 dBm

-130 dBm

IL=7.5 dBF=7.5 dB -17.5 dBm

-97.5 dBm

-130 dBm

17 dBm

-63 dBm

LO

RFIF

Noise Figure = 40 dB

80 dB

80 dB120 dB

Example: FT? Ts ? No ? Given IF bandwidth = 10 MHz

Noise

LNAMixer

LO

BPF

G = 10 dB

Ta = 15 KF = 2 dBL = 1 dB

L = 3 dBF = 4 dB

So , No

Si , Ni

1) dB to numerical valuesLNA G = 10 dB (10) BPF: G = -1 dB (0.79) Mixer: G = -3 dB (0.5)

F = 2 dB (1.58) F = 1 dB (1.26) F = 4 dB (2.51)

2) FT = [ 1.58 + 0.26/10 + 1.51 /7.9] = 1.8 (2.55 dB)

3) Te = To(F-1) = 290 (1.8 – 1) = 232 K

4) Ts = Ta + Te = 247 K5) No = KTsBG,

G is the overall Gain = G1×G2×G3×…. =10 × 0.79 × 0.5 = 3.95 (~6dB)

No = -98.7 dBm

Signal

The range of signal strength where the receiver will perform as expected

1. Minimum signal of interest: The input signal that produces unity signal-to-noise ratio (SNR) at the receiver output (minimum-detectable-signal).

2. Allowable deviation from expected characteristic: The maximum signal is one that will cause some deviation from expected performance. Linear receivers usually specify a 1 dB decrease in incremental gain (the slope of the output versus- input curve).

3. Type of signal: Three types of signals are of general interest in determiningdynamic-range requirements: distributed targets, point targets, and wideband noise jamming. If the radar employs a phase-coded signal, the elements of the receiver preceding the decoder will not restrict the dynamic range of a point target as severely as they will distributed clutter; the bandwidth-time product of the coded pulse indicates the added dynamic range that the decoder will extract from point targets.

The dynamic range can be defined in terms of:

Dynamic range,

Dynamic Range (DR), and 1-dB Compression Point

The operating range for which a system has desirable characteristics

Input power

1 dB compression point

Output

power

1 dB

Dynamic range

Noise level

Noise floor

MDS

The receiver dynamic range depends on the noise characteristics of the receiver as well as the type of modulation being used , and the required S/N

DR = Maximum allowable signal│dBm – MDS│dBm dB

Noise

(So/No)min

Receiver

TeFG

To demodulatorSi & Ta

No = KBG(Ta +Te) So = G Si

B is the IF BW (set by the IF bandpass filter)

MDS = Simi n = = Somin No So

G G No min

Local Oscillator

The superheterodyne receiver utilizes one or more local oscillators and mixers to convert the echo to an intermediate frequency that is convenient for filtering and processing operations. The receiver can be tuned by changing the first LO frequency without disturbing the IF section of the receiver. Subsequent shifts in intermediate frequency are often accomplished within the receiver by additional LOs, generally of fixed frequency.

Pulse-amplifier transmitters also use these same LOs to generate the radar carrier with the required offset from the first local oscillator. Pulsed oscillator transmitters, with their independent "carrier" frequency, use automatic frequency control (AFC) to maintain the correct frequency separation between the carrier and first LO frequencies.

In many early radars, the only function of the local oscillators was conversion of the echo frequency to the correct intermediate frequency. The majority of modern radar systems, however, coherently process a series of echoes from a target. The local oscillators act essentially as a timing standard by which the echodelay is measured to extract range information, accurate to within a small fraction of a wavelength. The processing demands a high degree of phase stability throughout the radar.

The first local oscillator, generally referred to as a stable local oscillator (stalo), has a greater effect on processing performance than the transmitter. The final local oscillator, generally referred to as a coherent local oscillator (coho), is often utilized for introducing phase corrections which compensate for radar platform motion or transmitter phase variations.

Oscillators are characterized by the following parameters:

1) Frequency

2) Frequency stability:

- Frequency pushing (frequency change with poor supply

voltage or current)

- Frequency pulling (frequency change with load mismatch)

- Temperature stability

- Shock and vibration effect

3) Spurious Performance

4) Phase noise

5) Power and efficiency

6) Tuning: Mechanical, Electronically, Tuning sensitivity (Hz/V or Hz/ mA), Settling time (the time it takes to respond to frequency control

signal) Post-tuning drift (drift after the oscillator reaches its desired

frequency )

The phase noise is defined as the power in 1-Hz bandwidth at a frequency fm from the carrier, and measured in dB below the carrier power.

The phase noise of microwave oscillator is 60 to 120 dB below the carrier power.

Phase noise

0

Phase noise

(dB)

1 KHz1

KHz

Effect of Phase Noise on Receiver Performance

How much phase noise can be tolerated in a given system design ?

IFIF IF

Desired LO

Phase noise

Noisy LO

Desired signal

undesired signal

Transmitted signal

Main signal Noise sideband

40 dB

Received signal

Main signal Noise sideband

40 dB

AirplaneMountain

How phase noise affect false detection for radar applications

In many early radars, the only function of the local oscillators was conversion of the echo frequency to the correct intermediate frequency. The majority of modern radar systems, however, coherently process a series of echoes from a target. The local oscillators act essentially as a timing standard by which the echodelay is measured to extract range information, accurate to within a small fraction of a wavelength. The processing demands a high degree of phase stability throughout the radar.

The first local oscillator, generally referred to as a stable local oscillator (stalo), has a greater effect on processing performance than the transmitter. The final local oscillator, generally referred to as a coherent local oscillator (coho), is often utilized for introducing phase corrections which compensate for radar platform motion or transmitter phase variations.

Mixers

LO + SIFLO

S

LO

V

I

IsVB

Non-linear I-V Characteristics of a diode

Mixers use nonlinear device to achieve frequency conversion of an input signal.

The drawback of nonlinear devices are that they generate many LO harmonics, and mixing products other than the desired one.

V = (VLsin wLt + Vssin wst), the mixer current will be

I = f(V) = ao + a1 (VLsin wLt + Vssin wrt) + a2 (VLsin wLt + Vrsin wst)2 + ……. + an (VLsin wLt + Vrsin wst)n

The primary mixing products (wL ± ws) come from the second order term and are proportional to a2 in amplitude

The third and higher order terms generate products of the form nwL ± mws and higher oredr harmonics

I = f(V) = ao + a1V + a2V2 + ……. + anVn

Where V and I are the device voltage and current

Mixer parameters

Important parameters are:(1) Conversion gain or loss

The ratio of IF power to microwave signal power. Conversion losses ranges from 5-6 dB for a single diode mixer to 9-10 dB for a double balanced mixer.

(2) Noise figure The ratio of signal to noise at mixer input to signal to noise at

mixer output. The noise figure determines the low-level sensitivity of the mixer. For passive mixers, assuming the devices do not generate any internal noise, we can use the conversion loss as the noise figure. Sometimes we can add 0.5 dB for account of the internal noise generation. For active mixers the noise figure and conversion loss are independently related.

(3) SWR at LO and RF inputs The mismatch at LO and signal ports

(4) LO/RF IsolationIsolation between LO and RF ports

(5) Harmonic suppressionsuppression of LO and signal harmonics

(6) Dynamic range It is the range of input power over which mixer provides required performance. The largest signal that can be handled by the mixer is determined by the acceptable level of intermodulation product, and is often specified by the requirement that the intermod products be below the noise level.

(7) 1-dB Compression point The input 1-dB compression point of a mixer is typically 5-8 dB below the input LO power. The output 1-dB compression point of a mixer can be calculated as the input 1-dB compression point minus the conversion loss of the mixer, minus 1 dB.For Example,■ LO power = 13 dBm ■ Conversion Loss = 8 dB ■ Input 1dB Compression point = + 6 dBm ■ Output 1-dB Compression point = +6 -8 -1 = -3 dBm

(8) Intermodulation product The amount of third order distortion caused by the presence of a second received signal at the output port (2f2 –f1 ± fLO). These products are extremely important because they will always occur with closely spaced input frequencies

(9) Intercept point The point at which the fundamental response and

the third-order spurious response curves intersect. The higher the intercept the point, the better the third-order compression. The input third order intercept point of a mixer is typically 4-5 dB above the input LO power. The output third order intercept point can be calculated as the input third order intercept point minus the conversion loss of the mixer.

Microwave input power

IF output

power1 dB

Input dynamic range

1 dB compression point

dBmIntercept point

Noise level

outp

ut

dyn

amic

ran

geIntermodulation

The sensitivity of the mixer in dBm is defined as the weakest signal that can be detected by the mixer and is given by:

S = -114 + NF + 10 log BW

Where NF is the mixer noise figure, BW the bandwidth in MHz and –114 dBm is the thermal noise in 1 MHz bandwidth

Selection of IF frequency:

fIF = |fRF - fLO|

For lower side band selection

fLO = fRF + fIF

Frequency Conversion and Filtering

Large IF eases the cutoff requirements of the image filter

FIF > BRF/2 Image frequencies outside RF BW

IF < 100 MHz Low cost

fLO

fRF Image

IF IF

Gain Controlled Amplifiers

Sensitivity Time Control (STC) The search radar detects echoes of widely differing amplitudes, typically so great that the dynamic range of any fixed-gain receiver will be exceeded.

Differences in echo strength are caused by differences :• In radar cross sections• In meteorological conditions• In range (the effect of range on radar echo strength

overshadows the other causes)

Also, many radar receivers exhibit objectionable characteristics when signals exceed the available dynamic range.These effects are prevented by the technique of sensitivity time control, which causes the radar receiver sensitivity to vary with time in such a way that the amplified radar echo strength is independent of range.

Most modern radars generate STC waveforms digitally. The digital commands may be used directly by digital attenuators or converted to voltage or current for control of diode attenuators or variable-gain amplifiers.

Clutter Map Automatic Gain Control

In some radars, mountain clutter can create echoes which would exceed the dynamic range of the subsequent stages of the receiver (A/D converter, etc.) if the STC attenuation at that range allowsdetection of small aircraft. The spatial area occupied by such clutter is typically a very small fraction of the radar coverage; so AGC is sometimes considered as an alternative to either boosting the STC curve (a performance penalty affecting detectability of small aircraft in areas of weaker clutter or no clutter) or increasing the number of bits of the A/D converter and subsequent processing (an economic penalty).

Clutter map AGC is controlled by a digital map which measures the mean amplitude of the strongest clutter in each map cell of many scans and adds attenuation where necessary to keep the mean amplitude well below saturation.

Automatic Noise-level Control

AGC is widely employed to maintain a desired level of receiver noise at the A/D converter. Too little noise relative to the quantization increment of the A/D converter causes a loss in sensitivity; too much noise means a sacrifice of dynamic range. Samples of noise are taken at long range, often beyond the instrumented range of the radar (in dead time), to control the gain by means of a slow-reaction servo. If the radar has RF STC prior to any amplification, it can achieve meaningful dead time by switching in full attenuation.

IF Gain Control circuit

IF Amp

Demodulator

LPF

Variable gain amp/attenuator

IF Input

DC AmpDC Ref

AGC detector

Gain Controlled Distributed Between RF and IF

IFdetector

IF amplifierFilter LNA MixerFilter

LPF

Comparator