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International
Academic
Journal
of
Innovative Research International Academic Journal of Innovative Research Vol. 2, No. 9, 2015, pp. 1-20.
ISSN 2454-390X
1
www.iaiest.com
International Academic Institute for Science and Technology
High Dynamic Range Receiver System Designed for High
Pulse Repetition Frequency Pulse Radar
Behrouz Kalate Meymaria, Reza Fatemi Mofrad
b, Morteza Shahidi Nasab
c
aMaster Student of electrical engineering of malek ashtar University, Tehran, Iran.
b Department of Electrical and Electronic Engineering, Malek Ashtar University of Technology, Tehran, Iran.
c Master Student of electrical engineering of malek ashtar University, Tehran, Iran.
Abstract
Microwave receivers play a critical role in Pulse Doppler radars, particularly in airborne radars and air
surveillance; and are extremely versatile; therefore their analysis and conceptual design is an important
issue. The receiver should have high dynamic range given that it is linked with a wide range of signals in
Pulse repetition frequency Pulse radars in term of input power; but we cannot use Sensitivity Time
Control (STC) or Automatic Gain Control (AGC). Therefore, the current paper provides a new conceptual
structure that yields high dynamic range (over 100 dB).
Keywords: Dynamic range, Noise figure, Sensitivity, Phase noise
Introduction:
Receivers are an integral part of radars receiving return signal echo. The receiver receives return signal
echo in the radio band and converting it to a digital baseband signal delivers it to signal and data
processing unit to be processed for detection, range and speed extraction, target tracking, and so. In Pulse
radar, the same antenna is used to send and receive signals, so low-loss Duplexer that switches between
the receiver and transmitter to keep them isolated is used to prevent the receiver saturation from the
transmitter with high power signals. In modern receivers input, a Low Noise Amplifier (LNA) is used to
reduce receiver noise figure sufficiently; and then the received signal is passed from a band pass filter to
prevent the arrival of signals outside the frequency band of mixer. Modern radar receivers do other tasks
such as switching frequency, bandwidth, and signals gain control for different radar modes, in addition
more sophisticated receivers have digital control networks to provide best efficacy of radar in different
working modes and meet different modes requirements. Moreover, sophisticated receivers include a
section that is usually used in automatic detection to test the receiver and prevent errors.
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The first section of the current paper (parts 1 to 3) describes basic concepts that are essential for receiver
design. Section 4 provides the receiver parameters calculation, and Section 5 is about phase noise
calculation. Section 6 shows analogue to digital converters considerations and Section 7 provides the final
structure of the receiver.
2- Radar receiver’s components and parameters
2-1 RF Pre selection As previously explained, Duplexer is used to protect receiver from strong signals leaking from
transmitter. In addition, we know many return signals that reach the receiver are not signals echo returned
from the target ; but are outcomes of jammers, environment clutter, and disturbing signals from other
radars at a different frequency with echo returning from the target and are placed outside radar band, for
example in the operation of a noise barrage jammer only a part of the jammer signal has the same
frequency of returned target echo. Therefore, the signals radio frequency pre selection is crucial in the
design of the radar receiver to minimize or prevent nuisance signals to the receiver.
Given two signals that are both in receiver passing band and show desired signal with the letter “S” and
jammer signal with the letter “J”, both signals are converted to baseband when passing from the mixer
with a local oscillator; thus jammer signal is on the desired signal and radar is not able to detect target. A
band pass filter is applied to the radio frequency (RF) after low noise amplifier and before the mixer to
prevent this, thus just a desired signal is seen at the mixer input and interfering signals in the baseband
greatly reduced. The presence of low noise amplifier at the beginning of the receiver enhances receiver
sensitivity and overcomes filters losses before mixer. A significant portion of jammer signal effect on the
target signal is neutralized by combining the RF signal and the frequency produced by local oscillator and
filtering its output at baseband. It does not only undermine jammer signal effect but other unwanted
signals, as well. [1]
2-2 Convert the radio frequency signal to an intermediate frequency band
A signal to be processed and detected in a processor must be converted from a radio frequency to the
baseband frequency.For example, assume that the radio frequency signal is 10 GHz received and it should
be 200 MHz in order to be processed, thus frequency conversion should occur in receiver . Mixers and
local oscillators do this.
Figure 1 shows that local oscillator frequency must be chosen very close to radio frequency, this causes
unwanted frequency components due to oscillator phase noise in base band and this prevents (or makes
difficult) designing narrow-band RF filter to remove unwanted signals caused by local oscillator
frequency offset, and locate image frequencies within the band, and if we can remove image frequency
components using only one mixer there will remain sidelobe surfaces; therefore, frequency conversion is
done in several steps to prevent producing troublesome harmonics[1].
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Figure 1: 10 GHz Radio frequency conversion to 200 MHz baseband, stage 1
2-3 Mixer Mixer is a non-linear plot that converts a primary frequency signal to a secondary frequency signal in
radar receiver. In fact, the mixer output is not the only desirable signal but there will be unwanted signals
at the output produced by harmonics and is not desirable. Assuming that the output voltage can be
described in terms of a mixer, in that case, only the second order power can produce the desired signal
and other orders produce unwanted signals or the so-called spurious signal (spurs).
Where, V is the sum of the radio bands signal voltage ( ) and local oscillator signal voltage (
)
1 1 2 2sin sin (2)V t v t v t
Where, and are the baseband and local oscillator signal angular frequency, respectively .
Therefore, the desired frequency in baseband will be ( ) and spurs that are unwanted frequency
will appear as ( ) [1].
2-4 Local oscillator and baseband frequency selection
Local oscillator and baseband frequency selection depends on the need for video signal bandwidth and the
signal effects caused by mixer inter modulation (spurs) which must be minimized. It should be noted that
different harmonics frequencies and local oscillator frequency should not be placed in the receiver pass
band to avoid or minimize spurious signals [1].
If we show radio frequency band signal with H and local oscillator frequency with, then H-L will be
desired frequency in baseband.
2-5 Receiver gain
The components of the receiver, in the simplest definition, can be assumed as a block that a signal with a
specified power will enter it and exit with a different specified power. Therefore, the receiver has
weakened or strengthened input signal.
The weakening or strengthening based on the design parameters of the radar receiver is known as receiver
gain. Weakening or strengthening of the input signal to the receiver is of the important when the receiver
1
0 0 1 (1)n
nI a aV a V
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output signal reaches from the analog to digital converter ; it can saturate the converter and actually
disable the radar receiver if it is with a power higher than converter input power tolerance [5], [20].
Figure 2: Schematic view of a receiver representing its gain
2-6 Thermal noise and receiver noise figure
The thermal noise power at the receiver input is where K is Boltzmann's constant
( , T is ambient temperature usually considered a standard temperature of 290 degrees
Kelvin and B is the receiver bandwidth in Hertz.
Power level in radar receiver is expressed in milliwatts, because it is usually low and is shown as dBm for
ease of calculation . The ratio of the noise output of a receiver to an ideal receiver noise output at the
temperature T is defined as the noise figure, showed as the signal-to- receiver input noise signal to signal-
to- receiver output noise ratio. Noise figure can be shown as (dB) using .
If we have N pieces of the series with the gain , and noise figure numerically; be total noise figure
is calculated as follows:
321
1 1 2 1 1
1 11... (3)
...
N
N
f fff f
g g g g g
2.7. Receiver sensitivity
Typical receiver sensitivity required minimum input signal in order to achieve a certain output signal and
is determined by the amount of signal-to-noise ratio (Equation 4). Antenna gain should be effective in
sensitivity for a receiver that binds to the antenna, thus the minimum operational sensitivity is defined in
the operating environment for the receiver (Equation 5).
0( ) ( ) (4)min minSS KT B NF dB
N
0( ) ( ) (5)
minS KT B NF
NMOS dBLiG
Where, in equation (4) and (5) is the minimum required signal-to-noise ratio for signal
processing, is the Boltzmann constant, is the absolute temperature in receiver input in Kelvin
usually assumed to be 290 degrees Kelvin, is the receiver bandwidth in Hertz, is the receiver
noise figure and is the antenna gain, respectively.
2-8 Dynamic range
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Dynamic range is the power range that the receiver can receive and process the signals. Usually the lower
limit of signal power for dynamic range is considered noise level and the mixer or limiter is considered to
be the upper limit based on the saturation level of the amplifier [1], [4].
Receiver dynamic range is entirely dependent on the components that are used in the receiver; particularly
analog components such as ADC.
Mixers and amplifiers are the most effective analog components in receiver that play a role in limiting the
dynamic range [1].
2-9 Receiver Bandwidth
The receiver bandwidth ( is one of the most important factors in radar design that is determined
according to the sent pulse bandwidth using the equation . [5], [21]. It is understood that
the broader bandwidth leads to more noise into the system.
2-10 Pulse Repetition Frequency (PRF)
The number of pulses sent by radar in a second is known as Pulse Repetition Frequency. This parameter
is important because it determines the rate of data sent and received, and its maximum depends on radar's
unambiguous range , as the receiver in mono static radar can receive reversed between two consecutive
pulses of a target in the maximum unambiguous range of , thus the maximum amount of PRF
is [3].
3 .Phase noise problem
In coherent radar where phase signal is measured target can be isolated from clutter in frequency by
analyzing the echoes received phase. The accuracy of this operation depends on oscillator stability that
produces a transmitter signal and used as a reference signal in RF to IF conversion in the local oscillator,
known as exciter. In fact, the target of exciter is to produce coherent signals to provide send and coherent
reference signal to the radar receiver.
Usually the received signal is processed and in the same phase (I) and orthogonal (Q) the each of these
two is a function of the range (R) and wavelength ( ).
Ideally, I and Q components are analyzed in processing with regard to the sustainability of sent reference
signal phase at the time of the sent signals production and received the returned echo. However, certainly
reference phase will not be sustainable in the operation and is with fluctuations; thus two components
I and Q occurs with respect to the phase difference in achieve the results are not, but are in turn a
consequence. Attempts of organizations are to improve performance and discover hidden talents among
staffs. They can also deal with the problems raised in crisis and prepare for the administration.
4πRI A cos( δ ) (6)
λ
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practice, although ideally ( ) is considered zero, and is as follows [12].
The minimum thermal noise that is causing volatility in phase and known as phase noise
is not white as noise in spectral characteristics and its power spectral density is not
uniform.
3-1 The effect of phase noise on Pulse Doppler processing
Radars designed to detect small targets at the presence of clutter is one of the things that make radar
systems performance phase noise facing difficulty . Assuming that the amount of signal that reaches the
noise to receiver is sufficient for processing, the important issue that significantly affects on radar
performance will be the amount of clutter signal with the phase noise also playing a role . Target echo and
clutter are shown in a range in the pulse Doppler radars faced with the problem of ambiguity in range,
however , there is different ranges given the main clutter echo results from close ranges to radar and the
target echo that is far.
Furthermore the problem of echoes accumulation in different ranges because of the ambiguity of the
range should also be considered. Therefore, in measuring the signal-to-clutter ratio, although target and
clutter appear at the same range of range, different ranges need to be considered. This is because that
clutter is usually in the first range and target is at a distance . Range in the signal-to-clutter ratio is
inversely by the power of 4, that leads to substantial decrease in the signal-to-clutter ratio.The next factor
that reduces the signal-to-clutter ratio is the target to clutter radar cross section that is always smaller than
one due to the larger radar cross section ratio.
Figure 3: Competition of target signal in distance range with clutter signal at close range in an ambiguous
system [12]
Figure 3 shows a distance range target and the clutter cells in different ambiguous ranges in the
competition with the target echo. Surface that radar clear s from clutter is proportional to non modulated
pulse width, 3dB bandwidth and clutter cells range.
3 (8)2
c c dB
cA R
Therefore, the signal-to-clutter that appears at first range ( ) will be:
4 21
0( ) ( ) (9)t c t
c t c
R GSCR
A R G
4πR
Q A sin( δ ) (7)λ
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Where and are antenna gain in the direction of the target and in the direction of clutter,
respectively .We face with an uncertainty in range in the pulse Doppler radar; thus we should consider the
folded effect of
multi-echo clutter in signal-to-clutter ratio. Thus, the total ambiguous range clutter in range between the
target and the closest clutter is calculated (n) . Given the clutter folded effect of we have:
2 4
0 4 21
(10)n
t t ci
it ci ci
G RSCR
R A G
Ideally, remaining Doppler power in clutter bin is zero after pulse Doppler processing (FFT), but clutter
spectrum will spread in all Doppler bins with phase noise.
The remaining amount of clutter in any Doppler bin is depending on the clutter power and inherent clutter
spectrum, phase noise power spectral density and Doppler bin bandwidth.
The remaining authorized amount of clutter in each Doppler bin depends on the probability of detection
and the system false alarm and is a function of the clutter probability density. Phase noise power spectral
density (PSD) requirements in each frequency offset are:
( ) ( ) (11)req d reqS f SCR dB B dBHz SIR dB
Where, is Doppler filter 3dB bandwidth that is inversely proportional to the radar Deauville time and
is and signal-to-interference ratio.
Power spectral density of phase noise should not exceed -110 dBc / Hz in each Doppler frequency in
pulse Doppler radar in detection mode and power spectral density of phase noise should not exceed -140 dBc / Hz if we want to design a radar system that detects targets with very low radar cross section and
Stealth.
Oscillator phase noise spectrum for each offset frequency is considered 2S f
f
and the desirable
amount for can be calculated. Also, normalized phase noise power ( ) to interference signal ( )
in the frequency range ( ) is calculated as follows [13]:
2
(12)H
L
f
PN
int f
P f
4- Calculate of the receiver parameters
In this section, we explain and calculate the basic parameter of a pulse Doppler radar receiver design
process.
4-1 IF signal frequency calculation
We select the first IF and oscillator frequency in X-band frequency given the input signal bandwidth
(frequency range 9.25 to 9.75 GHz). We try to go into the area with the smallest IF signal while at the
same time give us the greater spur harmonic content because it is much more weakened at these
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frequencies. That's why we select the area with [0.21,0.24]H L
H
f f
f
frequency ratio. This area is
between two sections of harmonic content.
If we select and for and based on the preceding explanations, thus,
and therefore, 0.22IF
RF
f
f , as a result, the first intermediate frequency of, 2062.5
MHz is obtained. So if we take the second oscillator frequency of 2 GHz, the second IF frequency is
equal to 62.5 MHz.
4-2 Receiver bandwidth calculation
As mentioned above, the receiver bandwidth is inversely related to the pulse width and it can be estimated
according to equation0.886 1
rB
, but the receiver bandwidth is multiplied further since we must
have sufficient bandwidth for FFT processing, here it is assumed to be four times higher. Since the length
of some air targets is 100 meters, so resolution should be considered about 150 meters for separation in
the range thus:
R (13)2
c
Therefore, given that is equal to 100 m, the pulse width will be equal to:
150 (14)2
c
1 (15)sec
So, this IF bandwidth can be considered 4 MHz.
4-3 Minimum and maximum input signal to a receiver calculation
4-3-1 radar antenna gain calculation
One of the important parameters in the radar equation that should be measured is radar antenna gain. The
higher this value returning echoes will come with more power to the receiver greater [14]. Considering
the types of antennas used in different radars, we will discuss parabolic antenna gain.
4-3-2 Parabolic antenna gain
Assuming antenna diameter is equal to two and a half meters and have 60 percent efficiency [5] gain will
be equal to [14]:
(16)
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(17)
Where, in Eq.(16) is the area of antenna that is equal to and d is diameter of the antenna
opening. In practice, antenna gain of 43 dB is obtained.
4-3-3 The radar data
Pulse Doppler radar to the authors of the paper includes:
Value parameter
75 KW
4-3-4 Targets RCS
One of the factors that determine the strength of the RCS returns signal is targets and objects that return
signal to the radar. In fact, bodies and different targets depending on the size and direction of the radar
reflectivity are different and that makes radar with different cross sections.
RCS can be obtained based on two types of targets, i.e., distributive target and point target. The current
paper uses point target for the ease
.
4-3-5 Minimum target return signal calculation
Returning echoes power to the receiver from various targets can be modeled by the following equation
[15]:
2 2
3 4[ ] (18)
(4 )
Tx trx
S
P GP W
R L
In equation (18) represents the losses that occur in Duplexer and transceiver modules.
The minimum signal is obtained at the maximum distance, in a 200 km range for a target with a 4 square
meters radar cross section, we have:
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Table 2: receiver received power from a target with a 4 square meters cross section in band X in 200
kilometers distance
dBm 78.8 dB 86
29.8-
6 dB 33
212- dB 10
dBm 114
4-3-6 Clutter power with the upward main lobe calculation
Figure 4: the radar main lobe is upward and clutter enters sidelobe level
Assuming that the radar main lobe is upward without dealing with the ground complications and only
sidelobe level have return of complications of ground (Figure 4) and given that block range created for
radar is 100 meters, therefore the clutter surface at a distance of 100 meters by sidelobe level (as the
shortest distance) is intended to calculate the level of clutter by antenna sidelobe level, we have: Thus, on the basis of radar data and the equation, for input clutter echo power to receiver at a range of 100
meters, assuming that return clutter echo intensity factor is we have:
sec (19)2
csl B
cA R
100 ( 1) 150 sec0 (20)180
cslA
2 2 261.8 24.2 (21)cslA m dBm
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Table 3: clutter echo power in the range of 100 meters that reaches the receiver from radar antenna
sidelobe level
dBm(75KW) 78.8 dB 46 (X band) 29.9-
20
24.2 dB 33
80 dB 7 L
20.9- Clutter
4-3-7 the clutter main lobe power toward the horizon calculation When the radar antenna main lobe is on the horizon, given 3 dB lobe width of antenna lobe width collide
with the ground complications in a calculated distance, therefore, clutter echo is received not only from
sidelobe level but main lobe (Figure 5).
Therefore, received clutter power for side lobe and main lobe must be calculated based on the range.
So we should first know at what range the main lobe collision with the ground complication begins, so we
have:
3
(22)sin(0.5 )
RSMlobe
dB offset
hR
4 1 146 (23)
sin(0.5 0.3)SMlobeR m
Figure 5 clutter enters receiver from the sidelobe and main lobe [16]
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So from the moment the echo clutter of the main lobe began to enter the clutter should be calculated for
the total sidelobe and main lobe that given the main lobe impact on input clutter power to the receiver, the
echo clutter in the range of 1146 meters will be -12.67 dbm.
Figure 6 input clutter power to receiver for one pulse
We face with ambiguity in pulse Doppler radars and clutter that enters each cell range is in a state of
ambiguity the total clutter entering receiver is accumulated in different distances (Fig. 7) and is obtained
using the following equation [16]:
1
1
0 0
( ) (24)N
c folded c
n
t nTP P Rect
Where, N is the number of pulses per coherent processing interval, T is inverted pulse repetition
frequency, is pulse width, and t is calculated in corresponding time delay due to the switching on or
off the transmitter and receiver and will be 0 0{ 2 1 ;0 1}nT t n T n N .
Figure 7: the input clutter to the radar from the side and main lobe piled of different ranges [16].
So, the amount of clutter obtained from pilled different pulse should be higher than clutter that arises
from a pulse.This problem is studied using MATLABF and shows the highest received clutter of -
11.22 dBm in 10 consecutive pulses. So, the highest received clutter in a total of two scenarios will be the
same amount.
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Figure 8: a) input clutter pilled from 10-pulses
.
Figure 9: echo Received from pilled received clutter for 10 consecutive pulses.
Thus, maximum and minimum input signal to the receiver are -11 dBm and -114 dBm.
The minimum input signal to the receiver is considered the minimum detectable signal that, according to
equation (25) and taking into account the bandwidth of 4 MHz for the receiver, the minimum signal to
noise of12 dB and noise figure of 4 dB, -90 dB is obtained. Here, noise level is considered the minimum
signal that is -102dBm.
4-4 Dynamic range receiver
As previously explained, the extent to which the receiver can do signal processing operation to the input
signal power to receivers is known as receiver dynamic range. The dynamic range of 103 dB for the
receiver will be necessary, with regard to the equation and the maximum and minimum receiver input
signal.
4-5 Receiver structure and gain to achieve high dynamic range
The receiver gain is adjusted according to the minimum receiver input signal, so as to reach the level of
full scale analog to digital converter, receiver gain is determined considering peak to peak voltage of
analog to digital converter that is 2 volts. However, since the noise system level of -102 dBm is
calculated, so this amount is considered as a minimum signal to gain.Therefore, an appropriate level for
the signal to reach analog to digital converter is a gain equal to 102 dB. However, since the maximum
signal is -11.2dB, the receiver is saturated in the primary classes and breaks. On the other hand, we face
with ambiguity in pulse Doppler radar; therefore we cannot use STC that is based on range, because the
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gain controlling maximum dynamic range is ultimately 60 dB, so we cannot use gain controllers, and thus
the best thing to Downconvert signals with the appropriate gain is switching.
We need a high dynamic range to receive the signal and convert it to baseband, and as described we
cannot use the conventional methods, so we have to divide signals using microwave switching between
the received signals divide receiver into two. Thus, we use a path for weak signals and another path for
strong signals that reach us to an ideal signal.
Thus, when low power signal is received over long distances and
this requires that high sensitivity receiver , we use a path with low noise amplifier (LNA), and a
restriction is placed before LNA to prevent saturation in case a strong signal to enter this path.
Signal that comes from close range is certainly high, which is calculated so we pass it from the path with
voltage variable attenuator (VVA) where a voltage controlled attenuator (VCA) is used. In this case,
P1dB will increase and we can achieve high dynamic range over 100 dB, but surely half or one dB will be
added to the receiver noise figure due to some increased receiver components that is predicted [17].
Figure 10: The proposed structure for a high dynamic range receiver with high gain
Thus, we consider the LNA gain amount of+ 25dB given the maximum input signal (-11dBm) and the
minimum noise (-102dBm) for the signal range of -102 to -70dBm and undermine the VCA from 0 to
35dB and take the total gains n other levels of 50dB, we will have the maximum and minimum IF signal
range between -30 to + 5dBm
.
5. phase noise requirements for receiver oscillator
As we explained earlier, phase noise originated from oscillators can have a devastating impact on the
receiver so that makes the clutter spectrum wide and prevent the discovery of the targets. We can be
considered six channels as Fig 11 indicates, if we consider 4 channels for the receiver with 500 MHz
bandwidth and assuming that the center frequency of each channel with adjacent channel is
approximately 167 MHz.
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Figure 11: Receiver channel
As explained, the presence of phase noise in oscillator signal leads to mutual mix. Considering the worst
case, i.e., we're going to work on mutual mix effect on channel 4 and the mixer output that the received
signal contains all 4 channels and the desirable channel is number 4 with minimal input signal with input
signals received in other channels the maximum amount. To simplify the calculations, each of the
interference signals is assumed to be hit and channel 4 bandwidth is considered to be 4 MHz, also, we
assume the ideal mixer and nonlinear effects are ignored and Let us examine only the effect of phase
noise .So, Figure 12 shows Mixer output after being transferred to the lower band signal, where an
amount of the spectrum of the each of the adjacent channels is within the channel 4 band and the noise
level will increase.
Figure 12: Effect of phase noise on the fourth channel reduced signal value -to-noise.
We're going to calculate the sum of the effects of interference signals that have been created due to the
phase noise. Oscillator phase noise spectrum can be considered as 2S f
f
and calculate the
appropriate value of .
Phase noise normalized power ( to be interference signal ( ) in the frequency range ( ) is
calculated as follows:
2(25)
H
L
f
PN
int f
P f
Also, interference signal can be considered k times of desirable signal, i.e., int sigP k P that Equation
26 is rewritten as:
1 1[ ] (26)PN
sig L H
Pk
P f f
So we can write the whole phase noise power to signal ratio for all interfaces as follows:
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1 2 3 4 5 6
( ) 1 1 1 1 1 1[ ] (27)PN
sig
P totalk
P f f f f f f
Frequency values are given below according to Figure 12. Also considering the problem assumptions 10.310 ( 103 )int sigk P P dB and for the amount of distortion created is minimal and ineffective, it
should be 27 dB below the desired signal power so the Eq. (28) is as follows:
2.7 10.3 1 1 1 1 1 110 10 [ ] (28)
165 169 332 336 499 503MHz MHz MHz MHz MHz MHz
Where, α value equals to . So now we can calculate receiver oscillator phase noise in any
given desired offset as follows
4
2 2
5.1 10(29)S f
f f
Thus, required phase noise for oscillator to offset 1KHz is:
PN@1 10log( (1 )) 93 (30)
dBckHz S KHz
Hz
Also we can add -20dBc / Hz to the obtained value and consider the value of phase noise in a kilo Hz
offset equal to -110 dBc/H z, taking into account other effects as well as approximation that we have in
our analysis.
6- Spur level required for the oscillator analysis
We should consider two issues to analyze Spur level required for the oscillator.The first thing is that
correct harmonics are usually generated from main harmonics in oscillators, that are unwanted and
attempts are made to remove them in design, because the presence of these harmonics also causes outside
the band interference signals Downconvert into the band. But in practice, It will not be very annoying in
receiver design because of interference signals outside the band, are attenuated to an acceptable level by
before the mixer RF filter also oscillator harmonic power is much less than its original harmonics. The second problem is that in some structures, especially structures of direct digital frequency synthesis
(DDS) with spurs in offsets near the oscillator frequency with problems in the oscillator signal due to the presence of the spur. The output signal from the mixer includes a combination of the original
frequency spectrum by shifting their findings If the spurs are present in an offset within the input channel
bandwidth and as a result, the signal-to-noise is affected [13].This is shown in Figure 12. In this figure,
for simplicity oscillator signal and its spur are shown as the hit.
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Figure 13: a) an input RF signal and the oscillator signal; b) output signals from the mixer
But we should assume another case that spurs are located in the offset higher than the bandwidth of the
original signal. Now, the spurs make unwanted interference signals downconvert into the band that this
case is much more important. The worst possible conditions should be considered to calculate the spur
level. First, we assume that the desired signal level is the minimum possible value (i.e., -114dBm) and
radar interference is the maximum possible value (i.e., -11dBm). Given the fact that the out of the filter
band weakening is considered to be 60dB, thus interference signals that reach the mixer will be -71dBm.
now, oscillator level of spur should be selected so that the interference signal are about 60 dB of lower
than the original signal after interference signals downconvert and transferred to the main band. The
oscillator spur level should be selected about 103dBc that we consider the value of -105dBc given that the
difference between the interference signal and the original signal is 43dB.
7- The analog to digital converter
According to Figure 14, we know that analog to digital convertor is used to convert IF analog signal to
digital signal in receiver structure. Generally in operation one or two bits should be added to the number
of effective bits based on theoretical calculations for analog to digital convertor to overcome constraints
such as quantization noise [18]. According to [19] the number of effective analog to digital converter bits
includes:
1.76 (31)
6.02
SINAD dBENOB
10 27 12 1.76 8 (32)
6.02
dBENOB
Therefore, we need a 10-bit digital converter given that we need to add two bits to the value obtained in
(32) for optimal performance, according to [18].
International Academic Journal of Innovative Research,
Vol. 2, No. 9, pp. 1-20.
18
8- Structure and receiver specifications table
Figure 1shows high dynamic range pulse Doppler radar receiver block diagram, based on the
calculations:
Figure 14: Structure of Pulse Doppler radar receiver with high dynamic range
According to our calculations X-band Pulse Doppler radar receiver characteristics is given in Table 4:
Table 4: Pulse Doppler radar receiver parameters
Parameter Value
Frequency band X-Band
RF bandwidth 500 MHz
Number of Channels 4
STALO Phase noise -110 dBc/Hz @
1KHz
STALO Spore levels -105dBc
The maximum and
minimum input power
-11.2~-114 dBm
Dynamic Range 103dB
Noise level -102 dBm
Noise figure 4~5 dB
Power
compression points 1dB
of output ( )
+20dB
The analog to digital
converter number of
effective bit
10bit یا 12bit
PRF changes and
received signal regime
Medium and high
Pulse repetition
frequency
International Academic Journal of Innovative Research,
Vol. 2, No. 9, pp. 1-20.
19
9- Conclusion In the current paper described the necessary concepts to receiver design and presented the receiver basic
parameters calculated values such as dynamic range, sensitivity, gain and phase noise analytically. A
new structure to achieve high dynamic range has also been suggested . This structure, as described in the
text, despite high dynamic range is very simple to implement. In addition, the analog to digital converter
was intended in term of dynamic range and the effective number of bits and as was observed we need up
to a 12-bit analog to digital converter despite receiver’s more than 100 dB dynamic range.
10- References
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