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Receiver-System Design, Part 2- System Specifications (cont.)
Thursday, July 1, 2004 - Dave Adamy
Receiver-System Design, Part 2— System Specifications
(cont.)
Dave AdamyJuly 2004
This is a continuation of the general discussion of receiver
specifications listed in the table lastmonth (see ”Receiver-System
Design, Part 1 — System Specifications,” JED, June 2004, p.
60).
Sensitivity Sensitivity can be defined in terms of the actual
minimum signal strength that must be received
to allow the receiver system to perform the necessary processing
to produce an output that meetsspecifications. Sometimes this
specification includes the gain of an antenna but requires
significantfurther definition because of the antenna’s gain pattern
and any of the antenna’s scanningcharacteristics. Also, the
sensitivity, including antenna gain, should be correctly stated in
terms of thereceived field strength (in µv/m) rather than the
received signal strength (in dBm). Sometimes thesensitivity of the
receiving system is specified independent of the antenna gain. Then
the requiredreceiver sensitivity is stated as a signal strength (in
dBm). The antenna’s gain pattern will be specified
in terms of peak gain, width of the main beam, and sidelobe
isolation. Another way to specify systemsensitivity is to define
the range at which a specified emitter will produce a specified
output from thereceiver system, as shown in Figure 1.
In every case, the sensitivity is specified as a function of
frequency, modulation type,modulation level, and receiver function.
Receiver functions can include detection of signal
presence,identification of signal type and/or mode, recovery of
modulation, or emitter location.
Dynamic Range and Instantaneous Dynamic Range Dynamic range is
defined as the difference in signal strength between the weakest
signal a
receiver system can receive and the strongest signal present
(usually within band). In communicationsand radar receivers, where
the concern is only the minimum and maximum signal strengths the
receiver
could handle, automatic gain control (AGC) provides an elegant
solution. As shown in Figure 2, AGCreduces an early gain or
introduces an early loss in the receiver system to prevent
saturation by a strongsignal. This has the additional benefit of
adjusting the level of the strongest signal present so that it
isoptimally processed. By reducing the gain, AGC also reduces the
level of weaker signals in band, sothat they are ignored by the
processing circuitry.
In receiver systems for electronic warfare (EW) and
reconnaissance, we often need to handlesignals over a very wide
range of received-signal strengths — from the threat with the
highest effectiveradiated power (ERP) at the minimum range to the
minimum-ERP threat at the maximum range.However, the important
target signal is often not the strongest signal present. Therefore,
AGC isvirtually never allowable, so the receiver must have a
relatively wide instantaneous dynamic range.Sometimes, when the
range of received-signal strengths exceeds the achievable
instantaneous dynamic
range, it is necessary to design a selectable attenuator into
the front end of the receiver system. Thisallows us to, in effect,
move the whole instantaneous dynamic range up in amplitude. When
performinga search, the receiver would cover the search range with
the attenuation deselected and then, ifnecessary, repeat part of
the search with the attenuation selected.
LO Radiation The local oscillator (LO) generates a signal that
is significantly stronger than received signals,
and it can pass back through the front end of the receiver. This
can cause two kinds of difficulty. Whenit is radiated from the
antenna, it may be detected by a hostile receiver. This is exactly
what ishappening when a car pulls up beside you at a stoplight and
you find yourself suddenly enjoying ateenager’s favorite “music.”
Since the radiated LO can also interfere with other receivers,
either withinthe receiver system or in other onboard systems, this
specification often requires careful considerationwhen designing
the receiver’s front end.
Selectivity Selectivity is a function of the types of receivers
included in the receiving system and the design
of their front-end filtering. It is an important consideration
relating to the signal environment in whichan EW/recon receiver
system must operate. If there are relatively few signals present,
little or no
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selectivity is required. On the other hand, in very dense signal
environments, it may benecessary to sacrifice sensitivity or
probability-of-intercept performance in favor of selectivity.
Therewill be an article on signal-distribution-network design later
in this series, following a detaileddiscussion of dynamic range.
This article will discuss filter characteristics, various
power-dividingschemes, and reverse path discrimination in various
types of components. This will support a discussionof the
achievement of both selectivity and LO-radiation
specifications.
DOA Accuracy and CEP A receiver system which provides only
direction-of-arrival information will be specified in
terms of its root-mean-square (RMS) accuracy. Where the system
includes multiple receiving stationsor platforms operating
cooperatively, the location accuracy is normally specified in terms
of circular-error probable (CEP), as shown in Figure 3. [For
discussions of the important emitter-locationapproaches and the way
that RMS accuracy and CEP are defined and predicted for each, see
the “EW101” columns in the September 2002-May 2003 issues of
JED.]
Probability of Intercept This is the probability that a specific
signal (typically a threat signal) will be successfully
received, given that it is present at the system antenna. The
required probability of intercept (POI) isoften 90% or 100% and is
associated with the detection time. This specification is related
to types ofreceivers in the system and its search features. [For a
look at the important search approaches used in
EW and reconnaissance receiver systems, see the “EW 101” columns
in the January-May 1998 issues ofJED.]
Detection Time Detection time is the elapsed time between the
arrival of the signal at the antenna or antenna
array of the receiver system (whether or not any antenna is
properly oriented to receive the signal) andthe output of the
required signal information to a display or digital output. This
will typically be a verysmall number of seconds, and the threat
signals to which it applies will be listed. There may be
differentdetection times specified for different classes of
signals.
In receiver systems associated with jammers, there will
typically be a specified look-throughpercentage. That is, the
jammer will be turned off for some (small) percentage of the time
to allow the
receiver to monitor the signal being jammed and to search for
other threat signals. This can make theachievement of an aggressive
detection-time specification extremely challenging.
Intercept-Throughput Rate The intercept-throughput rate is the
number of threat signals the receiver system can receive and
process during some specified period of time.Radar-warning
receivers will usually be required to accept several million pulses
per second.
There may be separate pulse rates per quadrant and/or frequency
band. There will also be specificationsfor the number of
continuous-wave (CW) or pulse-Doppler (PD) signals with high
pulse-repetitionfrequencies present at once. This is an important
factor, because these types of signals can saturatewideband
receivers that are only able to see one pulse at a time.
Since tactical communications bands are characterized by high
band occupancy (up to 10% ofall channels occupied at any instant),
the intercept-throughput rate is very challenging for EW
andreconnaissance receivers in those bands. Receivers and
emitter-location subsystems for these bandshave typically been
narrowband, handling only one signal at a time. Therefore, the time
required tocollect enough data to generate the necessary outputs
can be the limiting factor in the ability of a systemto receive and
report all of the signals present. This is one of the factors that
make digital receivers (tobe discussed later in this series) so
attractive.
What’s Next Next month we’ll review the types of receivers
available for EW and recon systems and their
characteristic specifications. For your comments and
suggestions, Dave Adamy can be reached [email protected] .
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