Page 279 - Radar Technology Encyclopedia

269 mixer, transistor modulation, amplitude

field-effect tetrodes are frequently used. The noise perfor- (1) Modeling the target and interference environment, the
mance of field-effect transistor mixers is slightly worse than propagation medium, and the major radar subsystems
for diode mixers, but they perform the conversion with some (antenna, transmitter, receiver, and signal processor), and then
gain (3 to 10 dB) reducing the IF amplifier noise figure simulating radar operation with this model. Typically this
requirements. The output power for transistor mixers is about approach is more flexible with respect to variation in the
an order more than for diode mixers, so the dynamic range is radar and environmental parameters, and it offers estimates of
about 10 to 20 dB larger. IAM the entire set of radar characteristics in detection, tracking,
Ref.: Fink (1982), p. 14.64; Gassanov (1988), p. 133. and as applicable in the target discrimination mode. However,
it requires the development of complicated and costly models,
The tunnel diode mixer is based on the tunnel diode as an
and is often based on Monte-Carlo simulation (statistical
active component, and has signal gain because of the negative
modeling) because analytical models (based on closed-form
resistance of the diode. Tunnel diode mixers are unstable
equations) cannot be applied. Algorithms for this approach
while operating. Power handling capability and dynamic
are described in Leonov (1979).
range are lower than for diode mixers, and noise performance
(2) Development of a model for the particular radar char-
is only slightly better. Consequently, they have no wide range
acteristic of interest (e.g., probability of detection, measure-
of practical application. IAM
ment error, subclutter visibility, etc.) as a function of the
Ref.: Gassanov (1988), p. 112.
factors affecting this characteristic (transmitter power,
An unbalanced mixer incorporates a directional coupler, antenna gain, number of received pulses, type of signal pro-
summing microwave signals and a single-band rectifier (mul- cessor, etc.). In this case the models can be less complicated,
tiplier) based on the mixing diode. The working point is or combined (analytical-statistical) models can be developed
located on the straight part of the volt-ampere curve, corre- (see ERROR model). This approach requires less develop-
sponding to the current of 0.5 to 1.0 mA produced by the rec- ment time, but is applicable only to estimation of the charac-
tified voltage of local oscillators. When the signal voltage is teristic for which the model was developed, and the
considerably less than the LO voltage, the mixer performs a applicability of results is limited by the assumptions of the
linear transformation of the signal spectrum. Typically, unbal- analytical model (e.g., operation in white, Gaussian noise,
anced mixers with high sideband and those with low sideband with a Rayleigh target, in Rayleigh clutter, etc.). SAL
are distinguished, depending on the location of the signal Ref.: Leonov (1979).
band relative to the LO frequency. Significant levels of sum
(w + w LO ) and image (2w LO - w LO ) frequencies are present (RCS) fluctuation model (see RCS fluctuation).
s
at the mixer output besides the difference (intermediate) fre-
Swerling model (see RCS fluctuation).
quency. These oscillations are reflected from the diode and
can be either absorbed by the matched load at mixer input, or MODULATION. Modulation is “the process by which some
reflected from the input by means of filters circuits. A mixer characteristic of the carrier is varied in accordance with a
matched at the input has conversion losses of 8 to 10 dB and modulating wave” and the result of that process. In radar
is broadband with linear frequency-phase response. Mixers applications three basic types of modulation are used: (ampli-
with reflections have losses which are 1 to 2 dB less, but their tude)-pulse modulation, frequency modulation, and phase
bandwidth is lower. modulation. SAL
Unbalanced mixers typically have waveguide, coaxial, or Ref.: IEEE (1993), p. 816.
microstrip structure. The main disadvantages are the higher
Amplitude modulation is “the process by which a continu-
LO power compared to signal power and the conversion of
ous high-frequency wave (carrier) is caused to vary in ampli-
LO noise to intermediate frequency. Consequently, they have
tude by the action of another wave containing information.”
a relatively high noise factor, 10 to 15 dB, (which can be
The analytical formula for amplitude-modulated signal is
reduced by cooling). IAM
Ref.: Gassanov (1988), p. 113. ut () A sin ( w t + f )
=
0 0 0
MODEL M A
0 0
– --------------cos [ ( w + W ) t + f + y ]
2 0 0
atmospheric model (see ATMOSPHERE). M A
0 0
+ -------------- cos [ ( w – W ) t + f – y ]
0
0
clutter model (see CLUTTER). 2
where A is the amplitude of carrier oscillation, M is the
0
0
detector model (see DETECTOR). modulation coefficient, w and f are the frequency and ini-
0
0
ar
tial phase of the carrier, and W and y e the frequency and
error model (see ERROR model).
the initial phase of the modulating signal.
The radar model describes a radar system operating against The first term is the underlying unmodulated oscillation
a specified class of targets in a specified environment. There with carrier frequency w ; the second and third terms are the
0
are two basic approaches to modeling of radar operation: description of oscillations due to amplitude modulation pro-
cess. The frequencies of these oscillations w + W and w + W
0
0