Page 277 - Radar Technology Encyclopedia

267 mixer, crystal mixer, Josephson-effect

capability. Conversion losses are about 3 to 10 dB. Typically, A Gunn diode mixer is a high power mixer using the nonlin-
it is not used in modern radars. ear resistance of a Gunn diode. This mixer has comparatively
Ref.:Ridenour (1947), p. 416; Fradkin (1969), p. 35. high efficiency (up to 9%) because of the descending part of
the volt-ampere curve of the diode and the low level of non-
A diode mixer is one using a microwave diode as the nonlin-
linear and frequency distortions. However, the level of the
ear component for frequency conversion. Most mixers use a
internal noise for this mixer is about 10 dB higher than for
nonlinear volt-ampere diode curve. The nonlinearity of the
low-noise mixer based on Schottky-barrier diode. IAM
diode differential capacitance is used only in power varactor
mixers for up conversion. Low-noise mixers, used in radar Ref.: Kukushkin, V. V., Radiotekhnika, no. 4, 1981, p. 9 (in Russian).
frequency converters and power mixers and to form transmit- A high-power mixer is one used to generate transmitter car-
ter frequency, are classified for application range. For opera- rier frequencies by conversion of IF oscillations to the micro-
tion principles, nonbalanced and balanced mixers are wave region, and is also used in local oscillator-exciters for
distinguished. The latter have wider range of application the conversion of carrier or LO frequencies. The main
because of sidebands suppression and a lower noise level. requirements of high-power mixer performance are to ensure
IAM specified output power with adequate uniformity in operating
Ref.: Tsui (1983), p. 439. band; to have sufficient suppression of spurious conversion
frequencies; and to have low conversion loss and high stabil-
The double-balanced mixer has two mixing sections,
ity. Typical high-power mixers use high-power varactors.
designed to provide image channel reception suppression.
These are comparatively narrowband and have rather compli-
The latter is provided due to the fact that the received signals
cated circuitry and tuning. IAM
are supplied to the two double-diode mixer in-phase and LO
Ref.: Gassanov (1988), p. 126.
signals are supplied out-of-phase. Phase suppression is imple-
mented through the addition of output IF signals. This mixer An image-recovery mixer is one using a reactive image ter-
has 20 to 30% improvement in bandwidth compared with mination without narrowband components. Typically it uses
unbalanced mixer. IAM two single-ended, balanced, or double-balanced mixers, and
Ref.:Tsui (1983), p. 443; Gassanov (1988), p. 119. two hybrid junctions as shown in Fig. M10. The left hybrid-
A double-diode mixer is a kind of balanced mixer suppress- Diode mixer
ing even harmonics of RF and local oscillator signals. Typi-
IF
cally it employes two single-ended mixers connected in
o
parallel and 180° ut of phase. Typically, the balanced mixer LO IF
RF 90 o 90 o
incorporates two diodes and coupling element with the signal hybrid hybrid output
input junction junction
source and LO. In a decimeter waves band, symmetric cou-
pling loop inserted in coaxial volume resonator of preselector
IF
is typically used. In a centimeter waveband, T-bridges, circu-
lar bridges, and slot bridges are used. To improve the decou- Diode mixer
pling factor of the signal and LO inputs, diodes are connected
Figure M10 Image-recovery mixer (after Skolnik, 1980,
to quadrature bridge (Fig. M9). Fig. 9.3, p. 349).
Low-pass filters junction produces a 90° phase shift, and the next one adds
another 90° phase differential. As a result, the IF signals from
two mixers add in phase and the images cancel. This kind of
l /4
Input signal Diode mixers has high dynamic range, less susceptibility to burnout,
and low intermodulation products. It is good as a receiver
l /4 front-end. SAL
Quadrature IF signal
bridge Ref.: Skolnik (1980), p. 349.
l /4 Diode
An image-rejection mixer is one having two conventional
LO signal asymmetrical mixers fed with the signals from a local oscilla-
,
tor shifted by 90° and received signals, and the bridge circuit
Low-pass filter
to suppress the signal at image frequency. If both mixers have
a balance configuration, the device is termed a double-bal-
Figure M9 Microstrip double-diode balanced mixer (after Gas-
sanov, 1988, Fig. 4.20, p. 117). anced mixer. IAM
Skolnik (1970), p. 5.10.
Because of partial circuit unbalance, LO noise is reduced only
A Josephson-effect mixer is one using a Josephson junction
by 15 to 20 dB. IAM
as the nonlinear component (see JOSEPHSON effect). The
Ref.: Gassanov (1988), p. 116; Skolnik (1980), p. 349. mixing component is a junction with a superconductor-insu-
lator-superconductor structure implemented with thermal