VOLTAGE
4. DETECTION METHODS AND RECEIVERS
distribution in these helices tends to be sinusoidal if the circumference c is less than21J3,but this is rendered uniform as c - ').. (Marsh, 1951). One must also use care in selecting the conductor material since its diameter affects normal mode radiation (Tice & Kraus, 1949). A large helix that radiates in the axial mode is typically 0.32').. in diameter and has an interwinding spacing of 0.25 A..In the axial mode, the diameter of the conductor has little effect on the helix characteristics.
1.MICROWAVE ENGINEERING FUNDAMENTALS AND SPECTROMETER DESIGN 43 detection procedures in which microwave power is measured via the change in resistance in a bias circuit as heat alters the conductivity of some material.
Spectroscopic applications of thermal detection methods are found in the older microwave spectroscopy literature (cf Gordy et al., 1953; Harvey, 1963), but were supplanted by diode detection methods. Bolometer methods are well suited to high-frequency(i.e. millimeter and sub-millimeter) EMR, however, and will therefore be briefly described here.
PtWIRE
BOLOMETER
SLIDING SHORT
Figure 18. The bridge method of power detection by using bolometers. The bolometer is positioned in a waveguide (or coaxial) section where it absorbs power from the incident electromagnetic field. The power absorption (heating) alters the resistance of the device, which is then detected on a conventional impedance bridge. This variant of the Wheatstone Bridge uses a low frequency AC signal to compensate for ambient temperature fluctuations;
other compensation techniques place matched temperature-sensitive resistors (those located immediately to the left of the bolometer in the figure) on the same mount as the bolometer, but not in the microwave field.
The simple bolometer is a thin platinum wire that terminates a transmission line and is mounted so that it maximally absorbs incident microwave radiation. A typical waveguide mount is shown in Figure 18, and the tuning plunger is used to optimize the microwave standing wave for maximal absorption by the wire. Other bolometer configurations are metal films deposited on a dielectric or grids, the latter of which are advantageous in the sense that the power may be partially transmitted. A thermistor is conceptually similar, but the device consists of two non-contacting wires that are bound within a bead of a conducting material such as a metal oxide matrix. Although bolometers work well at all frequencies, thermistors are contraindicated above 15 GHz.
The power measurement is made via bridge methods (Figure 18). The device is biased by a DC source and variations in itsR are measured via the bridge. Typical specifications of the older platinum wire devices were a minimal power detection of 0.01 JlW and a response of 50JlV/JlW. Modern bolometers that are manufactured for use as detectors in radioastronomy are semiconductor composites (Ge-Si) and cryogen cooled, yielding much higher responsivities . Bolometer mixers and pulse detection are also feasible with current technology.
4.2 Crystal Detectors and Autodyne Detectors
Early detectors of electromagnetic waves consisted of a resonator (i.e. a loop antenna) and a spark gap, and reception of a signal was based on the observation of the spark or the deflection of a galvanometer. The spark gap method of reception requires that the incoming signal be of high amplitude so that the gap breaks down, and it was gradually replaced by contact interfaces between oxide-forming metals, the so-called 'coherers'. The coherer was fashioned as either a point contact between two metals, for example, a whisker contacting a metal plate, or as an aggregate of fine metal particles. The metal had to form an oxide layer at its surface, and the principle of operation was similar to the spark gap, that is, the ordinarily high resistance dry joint between the two metals became a low resistance to an rf signal; both receivers allowed for reception of pulse-code modulated carriers .
The principles of coherer operation, namely, the breakdown of the metal oxide interface, are suggestive of the point contact rectifying diode, which has been a standard detector for high frequency electromagnetic waves. An early version was fabricated at the interface between a phosphor bronze whisker and iron pyrite (Cleeton & Williams, 1934), and later devices adopted semiconductor materials such as boron-doped silicon (Scaff& Ohl, 1947; see also Levine, 1964). Modern crystal detectors rely ona metal layer that is evaporated directly onto the surface of the semiconductor, the latter of which is itself a more sophisticated device that is fabricated as ann-i-p-i-n GaAs rectifier.
The whisker contact to a semiconductor forms a Schottky barrier (Sze, 1981 ), and the mode of operation follows from the current-voltage relationship. Figure 19 depicts a typical 1-V curve of a Schottky barrier diode. If the barrier is biased by an applied voltage, a current flows across the interface, and the magnitude of the current flux is specified by the conduction properties of the semiconductor. An oscillatory waveform is rectified, as indicated, because of the asymmetry of the 1-V curve, and this rectified current signal can therefore be used as a video detector (i.e. a measure of the current amplitude) of microwave and rf signals. The response
I.MICROWAVE ENGINEERING FUNDAMENTALS AND SPECTROMETER DESIGN 45 is linear insofar as theI-V curve is linear, and therefore the device is suitable for small signal detection only.
RECTIFIED OUTPUT
v
VIDEO (MODULATED) OUTPUT
...
v
MODULATED CARRIER
Figure J9. The current-voltage curves of a diode. Rectification of an AC signal is achieved due to the nonlinear profile of the curve (top), and if the AC signal is modulated (e.g. a sawtooth, as indicated in the bottom portion), this modulation, or ' information' is transferred to the diode output.
The barrier diode is essentially a building block of more complicated detectors. They can, for example, be combined in a bridge configuration to form a mixer (Section 4.3), but they can be used as stand-alone units in either waveguide or coaxial mounts. As a waveguide-based device, the diode is mounted in much the same way as has been described for limiters and attenuators (Section 3.3), the principal difference being that the diode terminus is outfitted with an output for metering the current amplitude (Figure 19). Most modern detectors, however, are packaged as coaxial devices, featuring an SMA connector for the high frequency input and a
BNC connector for the video output. Typical products are the Hewlett-Packard GaAs planar doped diode detectors (for high speed response) and traditional Schottky barrier detectors." The crystal is therefore used as a video detector of rf and microwave signals. The response is linear in so far as the 1-V curve is linear, which limits their application to small signal detection, although one tends to not operate in the linear region of the 1-V curve because at this region the diode contributes the most noise . The absolute amplitude of the output is affected by the impedance of the rf source and the load characteristics of the whisker.
The detector output voltage is determined from the characteristic 1-V curve, which is non-linear. The equation Vout
=
IR is modified by writing I as a power series, 1=ao +a\V+a2V2 ... , and substituting the time-dependent expression for the rf voltage, V(t)= Vocosrn t. If one rewrites the expansion ofI using the expression for V(t) in place of V, and then uses the substitution cos2wt=
t+tcos2wt, then the first two terms of the powers series expansion . become 1=t
a2Vo2+a, Vocoswt, and the DC output is proportional to the square of the peak input voltage. This is the so-called 'square-law' property of the detector, and one wishes to or,erate at voltage levels such that this term dominates the diode signal output. 24.3 Homo- and Heterodyne Detection
Figure 20 illustrates the placement of a crystal detector in a section of waveguide where a single carrier signal will impinge upon it. In many detection circuits or receivers, however, one will find a configuration similar to those illustrated in the figure, in which a second signal is imposed upon the diode by a local oscillator (LO), for example, in radar and some similarly configured pulsed EMR spectrometers. In other words, there are two carrier signals.fc.1 and.fc.2' In this case the rf input voltages are VIet)
=
V\ cose»t and V2(t)= V2 cosW]t, and the net voltage Vet)= VIet)+V2(t).Substituting into the series expansion for I yields two DC terms a2/2 [V\2 + V/] plus rf21 Part numbers of both diode types are similar. HP8474 series are GaAs planar diodes;
HP8373B& C are Schottkys. Wiltron likewise markets high frequency coaxial detectors as a 75 series.
22See also Ishii, 1990, Chapter 7 for a more rigorous analysis of the square-law response.
1.MICROWAVE ENGINEERING FUNDAMENTALS AND SPECTROMETER DESIGN 47
signals a2 /2V(2·cos2mJt, a2 /2V22·cos2liJ)t, a2VrV2·cos(WJt + liJ)t), and
"~ =f5DE
rf 0-0---I~""_o VIDEOIN 0 0 OUT CRYSTAL
Figure 20. Diode detection schemes for auto-, homo-, and heterodyne detection. As with the bolometer, the diode is positioned in a transmission line so that the E-field lines intercept the device. A crystal diode detector configuration is depicted in the top left comer, whereas homo- and heterodyne arrangement are depicted on the bottom left and right diagrams. The equivalent circuit of the diode detector and 'anatomy' of a diode cartridge mount are also illustrated.
a2VI'V2'COS(WIt - liJ}t). For homodyne detection purposes one ensures that
WI
=
liJ}, and the DC signal therefore becomes ar(V(/-./2+V2/-./2)2 . One ofthe signals is a reference signal and does not vary in response to some change in impedance or external modulation, whereas the other signal is the information carder. The effect of the second reference signal in this type of detection is that it supplies a bias to the diode and renders the current response ofthe diode proportional to V/+Vs,as opposed to simplyVs2 (Vs
denotes the signal voltage), and therefore there is a gain in sensitivity.
Heterodyne detection refers to the situation in which the two frequencies
WI and liJ} are not identical, and the information is carried on an intermediate frequency ImJ - liJ}
I,
The rationale behind the superheterodyne approach to detection stems from the performance of amplifiers at DC, as opposed to higher frequencies. There tends to be more noise produced in the amplification of a DC signal than a moderate frequency AC signal, and there was at one time an impetus to use superheterodyne receivers in order to maximize sensitivity (see Feher, 1956). But like the once eschewed solid-state microwave sources, the noise figures of solid-solid-state amplifiers has greatly improved with modem fabrication techniques, and homodynedetection is competitive with superheterodyne (and obviates the need to lock two sources to one another).
4.4 Mixers
The features of homodyne and heterodyne detection appear in a single semiconductor crystal because of its nonlinear response (i.e.I-V curve), but in most high-sensitivity applications two or more crystal diodes are used in tandem to improve performance (Pound, 1948;Tsui, 1983; Ishii, 1990). A so-called balanced mixer (cf Pound, 1948, Ch. 6) combines two crystal diode mixers that are simultaneously driven by the local oscillator(i.e. reference) and received signals . The diodes may be inserted into the circuit by using a magic tee (or microstrip hybrid junction) so that two IF output signals are obtained. For example, by placing the two crystals on separate branches of the hybrid (i.e. Ports I & 2), one can ensure that the signal from the local oscillator port (LO - Port 3; RF- Port 4) arrives at the two diodes 1800 out of phase, and therefore any random noise that modulates the source oscillator signal will be nulled as the output of the IF stages are combined. The symmetry of the device, namely the hybrid, is the determining factor in noise reduction by this detection device because the symmetry determines the extent to which each port (diode) is isolated from the others (cf Section 3.2.4).
One may also build on this basic balanced mixer by adding diodes pairwise in a symmetric manner and thereby further reduce spurious noise because in so doing one introduces more isolated ports in the detector circuit.
For example, one may combine two balanced (each containing two crystal diodes) mixers in parallel so that they are out of phase . Rendered as a pairing of two magic tees, the signal of the local oscillator is split between two branches by using either a simple tee or a magic tee . At the end of each branch is a balanced mixer of the type described in the preceding paragraph;
the received signal must likewise be split and passed to the designated RF port (number 4) of the two balanced mixers. Figures 6-20, 6-21, and 6-22 in the book by Pound (1948) illustrate the construction details of the double balanced mixer comprised of two or more hybrid tees . The advantage of using such a configuration is that it increases the isolation between LO and RF ports and suppresses harmonics, and in so doing reduces the conversion loss from the rf to IF signal. The amount of isolation, however, depends upon the symmetry of the device and how well fabrication methods can ensure symmetry for the frequency range specified.
1.MICROWAVE ENGINEERING FUNDAMENTALS AND SPECTROMETER DESIGN 49