Diode-rectifier with non-linear load resistance

[0002] This invention relates to a half-wave, full-wave or multi-path RF diode-rectifier circuit having at least one rectifier diode and at least one output-side charging capacitor.

[0003] Diode-rectifier circuits of this type, based on junction or Schottky diodes, are known in an extremely wide variety of embodiments. FIG. 1 shows a basic wiring diagram that is common to all these circuits, having a diode V and a charging capacitor C_r. These can be manufactured with very small time constants, and because of their thereby higher measuring speed, they are often used instead of thermal sensors for power measurements. Such rectifier circuits are also used for rms voltage measurements. A clear interrelationship, independent of a type of signal, between an average value of an output voltage and an rms value U_inof an input voltage is essential for power and rms voltage measurements, so that according to the equation $\begin{array}{cc}{P}_{\mathrm{in}}=\frac{U_{\mathrm{in}}^2}{Z_0}& \left(1\right)\end{array}$

[0004] power converted in a terminating resistor Z₀of a power sensor can be calculated. Narrow limits are set for the diode rectifier relative to a dynamic range, because of exponential voltage-current characteristics of the semiconductor. That is, in only a relatively small range of the input voltage, a so-called square-law range, is the output voltage dependent exclusively on the rms value of the input voltage, so that strictly speaking, only within that range are power measurements possible for all signal shapes and modulation types. Outside the square-law range, a peak value of the input voltage determines, to an increasing extent, a level of the output voltage, so that the clear interrelationship between output voltage and input power is lost. Only if one limits oneself to a specific signal type—for example unmodulated spectral-pure signals—can the specific interrelationship between the rms value of the input voltage and the output voltage for that signal be used to perform accurate power measurements also outside the square-law range. However, as soon as the input signal varies from a curve shape for which the rectifier was calibrated (for example, measurement of a sine-wave signal with harmonics instead of a spectral-pure sine-wave signal), or an envelope of the measurement signal is modulated (AM, π/4 DQPSK, QAM, etc.), measurement deviations occurs outside of the square-law range.

[0005] To expand a proportional zone between output voltage and input power beyond the square-law curve range, it is known to load the output of the rectifier with an ohmic resistor R_p(negative temperature coefficient (NTC) thermistor) that is selected to be appropriate for the video resistance R₀at zero bias of the rectifier diode, (Hoer, C. A., Roe, K. C., Allread, C. M.: Measuring and Minimizing Diode Detector Nonlinearity, IEEE Transactions on Instrumentation and Measurement. Vol. IM-25, No. 4, December 1976, pp. 324-329). If the suitable dimension is R_p=0.4×R₀, a 17 dB improvement in the dynamic range is achieved in comparison to an unloaded rectifier diode; however, because of the loading with the ohmic resistor, the output voltage is also simultaneously reduced to approximately ⅓ of the no-load value, so that an effective increase in the proportional area of only approximately 12 dB can be assumed. This known circuit, because of the inadequate matching between the rectifier and the load resistor, is also strongly temperature-dependent and therefore is only conditionally suitable for mass production.

[0006] It is an object of this invention to provide a diode-rectifier circuit whose dynamic range can be expanded beyond the square-law range using uncomplicated circuit elements and that has an improved thermal behavior so that, for example, average power value measurements of modulated sine signals are also possible, even into the GHz range.

[0007] This object is achieved by using, with a single- or multi-path RF diode-rectifier circuit of the preamble of the main claim, the characterizing features recited in the main claim. Advantageous enhancements are set forth in the dependent claims.

[0008] According to the invention, instead of a linear ohmic resistor (NTC thermistor), a non-linear load resistor, having a relative temperature coefficient that is selected to be equal to a video resistance at zero bias of the rectifier diode, is used. Because of the non-linearity of the load resistor, a higher output voltage is achieved than with a linear load resistor, and through temperature synchronism of the load resistor and the video resistance, the temperature coefficient of the output voltage corresponds to that of an unloaded rectifier, and an increase in the proportional area is approximately 10 dB. With relatively little circuit-technology expense such a diode-rectifier circuit is achieved, with which also measurements of average power value or the rms voltage value of modulated sine signals outside the square-law of characteristic curves of the rectifier diode are possible. Also, an input impedance remains largely the same outside the square-law range.

[0009] A non-linear load resistor is understood to be a resistor in which the relationship of applied voltage and current is not linear, as is represented, for example, by a conventional voltage-current characteristic of a diode. For this reason, the non-linear load resistor is preferably also realized by such a diode.

[0010] The invention is described in more detail below using schematic drawings of embodiment examples.

[0011] FIG. 1 is a circuit diagram of a prior art basic half-wave rectifier;

[0012] FIG. 2 is a prior-art equivalent circuit diagram for output currents far below the reverse saturation current of the diode;

[0013] FIG. 3 is a prior-art equivalent circuit diagram for input voltages of any magnitude;

[0014] FIGS. 4aand 4bare circuit diagrams of a first embodiment of the invention for a half-wave rectifier circuit;

[0015] FIG. 5 is an equivalent circuit diagram of a modification of the FIGS. 4aand 4bcircuit;

[0016] FIG. 6 is a circuit diagram of an embodiment having a series resistance;

[0017] FIG. 7 is a circuit diagram of an embodiment of this invention without a series resistance; and

[0018] Each of FIGS. 8, 9, 10, 11, 12a, 12b, 13 and 14 is a circuit diagram of a different embodiment of this invention.

[0019] For a basic circuit of a half-wave rectifier shown in FIG. 1, assuming an ideal diode V, the following relationship exists between voltage u_vand current i_v: $\begin{array}{cc}{i}_v=i_s·\left[\frac{u_v}{e^{{\mathrm{nu}}_T}}-1\right]& \left(2\right)\end{array}$

[0020] Herein, i_sis the reverse saturation current of the diode, for which in the case of a Schottky diode, the formula $\begin{array}{cc}{i}_s=\mathrm{AR}*T^2{e}^{\frac{-e_0{\phi }_{\mathrm{bn}}}{\mathrm{kT}}}& \left(3\right)\end{array}$

[0021] applies. n is a correction factor, also known as ideality factor, that is influenced by the technology used, and ranges from 1.0 to 1.5. u_Tis known as voltage equivalent of thermal energy and depends only on—in addition to two natural constants—the absolute temperature T as per: $\begin{array}{cc}{u}_T=\frac{\mathrm{kT}}{e_0}& \left(4\right)\end{array}$

[0022] A: cross-sectional area; R*: modified Richardson's constant; k: Boltzmann's constant (1.38·10⁻²³J/K), e₀: fundamental electronic charge (1.60·10⁻¹⁹C), T: temperature in K, e₀Φ_Bn: potential barrier.

[0023] As seen in equation (2), the voltage-current characteristics of a rectifying diode can be completely represented by the inverse saturation current i_s, and the ideality factor. However, instead of i_s, the video resistance of the diode at zero bias is more often used, which is linked to the ideality factor n and i_svia the relationship: $\begin{array}{cc}{R}_0=\frac{{\mathrm{nu}}_T}{i_s}& \left(5\right)\end{array}$

[0024] R₀has a direct physical significance, which indeed involves the ohmic resistance of the diode at small signal excitations, which is noticeable as an input and output resistance of the rectifier. Because of its connection with i_s, the video resistance has the same strong temperature dependency, and in fact—depending on the barrier level e₀Φ_Bn—a doubling or halving is to be expected approximately every 15 K.

[0025] The rectifier of FIG. 1 is fed from a source having internal resistance R_iand sinusoidal electromotive force having the rms value U_in, the output voltage is further processed in a circuit whose input is represented by a capacitance C₁(the ohmic input resistance of the following circuit should be so large that it can be disregarded). It is the task of a resistor R_dto decouple an input of the following circuit from the rectifier, high-frequency wise. Far above a lower frequency limit of the rectifier, which is determined by the video resistance of the diode at zero bias and the load capacity C_r, and disregarding R_i, the whole input voltage U_indrops across the diode, so that the output voltage u_outof the rectifier can be described by the equation: $\begin{array}{cc}{u}_{\mathrm{outt}}={\mathrm{nu}}_T\ln \left[\mathrm{IO}\left[\frac{\sqrt{2}{U}_{\mathrm{in}}}{{\mathrm{nu}}_T}\right]\right]& \left(6\right)\end{array}$

[0026] It can be seen from equation (6) that in a simple diode model, only one diode-specific parameter, the factor n, determines the level of the no-load output voltage.

[0027] IO( . . . ) is the modified Bessel function of first kind and zero order, that can be expanded is into a power series with regard to U_in. Disregarding higher-order components, in the square-law range one obtains $\begin{array}{cc}{u}_{\mathrm{out}}\approx \frac{U_{\mathrm{in}}^2}{2{\mathrm{nu}}_T}& \left(7\right)\end{array}$

[0028] and for very high input voltages

[0029] u_out≈{square root over (2)}U_in  (8)

[0030] For output currents far below the reverse saturation current of the diode, as may occur with a passive load and driving within the square-law range, the output resistance is identical to the video resistance at zero bias R₀of the diode, and the linear equivalent circuit diagram of FIG. 2 applies. Because of the linear circuit parameters and the proportionality between the no-load output voltage and U_in², it is ensured that, when there is a time-variable amplitude of the input signal, the average value of the output voltage u_outis always proportional to the average value of U_in²namely it is proportional to the average power value of the measuring signal.

[0031] In the equivalent circuit diagram for input voltages of any magnitude of FIG. 3, the rectifier diode V must appear in the place of R₀, and the no-load output voltage is, according to equation (6), no longer linearly dependent on the input power. The non-linear output impedance of the rectifier becomes noticeable as interference when the load impedance of C_r/(R_d+C₁) can no longer be disregarded relative to R₀, that is, for higher frequency components of the rectified voltage. For this a drive-dependent voltage division appears which leads to a kind of second rectification, and the output voltage can increase beyond the average value of the open-circuit voltage. The effect is particularly noticeable for broad-band RF signals having a high crest factor (for example, W-CDMA), which—in accordance with the high band-width—contain higher frequency parts of greater amplitude in the rectified voltage.

[0032] In order to increase the measuring range of the rectifier, in which a behavior like that of FIG. 2 is sought, two steps are necessary: On the one hand, the open-circuit voltage must be proportional to U_in²over a larger range than is provided through the square-law range, and on the other hand, the output resistance of the circuit must be linearized, and the second measure may be omitted if, for all spectral components of the rectified voltage, it is assured, that the associated load impedance is large relative to the video resistance at zero bias of the diode, that is, no drive-dependent division is to be expected.

[0033] FIG. 4 shows a first embodiment of the invention for a half-wave rectifier circuit, and indeed FIG. 4ashows an arrangement of the diode V for positive output voltage and FIG. 4bshows an arrangement for negative output voltage. According to the invention, a non-linear load resistance R_pis connected between the rectified voltage output A and a common reference point M of the input voltage. For current-voltage characteristics of an optimal load resistance R_p, the relationship $\begin{array}{cc}{i}_p=i_s·\left[\mathrm{IO}\left[2\sqrt{\frac{u_p}{{\mathrm{vnu}}_T}}\right]e^{\frac{u_p}{{\mathrm{nu}}_T}}-1\right]& \left(9\right)\end{array}$

[0034] can be derived from equations (2) and (6). Depending on the division ratio v, the relationship of the output voltage to the open-circuit output voltage of the rectifier in the square-law range, it delivers an optimal range of a current-voltage characteristic, which can then be approximated in various ways.

[0035] As derived from (9), the load resistance R_pmust always have the same relative temperature dependence as the video resistance of the rectifier diode V, so that the linearization is maintained across larger temperature ranges. Preferably, therefore, the load resistance will be structured as an individual diode matched to the rectifier, or as a network of matched diodes, that have the same temperature dependence as the rectifier diode V.

[0036] The non-linear output resistance of a circuit of this invention can also be compensated by a series-connected, non-linear, resistance R_saccording to FIG. 4, so that the linear output equivalent circuit diagram of FIG. 5 results, with a linear output resistance R_outand a power-proportionate open-circuit voltage.

[0037] With a non-linear load resistance R_pof this invention, that is preferably in thermal contact with the diode V and thus is at the same temperature level as the diode V, the proportional area is increased by approximately 10 dB, that is, essentially by the same value as in a known circuit with a linear load resistance, but with significantly improved temperature behavior, more favorable behavior of the input admittance outside the proportional area, as well as higher output voltage at the same power.

[0038] As a non-linear load resistance R_p, a diode V₁is preferably used, whose video resistance at zero bias and ideality factor agree as closely as possible to the corresponding parameters of the rectifier diode V. FIG. 6 shows an embodiment having a series resistance R_s, and FIG. 7 shows an embodiment without a series resistance. The video resistance at zero bias of the single diode differs by no more than −50%/+100%, particularly by no more than ±10%, from the video resistance at zero bias of the rectifier diode (V, V_a, V_b), and the polarity thereof is such that it is driven in reverse direction by the rectified voltage (FIG. 6).

[0039] In using paired diodes for the rectifier diode V and the load diode V₁, for example at adjacent positions on a chip, (commercially available in the form of diode pairs or four-diode sets), temperature tracking of all parameters can actually be assumed. It must, therefore, merely be ensured that all diodes are arranged at the same temperature level. The functioning of the circuit is then guaranteed across a very wide temperature range. Both properties, that is, the temperature tracking of the parameters and approximately the same temperature level, are automatically present if the diodes are structured as components of a monolithic integrated circuit.

[0040] In comparison to a circuit having a linear ohmic load (NTC thermistor), the input admittance of a circuit of this invention, outside the proportional area, is significantly less strongly dependent on the level of input voltage than in a circuit having a linear ohmic load. This is particularly advantageous in circuits in which it is impossible to prevent at least one rectifier from being driven far beyond the proportional area. The less an over-driven rectifier thereby changes its input impedance, the less the remaining circuit is thereby affected, and the less an input-side matching of the circuit changes. Additionally, in a circuit of this invention, the input admittance decreases when the proportional area is exceeded—the circuit becomes higher in impedance—whereas it increases in a circuit having a linear load resistance. The interfering signals emitted by the rectifier into the measured object are thus minimized, whereas they increase in the circuit having the linear load resistance.

[0041] Because of the non-linearity of the current-voltage characteristics of the load diode, a circuit of this invention can deliver a higher output voltage than can a circuit having a linear load resistance. If the rectifier and load diodes have the same video resistance at zero bias, the output voltage of a circuit of this invention in the square-law range amounts to exactly half of the output voltage of the open-circuit rectifier, whereas in the circuit with a NTC thermistor it comes to just 29% of the open-circuit voltage. Hence, the circuit of this invention permits measurements of somewhat smaller power than does a circuit having an NTC thermistor.

[0042] For wide band modulated signals, measurement errors can occur outside the square-law range because of dynamic voltage division by the charging capacitance (C_r) and the input capacitor of the connected circuit (C₁). While the charging capacitor can, for the most part, be kept sufficiently small—broad band modulated signals are commonly employed at frequencies above 500 MHz and charging capacitors of a few pF are sufficient for that purpose—certain limits are set for the reduction of C₁. The problems can be significantly reduced if a suitable non-linear resistance R_sis connected between the output of the rectifier and the input of the following amplifying circuit according to FIG. 4, with which the output resistance can be linearized. Then the effect of C₁can also be disregarded. In the simplest case, according to FIG. 9, the added non-linear resistance R_sis of two parallel-connected diodes V₂and V₃matched to the rectifier and load diodes relative to the video resistance, or as in FIG. 8, of only one diode V₄matched to the rectifier and load diodes relative to the video resistance and one parallel-connected NTC thermistor R₁. The resistance of the NTC thermistor should be selected to be somewhat larger than the video resistance of the diode, for example, 1.55 times. The temperature coefficient of the NTC thermistor R₁must be matched to the temperature coefficients of the video resistance of the diodes. The resistance of the NTC thermistor is selected preferably to be 55%±25% larger than the video resistance at zero bias of the rectifier diode (V), or the average value of the video resistances at zero bias of the rectifier diodes (V_a, V_b) The relative temperature coefficient of the non-linear load resistance differs by no more than ±25%, preferably by no more than ±2%, from that of the video resistance at zero bias of the rectifier diode (V).

[0043] The best results with respect to expanding the proportionality range are in fact achieved when the rectifier and load diodes exhibit absolutely identical properties. However, in some circumstances, it may be useful to select the video resistance of the load diode V₁at zero bias to be somewhat smaller than that of the rectifier diode, because then even an additional expansion of the proportionality range (at the expense of linearity at low power) is possible. Such asymmetrical dimensioning is possible in three ways: through suitable matching, through manufacturing diodes having differing geometries (according to the relationship of the video resistances) on a single wafer or as components of a monolithic integrated circuit, or through parallel connection of a diode having a multiple of the video resistance of the rectifier diode parallel to the load diode. If the video resistance of the parallel-connected diode is chosen, for example, 10 times as great as that of the rectifier diode, this corresponds to a ratio of r=0.9 of the video resistance. Therefore, when matching is discussed, a defined, small variation from absolute equality should be included.

[0044] The requirements concerning matching of other parameters vary. It is important that the ideality factor n and the relative temperature coefficient of the video resistance at zero bias of the rectifier and load diodes be in good agreement.

[0045] FIGS. 10 through 14 show additional embodiments of a rectifier circuit of this invention that have similar favorable characteristics relative to expanding the proportionality range as do the embodiments of FIGS. 6 through 9. In the circuit according to FIG. 10, the non-linear resistance is formed by three diodes V₅, V₆, and V₇as is also the case in the FIG. 11 embodiment. In FIG. 10, the one diode V₅, like the single diode V₁, is again connected between the direct voltage output A and ground M, and the other two diodes V₆and V₇are arranged in series, in opposite directions, parallel thereto, between the output A and ground M. In the embodiment of FIG. 11, these two diodes V₆and V₇are connected in series, having the same polarity, and are arranged between the output A and ground M. The non-linear load resistance could also be formed by an even larger number of matched diodes; with it being important to have as close agreement as possible between the relative temperature coefficients of the video resistances of all these diodes.

[0046] The circuits already discussed have been, without exception, half-wave rectifiers, that is, each has been an arrangement having one rectifier diode that delivers a positive or negative output voltage depending on its orientation. Of course, all the features of the invention are transferable to full-wave rectifier circuits. In the simplest instance, a full-wave rectifier, according to FIG. 12a, is of two nearly identical rectifier diodes V_aand V_b, that are fed with the same RF voltage and differ only in the orientation of the diodes, with one rectifier diode supplying a positive output voltage, the other a negative output voltage (full-wave rectifier with bipolar output voltage). Each of the two rectifier diodes can be associated with a corresponding non-linear load resistance. Often, however, only a differential output voltage between the two outputs is further processed, in order to attenuate the common-mode portion. In this case, the two load resistances are connected, or only one load resistance is connected between the two outputs A_aand A_b, that is twice as large as that for a correspondingly-dimensioned half-wave rectifier (FIG. 12b). The series resistances R_saand R_sbfor linearizing the output resistance remain unchanged.

[0047] Additionally, the load resistance may be dimensioned for full-wave rectifiers in ways that cannot be transferred to half-wave rectifier circuits, as shown in FIG. 13. In this circuit, two respective sets of two diodes, V₈and V₉or V₁₀and V₁₁, arranged in series, with opposite polarity, are between the outputs A_aand A_b.

[0048] FIG. 14 shows a full-wave diode-rectifier circuit having unipolar output voltage. The input voltage is fed via a coupling capacitor C_kto the two rectifier diodes V_aand V_b, to which a common charging capacitor C_ris allocated. Here again, a non-linear load resistance R_pand a series resistance R_sare provided, that are each, once again, dimensioned to be twice as large as for a correspondingly dimensioned half-wave diode-rectifier circuit, but are otherwise structured in accordance with the exemplary embodiments presented above.

[0049] The invention is also applicable for voltage multiplier circuits; in a simplest case, here again the dimensioning of a half-wave diode-rectifier circuit is used, and both the load resistance R_pand the series resistance R_sare selected accordingly to be N times as large as for the half-wave circuit, where N is the voltage-multiplication factor.