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.
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 Cr. 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 Uin of an input voltage is essential for power and rms voltage measurements, so that according to the equation                               P          in                =                              U            in            2                                Z            0                                              (        1        )            
power converted in a terminating resistor Z0 of 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 typexe2x80x94for example unmodulated spectral-pure signalsxe2x80x94can 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, xcfx80/4 DQPSK, QAM, etc.), measurement deviations occurs outside of the square-law range.
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 Rp (negative temperature coefficient (NTC) thermistor) that is selected to be appropriate for the video resistance R0 at 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 Rp=0.4xc3x97R0, 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.
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.
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.
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.
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.