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The Ubiquitous Microwave Diode. Chapter 3: Step Recovery Diodes

Posted on Wednesday, September 27, 2017

Diodes are remarkably simple yet extremely versatile devices, and each type has characteristics that make it well suited for specific applications. One of the less common but nevertheless very useful is the step recovery diode (SRD) whose characteristics lend themselves well to several applications for which other diodes aren’t well suited.

An SRD acts like charge-controlled switch in its forward biased (on) state, and when the diode it is switched from forward-biased to reverse cut-off, current flows briefly as stored charge is removed, exhibiting a low impedance of about 1 ohm. Once that charge is depleted, the impedance rapidly increases to its normal high reverse value, in 1 ns and sometimes less. When this occurs, an output signal is produced that consists of very fast, very short pulses that are harmonically-related to the input signal, appearing on a spectrum analyzer like the teeth of a comb.

So, not surprisingly, an SRD acting as a comb generator (Figure 1) is well suited to synchronize the phase-locked-loops in frequency synthesizers. The SRD’s output can be used directly, to synchronize phase-locked loop oscillators, or to generate a set of substitution channels for testing, each of which has the same baseband audio and video signals. Another use of the SRD’s output is for EMC testing in which the output is used as a broadband reference. Generally speaking, any application that requires frequency multiplication is a potential home for an SRD.

 Figure 1

The diode’s characteristics also allow it to be used for frequency multiplication in which the input frequency can be multiplied at the output by up to 20 times. An SRD can operate at reasonable high RF power levels that some other devices cannot match and can achieve levels of efficiency up to about 65% as a doubler and 50% as a tripler. Maximum operating frequency is about 20 GHz.

Similarly, an SRD is sometimes used in parametric amplifiers that are rather obscure types used primarily because they operate up to several hundred gigahertz and have extremely low noise figures when compared to a transistor amplifier. Typical applications include radio astronomy, satellite communications in space, and radar low noise amplifiers at very high frequencies. Rather than a transistor, they use in inductor or capacitor as the active element, the reactance of which is varied at a different frequency that is generated by a local oscillator called the pump. In addition to SRD’s, varactors diodes are also used in this application.

More obscure uses for SRDs our applications such as ground-penetrating radars and particle accelerators. In these cases, a Drift Step Recovery Diode (DSRD) his used, and SRD variant that was discovered by Russian scientists in 1981. A DSRD operates like an SRD but rather than having continuous forward current it is pulsed. In a DSRD, a short pulse of current is applied in the forward direction, which "pumps" the P-N junction, charging it capacitively. When the current direction reverses, the accumulated charges are removed from the base region. Once the accumulated charge decreases to zero, the diode opens rapidly as in standard SRDs. However, in the DSRD, a high-voltage spike can appear caused by the self-induction of the diode, and the larger the current, the shorter the transition from forward to reverse conduction, and the higher the pulse amplitude and efficiency of the pulse generator.

A DSRD has a specific level of doping that distributes the majority carriers and pulse signal conditions in such a way that extremely high switching speeds can be maintained up to the theoretical limit of silicon even at very high voltages and currents up to several hundred amps. The DSRD provides the benefit of high power-handling ability beyond that of a standard SRD while having a somewhat greater operating lifetime. DSRDs have been demonstrated with peak power levels greater than 1.6 MW into a 10-ohm load with a 2 ns rise time.

One of the key components in the transmitter of a grand-penetrating radar is its sub-nanosecond pulse generator that must handle RF power levels of 5 kW or more with rise times as low as 2 ns. The DSRD is sometimes used in this application because of its ability to deliver pulses with a high repetition rate and high shape stability. In a linear accelerator it’s used as the “beam kicker” or opening switch that separates a high-current electron beam, so it can be transported into two different streams whose energy outputs are several kilovolts.

As is probably obvious by now, the SRD and its DSRD variant are used in applications that are very specific and different from the environments that most microwave diodes are designed to serve. However, they are much simpler and far less costly than alternatives in some cases, and if designers pay attention to their peculiarities they can provide unique solutions for frequency multiplication and pulse generation.

Caption

1. Harmonically-related content generated by an SRD.

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