Diode step recovery

The design of a system for working with short pulses follows the same principles as other pulsed ultrasonic systems such as ultrasonic flaw detectors, but in this case it is necessary to achieve very much greater stability and higher bandwidth. A schematic circuit is shown in Fig. 5.4 A very short impulse is generated by a step recovery diode. The pulse has a width of half the period of the centre frequency of the lens if it is shorter than that the energy in the pulse is reduced without any improvement in the signal bandwidth. Thus the lens acts as a sort of matched filter with poor time resolution but optimal... 

Schematic representation of the step recovery diode mode of action. During the first part of the forward bias cycle the stored charge is abruptly returned to the junction, producing the step-recovery effect...

The amplified rf power is fed to a step recovery diode housed in a microwave structure with output in an intermediate frequency range, typically low GHz. That range is not critical and can be chosen to be compatible with existing or low cost equipment used in other fields, e.g. radars and satellite downlinks. The present authors, for example, chose the band 11-15 GHz as it permitted the use of low cost coaxial components, synchroniser and YIG oscillator for the locking steps. 

To achieve this there is a hi-stab crystal controlled 20 MHz reference oscillator in the synchroniser circuit that is summed phase-coherently in the mixer with the step recovery diode output containing the 20MHz beat to produce the correction signal. 

That correction signal has an amplitude proportional to the difference in phase between the reference 20 MHz signal and the 20 MHz beat frequency. This phase error signal is then amplified and fed back to control the YIG oscillator output fi-equency. The sign of the phase error signal is such as to force the YIG oscillator frequency towards the synthesiser harmonic frequency. When they become equal the beat frequency from the step recovery diode will be identical to the reference oscillator 20 MHz and the two sources will be locked to zero phase difference. 

In summary, what we have described is a system which recovers from the transmitter pulse in two distinct steps. The first part takes place through some low impedance path usually, but not always, associated with the output circuitry of the transmitter. In order to improve the S/N, the transmitter circuit is passively disconnected by a set of series crossed diodes after which the second phase of recovery proceeds according to an active damping circuit which, if present, leads to better S/N because of the improved recovery even though it is a source of additional noise. Neither sets of... 

As an alternative to the classical C18- or C8 apolar bonded phases, the use of normal phase supports has been proposed to improve the isolation of more polar compounds as well as to perform extraction and clean-up in a single step, prior to reversed phase LC determination. Following this trend, Kishida has developed a simple method for the determination of six sulfonamides in meat samples, using normal phase MSPD with alumina N-S and 70% (v/v) ethanol solution as extraction solvent, followed by the evaporation of the extracts and LC-MS/diode-array detection (DAD) determination. Average recoveries were >90% in all cases, and the LOQs were well below the MRLs established by the EU. 

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