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In this article, we describe a digital solid state double boxcar of infinite holding time and very high linearity. It uses a Hewlett-Packard (HP2212 A-M3) voltage-to-frequency converter (VFC) whose output is accumulated on a running counter, thereby providing signal averaging with an infinite holding time. The combination of FET signal gates with zero offset and the highly linear VFC results in a linearity of better than ±0.01% over an input range of ±1.0 V, which is a considerable improvement over that available with other digital boxcars. Integrated circuit pulse generators of high stability provide gating times from a fraction of a microsecond to many seconds. A unique monitoring system allows ease of adjustment with a simultaneous ``master'' display of all gating and signal functions. Other features of this boxcar are extreme compactness, low cost, and very high stability.

In this paper we describe a circuit for reducing the transmitter ringdown time and thereby improving the recovery time in pulse NMR experiments, The circuit uses only solid state devices and requires no external switching, For transmitter voltages less than O.S V peak to peak the effective resistance in parallel with the transmitter coil in a crossed coil spectrometer is only 60, whereas for larger voltages the shunting resistance is of the order of 1-2 kO. This circuit thus has the effect of significantly squaring the envelope of an rf pulse. It has the extra advantage of suppressing noise generated by the transmitter during the time interval between pulses.

A new me.thod. for preformi~g adiabatic demagnetization in the rotating reference frame in NMR experiments IS pr~sented III thIS paper. In this new method the frequency of a crystal oscillator rather than the actual magnetic field IS pulsed off resonance. Using simple circuits which are described in this paper, the effective field in the rotating frame can be pulsed by as much as 50 G, depending on the nucleus studied. This method completely avoids the problem of external magnetic field correction due to a pulsed field in the magnetic gap.