A digital interface for phase control of PTS-300 synthesizers

Abstract

Modern NMR spectrometers must be equipped to generate accurate, fast, reproducible radiofrequency phase shifts of essentially arbitrary angles under pulse programmer control. Many designs for phase shifting circuits have been used: analog circuits using delay lines ( I), RC networks (2)) hybrid networks (3)) digital designs based on clocked logic (4-7) or packaged phase shift units (S), high-speed analog designs using double-balanced mixers (9), and circuits using direct digital synthesis (DDS) techniques. The DDS designs using lookup tables have other advantages such as rapid phase-continuous frequency switching. Adding a phase shift option to such a circuit is relatively straightforward. Digital frequency synthesizers produced by Programmed Test Sources, Inc., are popular with designers of NMR spectrometers. In our homebuilt spectrometer we have used two PTS-300 synthesizers equipped with the “Type 2” frequency generation scheme that allows phase shifts in 0.225’ steps (2n/ 1600) using the DDS method. In this Note we describe a simple interface that allows the phase shift options of the PTS to be implemented in a convenient way. The design has the advantage that it allows the step size of the phase shifts to be preset, so that phase shifts can be expressed in multiples of a basic increment rather than in multiples of the basic resolution. The design can readily be adapted to other situations where such a feature is desirable. The PTS-300 uses 12 binary lines to encode the phase shift. Four lines specify the phase shift in multiples of 0.225” using binary coded decimal (BCD), and another four lines specify the phase shift in multiples of 2.25”, also using BCD. The final 4 lines specify the phase shift in multiples of 22.5”, but in contrast to the other lines, these 4 are coded in binary in the range 0 to 15 ,0 (in the older models the top 4 lines are also in BCD, giving 0.36” resolution). The relation between the desired phase shift and the requisite binary code is thus rather involved, although of course