Initially, the PT2026 development was delayed by “normal” problems, of the type known to every engineer: overheating, interface bugs, optimization of the RF circuitry, underestimated difficulties in firmware development, etc. Once all those were resolved, we were finally ready to discover the real challenges.
But to explain these, we have to review the operating principles of the previous-generation, analog NMR magnetometer - the PT2025 - and its digital descendent, the PT2026.
The PT2025 (show architecture diagram) is designed around a high-precision Voltage Controlled Oscillator (VCO). In the Manual mode, the VCO input voltage, and therefore the frequency, is set by front-panel controls. This RF frequency is divided down to match the probe range, and sent to a coil surrounding the NMR sample.
A separate oscillator and coil induce a slight modulation of the magnetic field at the NMR sample; this causes the NMR resonant frequency of the sample to oscillate. When it crosses the RF frequency, the resonance is detected and amplified with an ultra sensitive peak detector, located right in the probe.
Concretely, the peak detector detects a very slight dip in the RF signal, after it has been filtered by a finely tuned LC circuit formed by the RF coil and a varicap.
From analogue to digital, with the same probe
Still in the PT2025, this low frequency, peak-detected signal – the “NMR signal” – is amplified and sent back to the main unit, where it serves to drive the auto-tuning circuitry for the LC filter and, more importantly, to indicate that the NMR signal has been found. The strength of the magnetic field corresponds to the RF frequency, measured by a high-precision counter. Finally, in the Auto mode, an additional feedback loop automatically adjusts the VCO input voltage to keep the NMR signal centered within the field-modulation sweep.
The PT2026 (show architecture diagram) can use the same probes, but has a much simpler architecture. The RF signal is generated using a Direct Digital Synthesizer (DDS). This allows the frequency to be rapidly swept up and down (chirping), thus replacing the field modulation with a frequency modulation and rendering the field modulation coil unnecessary. The NMR signal is digitized and fed into a DSP, thus allowing much more sophisticated detection algorithms than a simple threshold. The DSP also controls the DDS, thus guaranteeing that the DSP knows exactly what part of the signal corresponds to what frequency. Finally, the tuning is also controlled digitally, to match the RF frequency at all times.
Two flies in the ointment: one fly removed…
A number of factors kept the DSP from properly tuning the LC circuit in the probe. First, the tuning frequency was hypersensitive to variations in DAC value, resulting in a very touchy control system. In addition, the existing probes have large time constants that prevent us from tracking the frequency during the frequency modulation cycle. Finally, the peak detector acts as a differentiator, resulting in a sloping baseline.