uTracer NXT — Vol 2: Hardware Architecture

2.1 The shape of the instrument

Read the block diagram above left-to-right. A host PC running the GUI talks over a serial link to a PIC microcontroller, which is the sequencer for everything. On command the PIC charges the high-voltage boost converters, sets the grid bias through a DAC-plus-HV-amplifier chain, fires the high-voltage switch to connect the charged reservoir cap to the tube for a pulse, and reads back the anode and screen currents through a sense-resistor / programmable-gain / ADC chain. A separate heater channel runs a PWM duty cycle. That skeleton is unchanged in spirit from the 3+ and the 6 — what changed in the NXT is the parts inside each block. This volume walks them.

2.2 Microcontroller and connectivity

The NXT keeps the PIC16F884 at its core — the same controller family used from the later uTracer3 revisions through the uTracer6 — with its 10-bit on-chip ADC. Keeping the proven MCU means the firmware, the timing, and the command protocol all evolve rather than restart.

Connectivity is a serial (RS232-style) link at 9600 baud, 8-N-1, exchanging fixed-length ASCII frames (Vol 4 covers the protocol). In practice you reach it from a modern PC through a USB-to-serial adapter, exactly as with the earlier models. As of the current build log there is no built-in USB, Bluetooth, or Wi-Fi — the NXT is not a “connected” instrument in that sense, and any claim of native wireless would be TBD — confirm; nothing in the source supports it.

2.3 High-voltage generation and the HV switch

Anode and screen voltages are each produced by a boost converter: the PIC drives an NMOS switch that pumps a 330 µH inductor to charge a reservoir capacitor (100 µF, 500 V class) up to the target plate voltage over a few seconds. When it is time to measure, a separate high-voltage switch connects that charged cap to the tube for the pulse. Vol 3 diagrams both switches and why there are two of them.

The single most important hardware change is which device does that high-voltage switching. The uTracer3/3+ used a high-voltage PNP transistor; those parts are getting hard to source and were the weaker point under fault conditions. The uTracer6 had already replaced its switch with a robust NMOS design that logged no field failures across its deployed units. The NXT adopts that NMOS high-voltage-switch topology — the reliability advance moves down from the kilovolt model into the mainstream instrument. Because the NXT targets a ~500 V envelope rather than the 6’s kilovolt one, it can use a 700 V-class NMOS (an Infineon CoolMOS-type part, the IPD70R360P7, driven from a low-voltage logic-level gate drive) instead of the 1000 V devices the 6 needed. Treat the exact device suffix as build-log-current and subject to revision, but the architecture — NMOS switch, direct low-voltage gate drive — is the confirmed and deliberate change.

2.4 The analog front end: the OPA227 problem, solved with a PGA

This is the block the whole redesign was arguably built around. The earlier uTracers sensed current with an OPA227 op-amp in a through-hole DIL package, and that part is going obsolete in that package. The NXT’s front end is rebuilt around parts that are current and will stay so:

• The current-sense buffer becomes a MCP6V86 zero-drift op-amp (roughly ±25 µV max offset, ~4 V/µs slew) in place of the OPA227.
• Gain/range is handled by a PGA113 programmable-gain amplifier, giving a ladder of software-selectable gains (1×, 2×, 5×, 10×, 20×, 50×, 100×, 200×). This is what lets the instrument measure both tiny grid/leakage-scale currents and full plate currents with good resolution — the software just picks the gain that fits the signal.
• Current is sensed across a 14.3 Ω shunt by default. (The uTracer3 used a higher-value sense resistor; the 6 used a much lower 4.7 Ω to reach an amp. The NXT’s 14.3 Ω sits between them, matched to its few-hundred-milliamp envelope.)

2.5 Grid bias chain

Control-grid bias is generated by a DAC feeding a high-voltage amplifier. In the NXT that is an MCP4921 12-bit SPI DAC (referenced to a 2.5 V precision reference) driving an OPA455-class high-voltage op-amp to swing the grid from 0 down to about −100 V. The grid amplifier is enabled only during the measurement pulse (through an optocoupler), so it dissipates almost nothing between pulses and is protected during faults. Note the envelope difference from the uTracer6 here: the 6 offered both negative and positive grid bias (to −100 V, and — on an optional extension board — to +100 V with grid-current measurement) because transmitter tubes are driven into positive grid. The NXT, aimed at ordinary receiving/audio tubes, provides negative bias only in the documented design — if you need positive-grid characterization, that is the 6’s job, not the NXT’s.

2.6 Power architecture — simpler by design

The redesign cleaned up the supply rails. The NXT runs from a ~20 V external brick and derives:

• a +5 V logic rail (a small linear regulator, 78L05-class) for the PIC, the DAC, and the low-voltage analog parts; and
• a −105 V rail (its own small boost converter with a high-voltage PMOS switch) for the grid bias.

Notably, the earlier designs’ auxiliary ±15 V supplies are eliminated — a deliberate simplification that removes parts and board area. The heater is driven as a PWM duty cycle at ~1.2 kHz (the same low PWM frequency the uTracer6 adopted for accuracy, versus the original uTracer3’s much higher ~19.5 kHz), producing 0 V up to roughly the supply rail at the heater terminals.

2.7 Voltage and current envelope

Pulling the numbers together, and being explicit about what is default versus a documented modification:

NXT measurement envelope (from the build log; treat extended figures as mod-dependent):

Table 1 — NXT measurement envelope (from the build log; treat extended figures as mod-dependent):

Parameter	Default	Notes / extended
Anode / screen voltage	~2–500 V	up to ~500 V with 500 V-rated reservoir caps
Anode / screen current	up to ~350 mA (≈250 mA with compliance)	up to ~750 mA is described with a parallel sense-resistor mod
Control-grid bias	0 to ~−100 V	a lower-range (0 to ~−25 V) option is a documented resistor mod for finer resolution
Heater	0 to ~supply, PWM	~1.2 kHz duty-cycle drive
Measurement pulse	~1 ms	boost charge pulses are tens of µs; the sample window is ~1 ms

Exact trip points, the precise sense-resistor and divider values, and the extended-range component values are laid out in the build log and should be taken from it (and re-verified against the shipped kit) before relying on the last digit — hence ”≈” above.

2.8 Kit form: still a hand-solderable board

Physically the NXT stays true to the family: a through-hole PCB deliberately dimensioned to the uTracer3’s footprint and terminal positions, so it is close to a drop-in on that mechanical layout, and it is meant to be assembled by hand rather than reflowed. Positions are reserved on the board (jumpers/optional resistors) for the range-extension mods noted above. Vol 4 covers the actual build and alignment. The net picture: same pulsed idea, same MCU and serial interface, same solder-it-yourself board — with a modernized, parts-available analog front end and the uTracer6’s NMOS high-voltage switch underneath it.