Pulse Compression Prototype Board

After almost 6 weeks of design changes, I have finally settled on a set of components to make up the analog portion of my project. It is far from what I originally envisioned, as this board relies on an FPGA or high speed microcontroller to be externally driven.

With up to 36W of transmission power to the transducer and ~66dB of sensitivity, the receiving side has a significant risk of coupling to the high voltage, high frequency switching noise in the transmission side. By using (possibly excessive) via stitching, the LNA and ADC driver are both heavily isolated and a very low impedance ground path is provided near the centre of the board, so theoretically the lower half should be relatively immune to noise from the upper half.

The board itself is very basic, and pretty far from my original vision from the project. Its intended use is generating and amplifying chirp functions, then amplifying and sampling the reflections.

The board can be used to generate and transmit arbitrary frequencies as the DAC can be drived up to 180MHz (clock path should be kept as short as possible), the high current driver amplifies the signal to 36V pk-pk with a slew rate of over 2000V/us, so it is possible to use square waves and PWM. The transmit path has impedance matching for a 4MHz, 75mm^2 transducer, which can be bypassed with jumper resistor if needed. There is very little damping provided by the circuit, so it may be difficult to drive very short pulses, such as the ~150ns pulses allowed by the HV7360 and similar pulser ICs. This is acceptable for pulse compression as the envelope width is ~200us. For this reason, open-air demonstrations will be preferable to imaging steel or some other material that permits a very high speed of sound. Signals up to 0.5V are isolated from the transmit side via diodes, while the receive sideuses the MD0100 to provide over 100V of isolation for large signals.

The LNA uses a trimpot instead of a digitally controlled gain. This is one of the major departures from the original design, as this is only a demonstration board under which the transducer will operate at similar distances from objects. The LNA is then followed by an ADC driver, which is connected to a 20MSPS ADC.

For this reason, a high speed digital device is required for interfacing, even if it is only to prepare the signal for transmission to a faster board or computer.

I welcome any critiques but am not willing to share the schematics or more detailed layout files until the conclusion of the project, as this board was created under heavy time pressure and has numerous caveats, as already discussed. Still, I look forward to using it and hope it will provide good data for my demonstration day.

Gerbers for PCUltra board

N.B. I somehow didn’t notice the pin headers were part of the silkscreen layer. I hope they are cropped off by the manufacturer, as there’s no copper below it in the GTL file.

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