Introduction
This
post continues a previous post that used a Microchip microcontroller and a buffer chip to generate a fast-rising edge. In this edition, the design is updated to suit an aluminium housing.
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| Assembled Pulse Generator Unit |
Another Release
To cater to those requesting a USB-powered unit in a ready-made enclosure, the same Microchip microcontroller (ATmega16U2) and driver design from the previous blog were used. Some design changes were made to improve manufacturability, such as using a right-angle PCB-mount SMA connector.
For a small sample run of the new design, the rise time is comparable to the previous design, at 1.1 ns and faster than the original design at 658 ps.
Design Changes
The Microchip microcontroller (ATmega16U2) can be USB-powered (5 V) and operate at a logic voltage of 3.3 V. An external linear regulator was chosen instead of using the UCAP 3.3 V output supply from the ATmega, as the UCAP supply is intended for only low-current loads.
The self-powered connection example is shown in Section 20.3 USB Module Powering Options of the ATmega16U2 datasheet (doc7799.pdf).
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| ATmega16U2 Bus Powered Example (Courtesy Microchip) |
With the power supply reduction to the ATmega I/O, the hex Schmitt-trigger power supply was also reduced to 3.3 V. The output performance was tested with Nexperia parts 74AHC04APW and 74LVC04APW.
To support DFM (Design for Manufacturing), the hand-soldered SMA connector from TE Connectivity (5-1814832-1) was replaced with a PCB-mount right-angle part from Molex (73391-0080).
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| Updated Pulse Generator PCB (2D) |
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| Updated Pulse Generator PCB (3D) |
Lastly, the board shape was resized to suit a metal case. A Hammond aluminium case (1455D602BK) was chosen for the PCB. This case was at the time less than USD 8 in quantities of 10 pieces and available at several online stores. A 3D-printed plastic case would also be an ideal, cost-effective alternative.
The plastic end caps supplied with the Hammond enclosure were replaced with PCBs. This change was aimed at a more turnkey solution, meaning the pulse generator PCB and end caps could come from the same PCB fabricator.
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| Pulse Generator PCB Assembly Modelled in Fusion 360 |
Designing the end caps was performed after the pulse generator PCB was completed, with some adjustments.
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| PCB End Cap (SMA End) in Fusion 360 |
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| PCB End Cap (USB End) in Fusion 360 |
After the pulse generator PCB was loaded into Fusion 360 and the end cap dimensioned, the PCB end caps were created in the PCB tool.
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| PCB End Cap (SMA End) |
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| PCB End Cap (USB End) |
Modelling in Fusion 360
The pulse generator PCB dimensions were chosen after making physical measurements of a Hammond aluminium case.
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| Pulse Generator PCB with Hammond Enclosure |
Hammond provides a CAD file for the aluminium case; however, a physical measurement for the PCB width was performed because of the powder coating.
Assembly and Tests
The pulse generator PCB Rev 3 and end caps were sent for manufacture. For the 50 Ω controlled impedance on the pulse generator PCB, a predefined layer stackup was selected from the PCB manufacturer's options.
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| Pulse Generator Blank PCB |
After assembling the pulse generator, programming and performing validation checks, the pulse generator PCA was slid into the Hammond case – almost. Checking another case, the PCA slid in very tightly. To make the PCA fit in all the aluminium cases, light sanding was required on the long edge of the PCB – tolerances and case anodising were possibly the culprit.
To fit the PCB end caps, the Hammond-supplied thread rolling steel screws #6 x 3/8" were changed to a set of salvaged pan head Philips screw. Using this type of screw is not ideal, and it should be replaced with a thread-cutting type to suit the C-shaped screw port in the Hammond case.
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| Pulse Generator SMA End View |
Changing the straight SMA to a right-angle SMA connector had a negligible effect on the rising edge.
With a 74AHC04PW fitted, the rise time (20-80 %) was 1.18 ns. Using a 74LVC04PW, the rise time (20-80 %) was 759 ps.
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Pulse Generator Output Rise Time (74AHC04)
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Summary and Future Updates
As noted by readers, this design is slower than other designs offered on the market. However, as readers continue to use this design, write their own USB interfaces and highlight the need for faster solutions, new posts on this subject will be made available.
Engineering Files
The same Atmel Studio project applies to this project. Below is the updated revision 4 schematic, PCB Gerbers and PCB BOM for download. The Rev 4 includes only a minor top-layer change near the SMA connector.
In addition to the PCBs, the Hammond case 1455D602BK and suitable screws are required, for example, M3 x 8 mm thread cutting.
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Pulse Generator Microchip Studio Project |
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| Pulse Generator Schematic Rev4 |
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| Pulse Generator Bill of Materials Rev4 |
