Method

The first step was to design the PCBs in a modern CAD software. Following the Q-meter's design documents, each of the five PCB's was reconstructed in KiCAD, a free and open-source CAD software for designing printed circuit boards.

Designing a circuit board in KiCAD follows several steps which will be briefly explained:

1. Schematics

The original schematic for PCB 1 in the design document	The recreated schematic for PCB 1 in KiCAD

First, one must construct the schematic for the circuit. Fortunately the design documents contained schematics for each PCB. Often, the user will need to construct a custom Symbol for one of the circuit's components, because the KiCAD library mostly contains common Symbols (resistors, capacitors, etc.) and likely will not have one tailored to the exact component. For instance, the 113A Symbol in the above schematic (rightmost square) had to be drawn from scratch, taking into account the number of pins and labels for each. KiCAD has a built in tool for drawing new components.

2. Laying out the PCB

The next step is to actually lay out how the circuit will look. The difference between this and designing the schematic is that, while the schematic tells KiCAD what components should be connected to which other components, the PCB editor actually determines how the board will be physically laid out.

To illustrate, the following image is what the PCB looks like when one first loads their schematic into the PCB editor:

There are white lines which denote connections between components - this information is received from the schematic (specifically the Netlist file, which is generated from the Schemati). The user must move these components around, and draw traces to complete a connection between components.

The completed PCB design is shown below, with the ground planes made transparent so that the traces (the thick green lines) are more clearly visible.

As with the schematic editor, there are representations of the circuit components. They are called "Footprints" here, and they are different from Symbols in that they must represent how the component will physically exist on the board. The Footprint will tell the KiCAD software (and, therefore, the manufacturer of the PCB) where the holes for the pins must be drilled and where to place copper for its connections. KiCAD has an editor to create custom Footprints.

The layout of each PCB was designed to match the original PCBs as closely as possible. Some things to keep in mind were the boards' dimensions, the positions of traces which connect to other PCB's (such as the two traces in the middle of the right edge, of the PCB pictured above), the thicknesses of the traces, the clearance of the traces (space between the trace and the ground plane), and the area of the ground plane.

3. Manufacturing

Once the PCB layout is complete, one must create "Gerber files" which are the industry standard format for designs of PCBs to be manufactured. KiCAD has the ability to create these from the PCB Layout editor, as well as to create "Drill files" which tell the manufacturer where holes should be drilled on the PCB.

The service used was called OSHpark. They have the advantage of decent prices, the ability to order a small number of PCBs at a time, and a simple and straightforward web interface. One simply must upload the Gerber files, and then place the order.

There are a few important things to note:

• PCB designs consist of "layers." For example, on a two-sided PCB, there will be a "Front Copper" layer and "Back Copper" layer, which have the traces meant for the front and back of the PCB, respectively. There is also the "Edge Cuts" layer which tells the manufacturer the perimeter of the board to cut around. There are "Silkscreen" layers which have useful symbols/text to be printed onto the board. There are also "soldermask" layers, explained below.
• A PCB typically has something called a "Soldermask" which is a layer of polymer that shields the copper traces underneath from oxidation. The Liverpool Q-meter design does not call for any soldermask, opting instead for gold plating. To match the original design, we ordered our PCBs with no soldermask layer. In CAD software, soldermask layers are typically "negative," meaning that rather than placing soldermask wherever the design is filled in, it actually places soldermask wherever the design is not filled in and keeps it away from filled zones. To order a PCB from OSHpark with no soldermask, one must go into KiCAD and place a filled zone over the entire board on both front and back soldermask layers.
• OSHpark offers gold-plated nickel over copper as a further way to protect the copper from oxidation. Even without soldermask, it is our expectation that this layer will protect the copper on our PCBs from oxidation.
  
  

List of Parts

Files

Liverpool Q-meter Guide: LiverpoolQMeterGuide.pdf

Q-meter design document: qmeter_blueprint.pdf

KiCAD files (Most recent update: 4/23/2019): qmeter Boards.zip (Note: pcb3_fabrication and pcb4_fabrication contain the PCB layouts which were ordered. pcb3 and pcb4 contain schematics which more closely resemble the design document, but which are less convenient to construct in KiCAD because of how the dual op-amps are separated).

Just the Gerber files (Most recent update: 4/23/2019): Gerber_Files_for_PCBs.zip