Uniquel Method Of DIY High Frequency Circuit Board

high frequency pcb

 

Don't get me wrong - I really like printed circuit boards (PCB). Printed circuit boards are definitely essential for mass-produced products. Even amateurs can make almost perfect, repeatable circuits with a little practice. Moreover, printed circuit boards with good ground planes are important for high frequency circuits operating at frequencies exceeding a few megahertz. The ground plane is a large area of copper layer that acts as a low inductance loop between the components in the circuit and the power supply. It prevents parasitic capacitance from smearing high frequency signals to form noise. If you don't have a ground plane, don't expect high-frequency circuits made from breadboards to work well, even if they are working properly.

 

However, there is a disadvantage to using a printed circuit board to make a circuit faster than the speed and ease with which a circuit board is used to make a circuit in factory. You can quickly make your own printed circuit board - as long as you don't mind the mess and the stains on your clothes, and you are willing to drill yourself. You can also send your printed circuit board layout to the producer and let them make it for you, but it is convenient and still is low cost, Only need extra shipping time.

 

So I started thinking about whether there is a viable alternative that would also allow producers to quickly create high-frequency boards that are easy to detect and modify. In this article, I will only show one of the key concepts; It should be said that there is no original content here: I just used some processes that have been forgotten for decades. I didn't expect them to be very effective in the era of working frequencies up to gigahertz and surface mount components.

 

In general, this approach first requires a copper foil untreated standard board, typically an FR-4 resin board. Instead of etching the coils, the circuit are used to connect the components, leaving a large ground plane. I made a "comb generator circuit" as a demonstration.

 

The comb generator circuit produces a set of harmonics with a wide frequency range and sharp boundaries. I have done this frequency up to 1 GHz and is a very useful module in microwave systems. The heart of the generator is a 74HC00 integrated circuit with four NAND logic gates. The signal generated by the 25 MHz surface mount generator, after two series of NAND gates, produces two slightly delayed square wave signals. These signals enter the last NAND gate, producing a narrow pulse that forms a harmonic spectrum.

pcb schematics

DIY Pcb

In order to make the circuit, I divided the copper layer into two pieces, and I intended to make the smaller area at the top serve as a 5 volt power rail, and the rest to form a ground plane.

 

In order to isolate the two areas, I stripped out three elongated rectangular copper foils as the boundary of the power rail. First mark the parallel lines with a scriber; then, align the steel ruler against the parallel line and use a cutter to cut the copper along the steel ruler (requiring considerable effort, usually several times to cut through). Finally, the copper foil between the parallel wires is heated by a soldering iron, and each copper piece is peeled off with a tweezers.

 

The PCB is usually a single ground plane without a through hole, so how do you install an integrated circuit? Bend the ground pin of the IC back to the surface, then place the ground pin in the proper position and solder it to the ground plane. Bend the other pins so they are parallel to the board and solder the leads directly to these pins. Since the integrated circuit looks like a bug that stretches out its legs, this method is sometimes called the "dead worm" method. The advantage of this method is that it is easier to solder the surface mount components than to use a conventional printed circuit board to make it easier to access the connection points. In addition, there is an area on the ground plane to facilitate connection of the power conditioner heat sink of the comb generator.

 

Continuously cutting and stripping the strip from the copper layer, an isolated area can be formed in the middle of the board as a connection point between the surface mount or via plug components. The capacitance between this isolated area and the ground is very small.

 

DIY rogers pcb

 

Another advantage of this type of mounting is that it is convenient to check if the high frequency circuit is actually operating as designed. A spectrum analyzer with a 500 ohm resistive probe (such as the Tektronix P6056) is ideal for this type of circuit by simply grounding the shield of the probe near the test circuit node. When the probe's ground shield is connected to the upper spring pin of the board ground shield, I can ground it regardless of which pin the probe is next to. (If you can't find a P6056 or similar probe, you can do it yourself: connect a 450 ohm resistor in series with a 50 ohm coaxial cable, but remember to use a 50 ohm terminal on the analyzer side).

 

The boards produced by these methods are not always very aesthetically pleasing, but I have achieved good results when using these techniques to make microwave circuits. If you’d like it, you can just try yourself. It is a good hobby.