Casual Microstrip Design 7
I have decided to continue this little impromptu series on microstrip design. This time I will attempt to build a bandpass filter (BPF) for the GPS1 band, at 1575 MHz. Using a little R2 program, I calculated the normalized values for a coupled line BPF with a degree of filtration (N) of three, a ripple factor of 0.0436 dB. I will use standard single-sided FR4 board, with a thickness of 1.57 mm, with the dielectric constant, \(\varepsilon_r\) of 4.5. Using a system impedance of 50\(\Omega\), I get these values,
The admittance inverter constants are an intermediate step to determine the impedance values for each pair of coupled lines. See this post for more details.
The values were adjusted a bit to bring the real values to the input needed. The ‘cl!’ changes the values to compensate for various effects, and needs to be brought back to the desired inputs.
After everything is optimized, the diagram looks like this. This particular layout size is 200 mm, which is a bit larger than I like to experiment with, so I may see if I can trim the ends of the diagram to make it fit on a 150 mm board. After printing the diagram, I will see if I can duplicate it using copper tape. Placing a strip of copper foil, then taping the printout on top worked okay. Using a sharp knife, I then traced the diagram, cutting through the foil. The final step was peeling the excess off.
Well, duplicating the circuit was fairly easy, however, getting the ends trimmed to account for dispersion was a bit harder. The value I wanted to remove from each end was about 0.18 mm. Controlling the knife was a bit more difficult, and I ended up cutting off more than I intended.
As can be readily seen, I trimmed a bit too much, so the circuit tuned ad 1590 MHz instead of the desired 1575 MHz. That was the operator, not the circuit. Also the edges came out a bit ragged, cutting the copper foil. Overall, however, the concept is realizable for lower frequencies. Smaller sizes and higher frequencies would be harder to cut with accuracy. The return loss was a bit higher than it should be, probably because of the ragged edges. A sharper knife would have been better.
But, we still praise God for everything!
References
Footnotes
Global Positioning System.↩︎