Application Notes: Tuning Screws
Written by Ernest Werbel on May 16, 2023
Tuning screws are a good practice in RF/MW circuits to add or remove capacitance or inductance.
Due to imperfections in circuit fabrication, such as solder joints, connections and over/under etched traces, there may exist parasitic effects on circuit performance. The impedance must be tightly controlled.
To alleviate this issue, tuning screws are positioned in strategic locations using both RF experience and intuition.
Here is where things get interesting. The act of positioning a set screw near or far relative to a section of RF circuit has the effect of increasing or decreasing capacitance or inductance.
Additionally, the implementation of lumped components or trimmer capacitors is not practical as the frequency is too high for them to be used effectively.
The tuning screw may be adjusted while the DUT is connected to the test set, and the performance is “dialed in.” This has several advantages.
Firstly, the technician can observe and tune performance in real-time. This allows precise phase trimming at 18 GHz as well as optimal return loss.
Second, the DUT does not need to be disassembled to access tuning points. As many technicians can attest, every time you open the DUT to fix one thing, something else changes.
Of course, not requiring the DUT to be disassembled saves an incredible amount of time. A tuning job that could easily take hours is accomplished within minutes.
Post-testing, the tuning screws are locked in place using automotive-grade enamel, to prevent further accidental movement that would de-tune the circuit.