What are Slide Screw Tuners?
A Slide Screw Tuner is a type of impedance tuner that is used to tune the impedance of a DUT for non 50-Ω measurements or to introduce a controlled mismatch into an otherwise matched transmission line. A slide-screw tuner consists of a slab transmission line with passive probes that can be inserted and moved to create different impedance conditions.
The transmission line is usually matched to 50 ohms so when no probes are inserted it acts like a thru line with minimal losses. As the probe is lowered (Y axis in the Image below) perpendicularly to the slabline, the electric field of the signal passing through the transmission line gets disrupted. This disruption results in some of the signal getting reflected back to the DUT and consequently, the magnitude of reflection, or the VSWR increases. Now, if we move the probe horizontally along the transmission line (X axis in the image below), the distance between the probe and the DUT gets modified, thereby rotating the phase of the reflection. Therefore, by moving the probe down and along the transmission line (left or right), a slide screw tuner can present any set of impedances to the DUT. Slide Screw Tuners usually have a small digital display which shows the impedance of the tuner based on the position of the probes.
X-Axis - When the probe is lowered into the slabline
Y-Axis - When the probe is moved back forth along the slabline.
In some slide-screw tuners, there are two probes used to cover the entire frequency range. The probes used are typically, wide-band in nature, so the addition of another probe results in little or no change in the reflective properties as they are designed to operate over different frequency bands. The probes used are also of two different sizes, which increases the overall useful bandwidth of the tuner. The low frequency probe covers frequency range up to the crossover frequency and then the high frequency probe covers the frequency from the crossover frequency to the maximum frequency the tuner can support.
While the probe is moving along the slabline horizontally, amplitude losses should be minimal. This is required to achieve accurate tuning circles on a smith chart. Other factors that should be taken into account when selecting a Slide Screw Tuner are repeatability, i.e the impedance error when the probes are placed at the same positions again and again and the error when a certain position is meant to generate a certain impedance. All these points should be evaluated before making a selection.
Types of Slide Screw Tuners
Slide-screw tuners were initially built as manual devices where the user would need to adjust the position of the probes to tune the impedance. A digital display would display the impedance generated by moving the probes vertically and horizontally. Over time, companies also developed automatic versions, which could digitally adjust the positions of the probes to set a particular impedance. Most impedance tuners used today are automatic, so they can be versions of a slide screw tuner or use a different technology.
