Engineering Problem:
A differential PCB channel may be electrically suitable for high-frequency measurement but still be difficult to probe because the available landing pattern does not match the probe configuration. The first decision is therefore not “Which probe has the higher frequency rating?” but “Which contact geometry matches the DUT?”
For differential PCB and PCBA measurements, SS and GSSG RF probes address different landing structures. Neither configuration is a universal upgrade or replacement for the other.
G-S-S-G pads designed as a controlled launch: start with GSSG.
Two accessible signal pads without practical local ground landing pads: consider SS.
Asymmetric, non-standard, or poorly documented geometry: review the pad layout and return-path structure before selecting either probe.
For a detailed discussion of signal-to-signal probing where local ground pads are unavailable, see SS RF Probes for PCB Testing Without Local Ground Pads up to 67 GHz. For the full platform, view Vinstronics RF probe solutions.
SS and GSSG Landing Patterns
S
S
S
G
| Selection Factor | SS Probe | GSSG Probe |
|---|---|---|
| Landing pattern | S-S | G-S-S-G |
| Dedicated local ground tips | No | Yes |
| Best starting point | Two accessible differential signal pads without suitable nearby ground landing pads | A differential launch intentionally designed with local ground contacts |
| Primary selection driver | Pad access and suitability of direct signal-to-signal probing | Correlation with the designed ground-referenced transition |
| Universal replacement for the other format | No | No |
When a GSSG Probe Is the Better Match
A GSSG probe uses a Ground-Signal-Signal-Ground contact arrangement. The two signal tips contact the positive and negative conductors of the differential pair, while the outer tips contact dedicated local ground pads.
When the PCB coupon, package substrate, or validation structure was intentionally designed with a G-S-S-G launch, those ground pads are part of the transition being characterized. Leaving them unused and landing only on the two signal pads changes the interface under test.
Start with GSSG When
- The DUT landing pads are arranged as G-S-S-G
- The local ground pads are part of the simulated or designed launch
- The test must correlate with an existing GSSG coupon or fixture
- The ground spacing and signal pitch can be matched by the probe
- Mode conversion or imbalance is part of the measurement objective
The presence of ground tips does not by itself guarantee an accurate result. Ground-pad dimensions, via placement, antipad geometry, planarity, touchdown repeatability, and calibration still influence the measured S-parameters.
For available Vinstronics configurations, see the GSSG and GSGSG differential RF probe platform.
When an SS Probe Is the Better Match
An SS probe uses two Signal-Signal contacts and does not require dedicated ground needles at the landing area. It is intended for suitable differential structures where two signal pads are accessible but practical local ground landing pads are limited or unavailable.
This can avoid redesigning the PCB solely to create a four-pad probing interface or adding temporary ground connections that change the original test condition.
Consider SS When
- The available landing pattern contains two differential signal pads
- Suitable nearby ground pads cannot be contacted safely and repeatedly
- Adding GSSG pads would require a PCB redesign
- The DUT is intended to operate as a differential structure
- The primary objective is board-level differential S-parameter measurement
- The probe pitch can be configured to match the signal-pad spacing
Important RF Boundary
An SS probe removes the need for dedicated local ground needles at the contact point. It does not remove the electromagnetic return path of the differential channel. Reference planes, vias, asymmetry, nearby conductors, and common-mode behavior can still affect the result.
Vinstronics currently offers the VST-D40G-SS-XXX DC–40 GHz SS differential probe and the VST-D67G-SS-XXX DC–67 GHz SS differential probe. Both use a 100 Ω differential configuration, a 120 µm tip size, and application-specific pitch configurations within the supported 350–3000 µm range.
When SS Should Not Be the Default Choice
SS probing is useful for a specific landing problem. It should not be selected merely because two tips appear mechanically simpler than four.
| SS Is a Reasonable Starting Point | SS Should Not Be Assumed |
|---|---|
| The landing area provides two accessible differential signal pads | The DUT already has a controlled G-S-S-G launch |
| No practical local ground pads are available | The path is single-ended or explicitly ground-referenced |
| The objective is differential S-parameter measurement | The choice is based only on probe availability in the laboratory |
| The DUT return structure and calibration plan have been reviewed | The pad geometry, reference structure, or measurement objective is unknown |
Final Engineering Checks Before Selection
After the landing pattern has narrowed the choice, confirm the following items before ordering the probe.
1. Pad Geometry
- Signal-pad center-to-center pitch
- Individual pad width, length, and solder-mask opening
- Ground-pad spacing for GSSG layouts
- Probe approach direction and nearby component clearance
2. Measurement Objective
- Differential insertion loss and return loss
- Mode conversion or common-mode analysis
- Correlation with a coupon, fixture, or simulation model
- R&D characterization or repeated automated testing
3. Complete Measurement Chain
VNA → test cables → adapters → probe → PCB landing transition → DUT
The usable bandwidth is limited by the complete path. A 67 GHz probe cannot compensate for a cable, adapter, calibration method, or DUT launch that becomes the limiting element at a lower frequency.
4. Calibration and De-Embedding
Calibration establishes a corrected measurement reference plane. De-embedding mathematically removes a characterized probe, fixture, or transition network. Both SS and GSSG measurements require a defined calibration and verification plan; a stored de-embedding file is not a substitute for VNA calibration.
Vinstronics SS Probe Options
| Model Series | Frequency Range | RF Interface | Tip / Pitch |
|---|---|---|---|
| VST-D40G-SS-XXX | DC–40 GHz | 2.92 mm Female | 120 µm tip; 350–3000 µm configurable pitch |
| VST-D67G-SS-XXX | DC–67 GHz | 1.85 mm Female | 120 µm tip; 350–3000 µm configurable pitch |
The pitch range describes the available product configurations. An individual probe is built for the specified pad pitch; it is not continuously adjustable across the entire range during use.
What to Send for Probe Selection Review
A target frequency alone is not enough to select a differential RF probe. Send the following information whenever possible:
- PCB pad drawing, Gerber excerpt, or clear close-up image
- Pad arrangement: S-S or G-S-S-G
- Signal pitch and individual pad dimensions
- Target frequency range and differential impedance
- Relevant stack-up or reference-plane information
- VNA, cable, and connector configuration
- Calibration or de-embedding approach
- Probe station, fixture, or automated test setup
Conclusion
Choose GSSG when the DUT provides a designed G-S-S-G launch and the local ground pads are part of the intended transition. Consider SS when the available landing area contains two differential signal pads and suitable local ground contacts are not available.
The probe should follow the DUT geometry—not the other way around. After the contact pattern is selected, confirm pitch, pad area, return-path structure, required bandwidth, calibration reference plane, and mechanical access.
Start with the landing pattern. Confirm the electrical transition. Then select frequency, pitch, interface, and calibration workflow.
FAQ
Can an SS probe perform a 100 Ω differential measurement?
Yes. The Vinstronics VST-D40G-SS-XXX and VST-D67G-SS-XXX are designed as 100 Ω differential probes. Measurement accuracy still depends on the DUT launch, calibration, cables, and the rest of the test chain.
Does an SS probe eliminate the need for ground?
No. It eliminates dedicated local ground needles at the landing area. The differential channel still has electromagnetic return paths through its paired conductor, PCB reference structures, and surrounding geometry.
Is GSSG always better at higher frequencies?
No. The usable bandwidth depends on the complete probe transition, pitch, connector interface, DUT geometry, calibration, and test setup—not on the configuration letters alone.
Can one SS probe cover every pitch from 350 to 3000 µm?
No. The platform supports configurations within that range, but each probe is built for the specified pad pitch.
What information is most important for selection?
The pad arrangement, signal pitch, individual pad dimensions, target frequency, reference structure, VNA setup, and calibration plan are more useful than the frequency requirement alone.
Need Help Choosing SS or GSSG?
Send your PCB pad layout, pitch, target frequency, VNA configuration, and calibration approach. Vinstronics engineers can review the landing geometry and recommend a suitable probe configuration.
