S-Parameter vs. Transmission/Reflection Test Sets
Filed under Test Sets
When configuring RF network analysis systems, engineers must choose between S-parameter test sets and transmission/reflection (T/R) test sets. The fundamental difference lies in their internal architecture and measurement workflow. S-parameter test sets contain internal coaxial transfer switches, allowing for automatic, bidirectional measurement of all four S-parameters (S11, S21, S12, and S22) with a single connection. Conversely, transmission/reflection test sets are typically passive instruments that require the operator to physically reverse the device under test (DUT) to perform reverse-path measurements. This guide compares their capabilities, calibration requirements, and operational configurations using industry-standard hardware examples.
- S-Parameter Path Control
- Automatic internal switching
- T/R Path Control
- Manual device reversal required
- S-Parameter Power
- Active (powered by host network analyzer)
- T/R Power
- Passive (no power supply required)
Bidirectional Switching vs. Manual Reversal
An S-parameter test set, such as the 85046A or 85047A, uses internal switching to direct the RF signal to either Port 1 or Port 2. This allows complete two-port calibration and error-corrected measurements in both directions without disconnecting the device. Active port LEDs on the 85047A front panel indicate which port is currently selected.
In contrast, a transmission/reflection test set like the 85044A or the 87512A does not contain internal RF switches to reverse the signal path. While they can perform simultaneous measurement of forward reflection and transmission characteristics, measuring the reverse characteristics requires physically reversing the device under test.
Power and Control Integration
Because S-parameter test sets require active switching and often include programmable features, they are active systems. For example, both the 85046A and 85047A draw their operating power directly from a compatible host network analyzer via a specialized interconnect cable. These models also include DC bias capabilities; the 85046A features rear-panel BNC inputs supporting up to 30 V DC bias, while the 85047A includes standard internal bias tees.
Transmission/reflection test sets, such as the 85044A and 87512A, are passive instruments requiring no external power supply. While simpler and lighter (the 87512A weighs 1.5 kg and the 85044A weighs 7.3 kg), they do not support integrated DC biasing or programmable step attenuation.
Frequency Range and Interface Options
The choice between these test sets also dictates the frequency coverage and connector types. The S-parameter 85046A and the T/R 85044A both cover 300 kHz to 3 GHz and use precision 7 mm test port connectors, which optimize directivity (the 85044A achieves directivity of 40 dB up to 1.3 GHz).
For higher frequency applications up to 6 GHz, the 85047A provides a standard range of 300 kHz to 3 GHz, extending to 6 GHz with its integrated frequency doubler. For low-frequency and general-purpose RF applications, the passive 87512A test kit operates from DC to 2 GHz using Type-N female connectors, introducing a nominal insertion loss of 10 dB across its signal paths.
Example instruments
Frequently asked questions
- Do transmission/reflection test sets require an external power supply?
- No. Transmission/reflection test sets like the 85044A and 87512A are passive devices and do not require any external power supply.
- How do S-parameter test sets apply DC bias to a device under test?
- S-parameter test sets feature internal routing for DC bias. The 85046A provides two rear-panel BNC female ports supporting up to 30 V DC bias, while the 85047A includes internal bias tees as a standard feature.