Safe RF power measurement chain for a 2 kW TWT amplifier — choose correct connection order Components available: (1) TWT amplifier (2 kW output), (2) Low-pass/High-pass filter, (3) Low-power 20 dB attenuator, (4) 40 dB directional coupler with matched load on the through line, (5) Power meter. What is the correct sequence from source to meter?
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A1, 4, 2, 3, 5
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B1, 3, 4, 2, 5
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C1, 2, 4, 3, 5
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D1, 4, 3, 2, 5
Answer
Correct Answer: 1, 2, 4, 3, 5
Explanation
Introduction / Context:Measuring the output power of a high-power traveling-wave tube (TWT) amplifier requires a chain that both protects low-power instruments and provides a representative sample of the fundamental signal. The right order prevents damage and ensures accuracy.
Given Data / Assumptions:
- TWT output power ≈ 2 kW (dangerous for small components).
- Available parts: filter, 40 dB coupler, 20 dB attenuator, power meter.
- Directional coupler is assumed to be high-power rated on the main line.
Concept / Approach:First, place the harmonic-conditioning filter directly after the TWT so the sampled signal represents the fundamental and to keep out-of-band energy from the measurement path. Next, use the high-power directional coupler on the main line; its coupled port extracts a small, known fraction (e.g., −40 dB) of the forward power. Then, because even the coupled power from 2 kW can still be substantial, insert a low-power 20 dB attenuator in the sample path before feeding the power meter. Finally, terminate the through line properly with the matched load or the system under test.
Step-by-Step Solution:
1) 1 → 2: TWT → Filter to clean harmonics at the source.2) 2 → 4: Filter → Directional coupler on the high-power through path.3) Coupled port → 3: Add 20 dB attenuator to further reduce level.4) 3 → 5: Feed the power meter safely.Verification / Alternative check:With 2 kW (≈ 63 dBm), a −40 dB coupler gives ~23 dBm at the coupled port; adding −20 dB attenuation gives ~3 dBm to the meter, well within many sensors’ ranges.
Why Other Options Are Wrong:
- A/D: Sampling before filtering yields a measurement polluted by harmonics and may stress the coupler differently.
- B: Placing a low-power attenuator directly after a 2 kW TWT risks immediate damage.
Common Pitfalls:Connecting the low-power attenuator on the main line; omitting the filter so the meter over-reads due to harmonics.
Final Answer:1, 2, 4, 3, 5