Instruments such as power meters and power sensors serve many applications in high RF-power measurement. Mishandling or applying high RF power to the power meter and/or power sensor will cause these instruments to malfunction and eventually damage them. As a result, there could be system downtime, reduced measurement quality, and high repair costs. In this article, we will look at practices to protect power meters and power sensors from faults that will prolong the life span of these instruments.
Table Of Contents
- The Basic Scenario
- Practices #1: Avoid Overpower
- Practices #2: Avoid Overvoltage
- Practices #3: Adhere To Warnings And Specifications
- Practices #4: Protect RF Connectors And Adapters
- Practices #5: Ensure Proper Grounding
- Practices #6: Take ESD Precautions
- Practices #7: Check For Temperature And Humidity
There is a basic method for measuring high-frequency power using a power meter and power sensor (Fig. 1). The power sensor converts high-frequency power to a dc or low-frequency signal that the power meter can measure and relate to an RF power level. The meter displays the detected signal as a power value in dBm or watts.
RF power meters and sensors are very reliable and rugged if handled correctly. However, service centers receive a significant number of damaged power sensors every year due to carelessness or users’ ignorance.
Mishandling may degrade measurement quality. Depending on the degree of degradation, it may result in the user having to retake many measurements. It may also lead to incorrect measurements, which may cause a product recall.
Furthermore, mishandled equipment may require costly repairs. Repair turnaround times in service centers are typically three weeks or more, leading to system downtime as well as a schedule delay.
Maximum measurable power for a given power sensor can vary from low power (say, –60 dBm) to high power (say, +40 dBm). The maximum power that a power sensor can handle typically will be a little higher than the specified value, giving users a safety margin. Yet you would be surprised to learn that overpowering causes well over half of the sensor failures seen by service centers. Inspection will reveal a fried component in the bulkhead thin-film circuit.
The bulkhead happens to be the most expensive module in a power sensor, costing around 80% of the price of a brand new unit of the same model. The bulkhead is the metal part of the power sensor that includes the connector (Fig. 2). Inside, there is a plastic bead supporting the highly precise, delicate structure of the RF input connector center pin; a cartridge with a thin-film circuit populated by termination and attenuator resistors; and a sensing element. The bulkhead’s function is to convert the RF input to a low dc voltage that varies in proportion with the input power.
To prevent blowing the bulkhead:
- Know the approximate signal level you’re measuring
- Make sure the measured power is well within the dynamic range of the power sensor
- Use an RF limiter to attenuate power that exceeds the power sensor’s limits.
One can destroy the bulkhead of a power sensor if the direct current voltage (DCV) in your signal exceeds the maximum handling voltage of your power sensor. A typical dc-blocked power sensor can handle up to 20 DCV. Also, dc-coupled sensors have much lower handling voltages, usually 5 V.
DC-Blocked Versus DC-Coupled
A dc-blocked sensor has an in-series capacitor placed in front of the sensing element to suppress low-frequency signals that may cause damage or affect measurement accuracy (Fig. 3). A dc-blocked sensor is good for accurate power measurements in applications where dc bias shares the same path with a RF signal. For a dc-blocked sensor, the lower the frequency, the larger the capacitance needs to be. This poses a design challenge in achieving a good match because of the discontinuity in transition from a tiny 50-Ω coaxial transmission line to a much larger capacitance structure.
Because they do not contain a blocking capacitor, dc-coupled sensors have better voltage standing-wave ratio (VSWR) performance and can measure down to really low frequencies. Sensors that are dc-coupled are good for metrology applications in which the sensor is calibrated by measuring the voltage from it with a voltmeter for a direct comparison.
Every power sensor will have a power sensor label printed on its body (Fig. 4). The sensor will have a minimum/maximum power measurement range, known as the dynamic range. The same goes for frequency range. If the power sensor can also measure peak power, the label will specify a video bandwidth. In fine print will be some warnings and reminders. Never exceed the values provided in the specifications guide or as indicated by the yellow warning labels on the power sensor and meter.
With care and proper technique, it is possible to minimize the degradation of connector performance, especially for the more expensive measurement instrument connectors. A damaged or out-of-specification connector can destroy a good connector attached to it even on the first connection. Therefore, any damaged connector or adaptor should be disposed of or sent out for repair immediately. A bad connector can result in transmission and reflection losses that may change when the connection is removed and reconnected or may be intermittent, resulting in measurement error and repeatability problems.
When making a connection, align the center axis of both connectors. Then push the connectors straight together so the male pin slides smoothly into the female fingers (Fig. 5). The internal surfaces of the female center conductor make electrical contact with the external surface of the male pin and physical contact with the mating plane. To tighten the connection, rotate the connector nut of the RF input connector and not the device (Fig. 6).
When breaking a connection, hold the connector body to prevent imposition of any rocking or bending force on the connectors. If necessary, loosen the connector nut with an open-end wrench (Fig. 7).
There are several ways to damage the female fingers on various types of connectors (Fig. 8). Causes of such damages include rotation of the male center conductor and misalignment during connection and disconnection.
Though many RF/microwave connectors are designed as rugged mechanical interfaces, users must be aware to take care in applying torque to the connector nut. This is crucial to long life and full signal performance (see the table). Too much torque will deform the connector parts and probably cause a mismatch problem, while too little torque will yield a lousy connection with poor VSWR.
The correct torque also improves measurement repeatability. When using a Type-N torque wrench, hold it lightly at the handle end when tightening (Fig. 9). Stop tightening when you reach the breaking point of the wrench handle.
An adapter can protect the power sensor RF input connector (Fig. 10). The most obvious reason for using an adapter is if the device under test (DUT) does not use the same connector family as the power sensor. However, even if the DUT has the same connector family, an instrument-grade adapter for protection can prevent damage and costly repairs.
Connectors have very precise mechanical tolerances. Thus, visual inspection is an important safeguard. Minor defects, damage, and dirt can significantly degrade repeatability and accuracy. Before each connection, visually inspect for obvious defects such as badly worn plating, deep scratches, or dents. Gold-plated connectors will be more susceptible to mechanical damage because gold is soft.
Damaged threads will deposit metal flakes into other parts of the connector, causing severe damage. Discard or send for repair any connector with an obvious defect. For dirt or stubborn contaminants that will not yield to compressed air, use a lint-free swab moistened with isopropyl alcohol.
Always use the three-prong ac power cord supplied with the power meter to ensure proper grounding. The Earth pin connects exposed metal on the equipment to ground. When the instrument is working properly, there will be no current in the equipment ground. If a malfunction occurs, the equipment ground provides a path for the current to flow. This protects the instrument and the operator. Check ac power quality and polarity. Typical ac voltage required is 100, 120, or 220 V ±10% or 240 V +5%/–10%.
Static electricity can build up on your body and can easily damage sensitive internal circuit elements when discharged. Even static discharges too small to perceive can cause permanent damage. You’re bound to find an electrostatic discharge (ESD) symbol on a power sensor label because power sensors are extremely static-sensitive devices (Fig. 11). An electrostatic discharge to the center pin of the connector can render the power sensor inoperative.
- Always replace the connector cap on an unused power sensor.
- Conduct power measurement at a static-free workstation whenever possible.
- Any cleaning or inspection of the power sensor should only be done at a static-free workstation.
- Avoid bringing sources of static electricity within 1 meter of your static-safe workbench.
All power meters and sensors must be properly stored in a clean and dry environment. Exposure to extremes of humidity, temperature, or dust degrades performance. It is critical to ensure proper ventilation in the equipment rack. Inspect and clean cooling vents and fans on a regular basis.
Refer to the datasheet for the recommended operating and storage environmental conditions. Optimal operating temperature is 23°C ±5°C. Always keep instrument ambient temperature at <30°C. Using the equipment at ambient temperatures outside of this range may produce measurement errors. The internal electronics of the measuring instrument may be destroyed at extreme temperatures.
The recommended practices outlined in this article will extend the service life of power meters and sensors while reducing downtime and maintenance costs. Maintaining the instruments in good condition will ensure accurate power measurement in your applications, reducing the test time and maximizing the overall throughput.
- “Choosing the Right Power Meter and Sensor”
- “4 Steps for Making Better Power Measurements”
- “Coaxial Systems: Principles Of Microwave Connector Care (AN 326)”
- “Fundamentals of RF and Microwave Power Measurements (AN 1449)”
- “Agilent Power Meters and Sensors”
- “Understanding DC-coupled and DC-blocking Power Sensors and How Your Choice of Sensor Would Impact Measurement Accuracy”