Remote Testing with RF Power Sensors

Introduction

Performing power measurements of RF devices and systems is an essential part of ensuring accurate, repeatable characterization. A variety of instruments are capable of measuring RF power, including power meters, vector network analyzers, spectrum analyzers, and power sensors. In many cases, the Device Under Test (DUT) and the measurement instrument will be some distance apart due to physical constraints, electrical isolation requirements, a hostile environment or other possible restrictions. When this is the case, awareness of remote testing and its implications comes into play.

Measurement at a Distance

For the purpose of illustrating some of the key topics of remote testing, this paper will focus on power sensors. Due to their small size, high accuracy, portability, broad bandwidth and excellent price/performance ratio, power sensors provide a quality use case for discussing remote testing.

A modern RF power sensor, such as the Mini-Circuits PWR-18RMS-RC, has the aforementioned characteristics as well as a USB connection for both power to the sensor and communications with the host PC. USB connectivity is widely implemented, simple, and works well on the benchtop with a maximum distance of 5 meters (USB 2.0) with standard passive cables.

For greater distances, USB may not be practical or reliable. For example, a manufacturing floor may use power sensors across the facility. In this case, an ethernet (LAN) connection is the better choice. Mini-Circuits’ power sensors are equipped with ethernet connectivity to support greater distances – up to 100 meters in theory, but easily extended with network switches. Multiple Mini-Circuits’ power sensors can also be supported from a single PC. An example GUI is shown in Figure 1.

Figure 1: Interactive GUI utilized with Mini-Circuits’ power sensors

Measurement Automation

In addition to a GUI, Mini-Circuits’ power sensors feature a flexible API (Application Programming Interface), which opens up the power sensor’s full feature-set for use in a custom control program written in any modern programming language. This can be particularly relevant for remote monitoring applications where power measurements must continue unaided, over long time periods, or outside of normal working hours.  The Python script shown in Figure 2 is a basic example of how to connect to a remote power sensor using its ethernet connection, periodically read the power, and generate an alert if a measurement falls outside of a set range.

from urllib.request import urlopen
from urllib.error import URLError, HTTPError
import sys

#######################################
# Define a function to send an HTTP command and get the result
#######################################
def Get_HTTP_Result(CmdToSend):
    
    # Specify the IP address of the power sensor
    CmdToSend = "http://192.168.9.61/:" + CmdToSend
    
    # Send the HTTP command and try to read the result
    try:
        HTTP_Result = urlopen(CmdToSend, timeout=2)
        PTE_Return = HTTP_Result.read()

        # The sensor returns "-99..." for unrecognized commands
        if PTE_Return[0:3] == "-99":
             print ("Error, command not found:", CmdToSend)
             PTE_Return = "PWR: Invalid Command!"

        # Catch an exception
        except URLError as e:
            sys.exit('URLError:' + str(e.reason))
        except HTTPError as e:
            sys.exit('HTTPError:' + str(e.code))
        except Exception as e:
            sys.exit('Exception:' + str(e))
        else:   # Command was successful so return the response
            PTE_Return = str(PTE_Return, 'utf-8')

        return PTE_Return

############################################
# Start the automated power measurements
############################################

test_limit = -20    # Set the test spec

# Print the model name and serial number
sn = Get_HTTP_Result(":SN?")
mn = Get_HTTP_Result(":MN?")
print (mn, sn)

#Set & confirm the measurement frequency
Get_HTTP_Result("FREQ:2000")   # 2000 MHz
print ("Frequency (MHz):", Get_HTTP_Result("FREQ?"))

# Loop through the following steps indefinitely
    while true:
    # Measure power
    power = int(Get_HTTP_Result("PWR?"))
    print ("Power (dBm):", str(power))

    if power < test_limit:
        # Power below limit so send an alert
        print ("ALERT - power out of spec")

Figure 2: Python script example for controlling a Mini-Circuits’ power sensor

Additional Features

Often, a Mini-Circuits’ power sensor such as that shown in Figure 3 includes a small display on the outside of its housing, which allows a user to view RF power, frequency, sample time and other parameters locally. Additionally, all Mini-Circuits’ power sensors are equipped with a high-grade RF connector and separate connections for Trigger Out and Trigger In. These additional features enhance a user’s ability to configure the Power Sensor at its location or remotely via the GUI.

Figure 3: Mini-Circuits’ power sensor.

Summary

Remote testing is often a requirement in modern test systems. Whether the distance that needs to be covered is a manufacturing floor, a temperature-controlled chamber, or a different environment altogether, Mini-Circuits offers a range of capabilities and features that provide users with convenient and customizable options. While this paper focuses on power sensors, many of these same features are found in a variety of Mini-Circuits’ products, such as the LAN capability found on Mini-Circuits’ line of programmable attenuators. As test demands grow, Mini-Circuits will continue to develop innovative solutions to meet customers’ unique needs and evolving industry standards.

Courtesy of Mini-Circuits