Accurate High RF Power Measurement
By Jin Yu and Henry Szczurko, Berkeley Varitronics Systems, Inc.

Abstract
Accurate measurement of base station transmission power is necessary to provide good coverage to the desired area and not to interfere with other cells using the same frequency. The accuracy requirement of transmission power level is typically ±10%, which is equal to ±0.4 dB. Usually a base station transmits signals at power levels about 40 Watts. However, both thermal power meters and spectrum analyzers cannot handle high power. The use of test equipment with the addition of couplers and attenuators will easily introduce other errors to the measurement and thus reduce the accuracy. The Yellow Frog 2™ is the latest power meter from Berkeley Varitronics Systems which can be connected directly to base station and measure up to 50 dBm (100 Watts); the accuracy is about ±0.25 dB.

Accurate Base Station Measurement
Measuring base station transmission power is very important when installing and monitoring wireless networks. Usually a survey study is implemented to validate and predict RF coverage for the cell. The transmission power should be constant during a site survey. When RF coverage is validated and the base stations are installed, the output power is verified by the system engineers. The base station transmission power is expected to cover the area inside the cell and not to interfere with other cells. A lower transmission power will cause poor coverage at the edge of the cell and a higher power will transmit the signals to other cells and cause interference if same frequency is used at those cells. Therefore, the accurate control of base station transmitter power has an impact on the overall system performance. The transmission power levels typically have a total accuracy requirement of ±10%, which is about ±0.4 dB [1].

Thermal Power Meters and Other Test Equipment
A traditional base station has a transmission power level about 40 Watts (43 dBm). Most of the thermal meters on the market can only measure up to 20 dBm. Therefore, a thermal meter must be used with couplers or attenuators to measure the base station transmission power. Considering the losses and variations of couplings, this setup can introduce considerable measurement errors if not properly calibrated. Another drawback of a thermal power meter is that the power heads are easily damaged after being dropped from the top of an equipment rack [1].

A spectrum analyzer can also be used to measure the power. However, there are several things which make it difficult to use spectrum analyzers to measure base station transmission power. First, spectrum analyzers are geneally more expensive than power meters. Secondly, the absolute power measurement accuracy of a spectrum analyzer is typically about ±1dB to 1.5dB (a ±41% error) although, that being said, spectrum analyzers have excellent relative power measurement capabilities. Power-handling capacity is another issue for spectrum analyzers. Many spectrum analyzers cannot handle several watts of power. It means the combined use with coupler and attenuator will further reduce the measurement accuracy.

BVS Yellow Frog 2 Power Meter
Yellow Frog 2 is Berkeley’s latest handheld power meter product. There are two models for this product. The first model is made for frequencies ranging from 700 MHz to 2.7 GHz and for power ranging from 0 dBm (0.001 Watts) to 50 dBm (100 Watts). The second model extends the frequency range of the first model. It can also measure frequencies from 150 MHz to 700 MHz in which the power can be measured from 10 dBm to 50 dBm. The Yellow Frog 2 has a resolution of ±0.1 dB, an accuracy of ±0.5 dB, and a maximum input voltage standing wave ratio (VSWR) of 1.20. In a practical test or measurement, the accuracy for most of the frequencies is within ±0.25 dB. The temperature range of compensation is from 0 to 50ºC. The specifications of the Yellow Frog 2 are shown in Table 1.

The Yellow Frog 2 can operate continuously for over 8 hours using an internal Lithium Polymer battery. A discharged battery can be fully charged in less than 3 hours using the built-in fast charger and external 12 VDC power supply. The portable instrument weighs less than 2 pounds and dimensionally fits in the palm of your hand (3"H × 3"W × 5"L). The unit features a large LCD display and USB port which also provides power when connected to any PC running Windows XP or Vista operating systems. While the Yellow Frog 2 includes PC controller/monitoring software for setting frequencies and recording power out, it does not require a PC to operate. Additional accessories include N-type adapters, 6-ft. long USB cable, and optional 20W or 40 W 50Ω load. Figure 1 shows a Yellow Frog 2 and the application software.

The Yellow Frog 2 is able to measure RF power from CW as well as more complex signals, such as GSM, CDMA, and WiMAX over the 50 dB dynamic range. Using one or two Yellow Frog 2 units, engineers can easily measure VSWR at the base of the antenna or load. It can be connected directly to Gator™, Dragon™, Lizard™ or any transmitter with a standard type “N” connector for instant verification of wired power levels in dBm or Watts. The direct connection reduces the errors caused by the combined use with couplers and attenuators.

Power Measurement
There are two parameters that determine the exact transmission power to an antenna. One is the input power to the antenna; another is the VSWR, which represents how much power is reflected by the antenna. How to measure these two parameters is addressed in the following text.

CW Signal Measurement
Typically, a site survey study is used to validate RF coverage. Usually a CW transmitter is set at the position of base station and then CW receivers are moved around to measure and record received power levels. The measurement data will be mapped to the survey area to validate and predict the RF coverage. Monitoring the transmission power level is extremely important at this stage because varying transmission power would cause erroneous coverage validation. The Yellow Frog 2 can also record the power levels by using the supplied application software. Berkeley Varitronics Systems (BVS) provides a complete package for drive study and propagation analysis, including Gator transmitter, Yellow Frog 2 power meter, Coyote modular receiver, Forecaster mapping software, and Sieve 40λ conversion software.

Figure 2a shows how to measure the transmission power. In this instance, the Yellow Frog 2 is connected to the antenna directly. This reduces the uncertainties in measuring the final transmission power from the antenna. The Yellow Frog 2 can show both dBm and Watts on the screen and updates 4 times per second.

Modulated Signal Measurement
When a base station transmits modulated signals, the modulation factor or crest factor of the waveforms should be known for the measurement. The Yellow Frog 2 power meter fundamentally responds to voltage and not to power. A direct consequence of this characteristic is that input signals of equal Root Mean Square (RMS) power but different crest factors will produce different results at the output. The effect of modulation is to shift the readings up or down. However, the shift is not affected when the modulation format is fixed. For example, comparing a sine wave and single CDMA channel of the same RMS power, the Yellow Frog 2 power meter will differ by the equivalent of 3.55 dB over the complete dynamic range of the device (the output for a CDMA input being lower). Table 2 shows the correction factors that should be applied to measure the RMS signal strength of various signal types. A sine wave input is used as a reference. To measure the RMS power of a square wave, for example, the shift 3.01 dB should be subtracted from the reading. Therefore, it is necessary to load the modulation factor into the unit to accurately measure the power level of modulated signals.

VSWR Measurement: Measurement with One Unit
In Figure 2, if the antenna is not matched well to the Yellow Frog 2 unit (50 Ω), the antenna will reflect the input power to the transmitter and the output power level will be lower than that shown on the screen of the unit. It is necessary to know the VSWR parameters to calculate the exact transmission power from the antenna. When only one Yellow Frog is at hand, we can do it in the following steps:

1. Set up the parameters in the unit, such as frequency, modulation factor and so on.

2. Connect the devices as shown in Figure 2a and read the data on the screen. Assume it is p1 and p1 = 30 dBm.

3. Reverse the Yellow Frog 2 unit and then read the data on the screen again. Assume it is p2 and p2 = 10 dBm.

4. Calculate the VSWR by using Equation 1. Note here p1 and p2 are in dBm and they have to be converted to Watts to do the calculation in Equation 1. In this example, VSWR = 1.22. The plot of VSWR versus p2/p1 is shown in Figure 3.

From Equation 1, we can express p1 in terms of p2 and VSWR,

Since Yellow Frog 2 can measure down to 0 dBm from 700 - 2700 MHz and 10 dBm from 150-700 MHz, the curves in Figure 4 show the minimum requirements of p1 to be able to measure VSWR for 0 dBm and 10 dBm.

Measurement with Two Units
If there are two Yellow Frog 2 units available, they can be used jointly to measure the VSWR. Connect these two units as shown in Figure 2b and then replace it with the unit in Figure 2a. Similarly, we have two readings from the two units. The reading of the first unit is p1, which represents the input power level. The reading of the unit connected to the antenna is p2, which is the reflection power from the antenna. The VSWR can be calculated using Equation 1.

Conclusion
Accurate measurement of base station transmission power is necessary to install and maintain wireless cellular networks. Yellow Frog 2 power meter, which is a new product from Berkeley Varitronics Systems, provides the solution to measure high RF transmission power (up to 100 Watts) with high accuracy.

Reference
[1] Bill Rau, Power Meter Parameters, http://telephonyonline.com/wireless/mag/
wireless_power_meterparameters/, Aug. 1998.

Berkeley Varitronics Systems
www.bvsystems.com
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