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Real-time answers to technical questions

Frequently Asked Questions

1. How do you provide an accurate EIRP estimate despite the fact that only one side of DUT is measured?
Effective Isotropic Radiated Power (EIRP) is the power that must be supplied to an isotropic antenna to achieve the same field strength. EIRP is determined traditionally by identifying and measuring the peak radiated value on the surface of the "measurement sphere". Cell phones are, by design, deliberately highly directional into one hemisphere (driven by concerns about SAR readings and by interference in the receive mode). By measuring in that hemisphere, there will be no difficulty in identifying and precisely measuring the peak value. So even if you had other devices under test that were not so specifically dominant in one direction, you could still accurately estimate the EIRP value. What gets potentially compromised in scanner measurements for omni directional sources, is accurate far-field pattern. Radiated power (PRAD) is obtained by integrating power density (obtained from the NF to FF transform) over the hemisphere. The hemisphere is divided into 50x100 pieces. Integration is carried out by summing the power densities over the hemisphere. In the current implementation, the theta and phi increments are 1.8 and 3.6 deg. So in the worst case, the PRAD error is 3 dB. Both "sides" can be measured and the power sum taken, which will increase the precision. As a design tool, differential changes on the peak side (or the weak side in the case of SAR optimization) are probably all that are necessary for optimizing between options.
2. How fast is the full measurement cycle?
Each measurement including processing and display takes 1 second. RFxpert collects the data in 40 milliseconds and then the PC may require an additional 1 second to process and display the data. The overall measurement time can be reduced to 300 milliseconds for a simple go/no go scan result (e.g. measuring EIRP against a spec limit). In TDMA mode the scan time will be longer, due to the fact that the system must wait for a pulse to appear. Currently, the total scan time for a GSM phone is approximately less than 3 seconds.
3. When measuring GSM power how do we address duty cycle?
We tested GSM sources at 1/8 duty cycle. At 1/8, it slows down the process. GSM has a 500 µs pulse. The system waits for the rising edge to level out and then measures it.
4. How do we capture quickly changing modes for transmitters with rapidly changing sources?
RFxpert requires the power level to remain constant for 40 ms.
5. What is the measurement sensitivity? (Accurate power measurements)?
RFxpert can measure down to at least 0 dBm for GSM and CDMA. To achieve the required accuracy, place the DUT on the surface of the spacer. For most frequencies this keeps it out of the extreme near-field region, but keeps it close enough for sensitive measurements.
6. Are the measurements reliable in noisy ambient environments without an anechoic enclosure?
In cell phone tests, we put an intentionally radiating device 30 cm away from the scanner and it did not compromise the cell phone results. If the interfering element is 20 dB higher than DUT, at 1 meter, the effect is minor. In more extreme cases where there is predictable interference, frequency and level coordination can be used. For adjacent test systems operating at very different power levels, frequency discrimination isolates the measurement systems. The scanner and DUT do not need to be put into an "anechoic" enclosure.
7. Can you measure multiple antennas on different planes?
Yes. This can be done if you know the location of each antenna on the DUT, their locations in relation to the sensors, and if each antenna is exercised independently. The measured data will then be adjusted based on distance tables.
8. How do you support Tx/Rx tests?
External test sets can measure round trip frame or bit error rate through the system at a variety of power levels. Traditional Tx/Rx measurements are conducted "over the air" through the scanner - which does not affect outcomes. External test sets can measure many parameters besides radiated power. For example, round trip frame or bit error rate through the system at a variety of power levels. These traditional Rx/Tx measurements can be conducted "over the air" using the scanner as a coupling device. To do this, the test set is connected to the RF pass-through at the back of the RFxpert. The DUT is placed on the scanner and set to maximum power. Over the air, path loss can be estimated by comparing the new channel power, as measured by the test set, to the expected value. All parametric tests that can be done with galvanic connection can now be done "over the air" using the new path loss value.
9. What is the measurement sensitivity BERT (or Frame error rate) test?
Depending on the ambient environment, BERT or FER tests can be done down to levels within 0 dB of galvanic tests.
10. What kind of power supply can be used?
Use the 6VDC power supply that is shipped with the system only. It is a "global" 50/60 Hz and 110v/220v converter.
11. What is the range of supported power levels?
The supported power level range is from +30dBm to 0dBm. There is no risk in running the RFxpert with less than the minimum supported power. RFxpert software will indicate if the signal level is too low to be detected or measured accurately. For power levels greater than 10dB over +40dBm, RFxpert may be damaged.
12. Where do I place the reference board?
To test a device it should be placed on the white plastic spacer. The device should be placed with the radiating surface down, and ideally positioned in the middle of the spacer. The maximum device size can be larger than the scanner area, but the radiating surface should be small enough to leave 5cm on each side to the edge.
13. Does RFxpert require annual calibration?
No. It does not require any annual calibration.
14. What is the accuracy of the RFxpert measurements?
For high accuracy radiated power measurements, TRP is typically +/- 1.5 dB
15. I have a network analyzer that is not on the list of supported analyzers, can it still be supported?
If your network analyzer is not on the supported network analyzer list, please contact your local representative or info at emscan dot com for details.
16. What are the minimum system requirements?
Intel Pentium 4 3.0 GHz
512MB recommended
Windows XP SP3 OS
CD ROM Drive
17. What is the maximum power level that RFxpert can receive without being damaged?
The maximum power level for accurate measurement is approximately +35dBm into the antenna. Damage may occur above +40dBm.
18. Impact of Separation: what is the ideal radiating source distance from surface of scanner?
The best set up is when the antenna element is placed on the surface of the scanner. Sometimes this is not possible because of the position of the antenna in final packaging, etc. If the separation between the scanner and the antenna is kept to less than 10mm it will have only a small effect on the accuracy of the far-field. For separations much larger than this the far-field accuracy could be significantly compromised.
19. What limits the frequency range on either end of the specification?
The frequency is limited on the high end by the receiver architecture and on the low end by the difficulty in setting up an isolated test environment, i.e. at low frequency the surrounding environment influences the set up more significantly.
20. What are the effects on the measurement from driving the antenna with lower power (e.g. dBm vs. 20 dBm)?
As long as the power received at the reference probes is high enough, there should be no change other than a lower power level. As the source power goes out of range, you will start to see phase errors first which will influence far-field results.
21. The features/specifications indicate measuring dipoles and helical antennas. I assume that this is done by laying the DUT flat on the scanner bed? How do you feed the antenna without having the cable on the scanner bed, without some sort of de-embedding? Both of these conditions would seem to compromise the integrity of the near-field and alter the measurement accordingly.
The system always provides measurement results as is, in the near-field. If a cable is present, then very often currents can be seen on it. The onus is on the tester to set up the conditions so as best to represent what they want to measure. If the cable cannot be moved away from the scanner surface then absorbing foam may help to reduce the impact on the test.
22. When selecting "Pattern Settings Properties", "Amplitude Scale", "Comparison Type", switching from linear to Log10 seems to have no effect. What is this supposed to do? I thought it changed the scale of the near-field and far-field patterns.
The linear/log switch applies to the difference mode of the comparison scan. When comparing the near-field of two scans that have different power output, it is often more instructive to compare the fields in a log sense.
23. If I put my cell phone on the scanner does it scan?
You cannot measure the cell phone from your pocket since it is being managed by a live network. The output power and transmit times are too variable to get a good measurement. However, if you were to connect to your phone using a base station emulator, RFxpert would be able to measure it accurately.
24. How can I measure each side of the DUT in order to get a full spherical pattern?
The DUT should be positioned along the longer axis of the scanner to ensure the aggregate node combines them together properly. First, scan one side, flip the DUT, position it along the longer axis, and run another scan. Then simply go to the Radiated Power "Data list" window - highlight one sample (representing one side of the DUT), then using the CTRL key highlight and click on another sample (representing the other side of the DUT). What is displayed is a full slice through the sphere that reflects a merging of two hemispheres (one for each side of the DUT). This merging will also apply to Bisection Polar View - Theta; and Bisection Polar View - Phi. TRP is also calculated.
 

25. What is the resolution bandwidth of RFxpert?
The built in receiver has an IF bandwidth of 60 MHz. For modulated signals that are wider than 60 MHz there may be slight offset in the reported far-field values. This offset can be determined by comparing a modulated signal to a CW signal for the same antenna or by comparing a chamber measurement to the RFxpert reported value.
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