Accurate power measurements using spectrum …

1 For the very latest specifications IFR 239X series spectrum analyzers from Aeroflexrun automatic power measurement functions dedicatedto modern digital radio systems power measurements usingspectrum analyzersby Jean No l Payen & Jean Jacques PerretThese systems use modulation techniques that makegenerated signals difficult to measure using standardmethods, and quite impossible with RF power meterswhen interferers or other channels are present togetherwith the signal. In this case, a spectrum analyzer is thebest tool to use if fitted with dedicated power measure-ment cover these requirements, Aeroflex has developed acomprehensive range of spectrum analyzers, the 239 Xseries, with models from 9 kHz to 3, 13 and the instruments of this family have a set of semi-auto-matic measurement functions that include the following.

2 - X dB Down, measuring the X dB bandwidth of a system- Adjacent Channel power , measuring the channel powerand the difference between channel power and adja-cent channels power - Channel power - Occupied Bandwidth; measuring the system bandwidthcontaining X% of the channel power - Harmonic Distortion ; measuring up to 5 harmonic fre-quencies within the signal, and calculating the THD ofthe signalThe aim of this application note is to provide information allowingfor better use of the power measurement functions, to explain themeasurement method employed and to highlight the causes principlePower measurement is performed in a spectrum analyzer by inte-grating several elementary power samples taken at different fre-quencies over the resolution bandwidth of the analysis filter(RBW).

3 Theoretically, it is necessary to know precisely the equiva-lent noise bandwidth (ENB) of each analyzer's filter. In the realworld, this bandwidth is very close to the analysis filter's band-width (RBW) but it can vary according to the shape of the it is necessary to take the samples at frequency intervalsequal to the equivalent noise bandwidth of the filter in use. In thereal world, the integration bandwidth does not exactly match anentire number of ENB's and therefore, must be much higher thanENB in order to limit uncertainty to following example shows the RBW filter set at 100 kHz whilethe integration bandwidth is set at 20 MHz.

4 The span has beenadjusted around 4/5 times higher than the signal top of that, each sample must represent a real sample: whenentering this measurement mode, the spectrum analyzer auto-matically switches to the "Sample" detection mode. The powermeasurement is processed using real samples taken at the dis-play points (different from the normal spectrum analyzer modewhere "Peak Pos" is normally used).Every sample must be statistically independent therefore thevideo filter must be switched off (or set at the maximum value) andtrace averaging not soft keys of the IFR 239X series spectrum analyzers allow theoperator to set up integration bandwidth ( ) and channelspan ( Span) parameters using the keyboard or scroll integration bandwidth is shown on the screen using two redvertical lower part of the screen is dedicated to the digital display ofthe measurement results.

5 A more stable display is obtained usingmeasurement averaging, without any negative effect on rate can be reduced by lowering the sweep case - Low signal measurementWhen the signal level is not far from the noise floor, accuratemeasurement is possible if the noise power is taken into accountand the measurement result corrected. In this case, the "AdjacentChannel power " measurement will be used as it will allow formeasuring the noise power in the same bandwidth. The operatingmode is identical and the same precautions as for ChannelPower" are soft keys of the IFR 239X series spectrum analyzers allow theoperator to set up the main channel bandwidth (MainChBW), theadjacent channel bandwidth (AdjChBW) and the channel spacing(ChSpacing).

6 The first two parameters must be set up at the samevalue and spacing must allow for the display of adjacent channelson the measurement results are displayed on the lower part of thescreen. Correcting the measurement will start with calculating theaverage value of adjacent channel power differences. = x [( )+( )] = dBThe correction factor C is computed using the formula:C = 10xlog (1-10 /10)The following table gives some of the most useful values: power differenceCorrection to this table, it is not necessary to use any correctionfactor when the power difference is higher than 15 dB. For low dif-ferences, correcting the result becomes mandatory.

7 For the very latest specifications our example, the corrected value of the channel power is :ChP = = dBmComparing with an RF power meterAdvantages of using the IFR 239X series:- Selective measurement : it is possible to measure one particu-lar channel among others (satellite reception) or when interfer-ers are present- High dynamic range (typically +30 dBm to -80 dBm), no sensorto change- Better accuracy at low levels, due to correction capabilityDrawbacks:- Slightly less Accurate for middle or high levels- measurement time slower- Not usable on pulsed signalsIFR 239X series spectrum AnalyzersThe family includes 4 units in the same mechanical design.

8 22339977-9 kHz to 3 GHz, monochrome display with optional DCpower supply and battery22339999AA- 9 kHz to 3 GHz, color display, switchable low noise pre-amplifierThese two units may be fitted with an optional 100 kHz to 3 GHztracking generator to allow VSWR and Cable Fault measurementfunctions to be performed with the spectrum kHz to GHz, color display , designed for spurioussignal identification on digital radios22339955-9 kHz to GHz, color screen fitted with Planar Crowntype N or APC7 compatible SMA input connectorAll these units can be delivered with the High Stability Time Baseoption, EMC filters and quasi-peak detectors option and EMC soft-ware facilities.

9 A digital filter option, reducing the analysis band-width to 10 Hz is available for all models except the 2397. Theyall have a 3 1/4 floppy disk drive as standard, RS232, IEEE488,parallel and VGA interfaces, and are delivered in a soft dumps can be produced either on a suitable printer orfloppy disk using Bitmap or JPEG EasySpan application software from Aeroflex allows for filetransfer to PC and export of data in many formats. More information on Aeroflex products can be obtained fromyour local Aeroflex rep or on the Aeroflex web site No. 46891/916, Issue 1, 07/03 CHINATel: [+86] (21) 6282 8001 Fax: [+86] (21) 6282 8002 EUROPETel: [+44] (0) 1438 742200 Fax: [+44] (0) 1438 727601 FRANCETel: [+33] 1 60 79 96 00 Fax: [+33] 1 60 77 69 22 HONG KONGTel: [+852] 2832 7988 Fax: [+852] 2834 5364 SCANDINAVIATel: [+45] 9614 0045 Fax: [+45] 9614 0047 SPAINTel: [+34] (91) 640 11 34 Fax: [+34] (91) 640 06 40 UNITED KINGDOMTel: [+44] (0) 1438 742200 Toll Free: [+44] (0800) 282 388 (UK only)Fax: [+44] (0) 1438 727601 USATel: [+1] (316) 522 4981 Toll Free: [+1] (800) 835 2352 (US only)Fax.

10 [+1] (316) 522 we are always seeking to improve our products,the information in this document gives only a generalindication of the product capacity, performance andsuitability, none of which shall form part of any con-tract. We reserve the right to make design changeswithout notice. All trademarks are acknowledged. Parent company Aeroflex, Inc. Aeroflex 2003.