8/14/2019 p.o. Box 64 Parker Co 80134 Usa

1/3

P.O. Box 64Parker CO 80134 USA

Phone: (303)768-8080; FAX: (303)768-8181TM

A p p l i c a t i o n

N o t e # 1 9 0 0 - 2High IM LevelPassive IM Measurements

IntroductionThe Passive IM Analyzers provided bySummitek Instruments are optimized forreceiving IM signals between approximately 75and 140 dBm. Signals which compress theinstrument in excess of approximately 0.5 dBwill result in an IM OVERLOAD error beingdisplayed. However, if the IM signal is more than20 dB higher than the instruments maximummeasurable power, the internal receiver actuallybegins to shut down. The result is that a very,very strong IM signal (-50 dBm or higher) canerroneously be reported by the instrument as avalid IM power level anywhere between 70 and140 dBm (depending upon the shutdown modeof the receiver).

This application note demonstrates how to

configure the test setup to ensure a strong IMsignal can be accurately measured by theanalyzer without the risk of over-driving theanalyzers internal receiver.

Configure the Test SetupFigure 1 illustrates the connection used tomeasure the forward IM of a device which isexpected to produce IM levels in excess of 80dBm. Such a device might be a ferrite isolator. Inthis configuration, the two carrier tones areinjected directly into the device under test

(DUT). IM generated within the DUT propagatesback towards Port 1 of the test set and also intothe directional coupler (connected to the DUToutput port).

The directional coupler is oriented so that asample of both the carrier tones and the IMsignal are routed to the coupled port. Thethrough arm of the coupler is terminated into alow IM termination such as the Summitek Instruments Model SI-20A.

Passive IM Analyzer

Port 1 Port 2

In

Thru IM Out

Low-IM,High PowerTermination

2-PortDUT

Low-IMDirectional

Coupler

SI-20A

Figure 1. Measuring High Forward IMLevels with the Passive IM Analyzer

Figure 2 shows how the same approach may beused to measure the reverse IM from a DUT.Notice the coupler direction is reversed in thisapplication compared to Figure 1. This allowsthe IM signal travelling towards Port 1 of theanalyzer to be sampled and eventually measuredat Port 2.

In both figures, a fixed attenuator is attached tothe coupled arm of the directional coupler. Thevalue of this attenuator is determined by the

8/14/2019 p.o. Box 64 Parker Co 80134 Usa

2/3

P.O. Box 64Parker CO 80134 USA

Phone: (303)768-8080; FAX: (303)768-8181

Page 2

SummitekInstruments

expected IM level. If unsure, start with two 30dB attenuators placed back-to-back. The purposeof the attenuators is to further reduce the IM

power below the level achieved with thedirectional coupler alone.

Passive IM Analyzer

Port 1 Port 2

InReverse IM Out

Low-IM,High PowerTermination

2-PortDUT

Low-IMDirectional

Coupler

SI-20A

Figure 2. Measuring the IM of a PowerCombiner Using Individual CW Tones.

The output of the fixed attenuator is thenconnected to Port 2 of the analyzer. Using themouse (or by pressing the F10 key), change theanalyzers measurement mode to Forward/ Thru. The green indicator above Port 2 shouldbe illuminated for this test. This means that theIM power incident on Port 2 of the analyzer willbe displayed on the screen.

In both forward and reverse test setups, ensurethe IM generated by the test setup does notgenerate significant IM levels (relative to theDUT) for the power levels used in the testing.This may be done by simply removing the DUT

and checking the residual IM prior to making themeasurements.

Making the MeasurementTurn on the carrier power and begin with thecarriers set to a very low level. Be sure anddisable ALC (power leveling) if you are workingbelow a carrier power level of 1 Watt (30 dBm).Increase the power in small increments (3 dB perstep) until an IM signal is detected. If an IMsignal is not detected at the maximum desiredcarrier power, remove some attenuation from thecoupled port of the directional coupler and repeatthis test.

It is desirable to find a combination of couplingcoefficients and attenuator values which willresult in an IM signal which ranges between -130and 75 dBm over the range of carrier powers of interest. For example, if the indicated IM levelon the analyzers display is 80 dBm for thelowest carrier power level of interest, increasethe value of the attenuator by at least 40 dB.

Once the IM power level has been placed withinthe dynamic range of the analyzer, you mustmanually add back the combined loss inserted bythe directional coupler and the attenuator(s). Forexample, if the display indicates a forward IM

level of 90 dBm, and you are using a 30 dBdirectional coupler and 40 dB of attenuation, thetrue IM level is 20 dBm (-90 dBm + 70 dB).

Helpful HintsThese quick tips can help reduce measurementuncertainty:

Choose the IM Units to be displayed indBm for the best measurement accuracy.This removes the uncertainty associated withthe carrier power level measurement fromthe receive IM power level results.

Minimize the length of cable between Port 1of the analyzer and the DUT.

Make sure the IM signal decreases when thecarrier power decreases. If not, the receivermay be in hard compression and notproperly reporting an IM OVERLOAD.

Use the Log to ASCII capability of theanalyzer to generate an ASCII data file. Aspreadsheet program may then be used to

8/14/2019 p.o. Box 64 Parker Co 80134 Usa

3/3

P.O. Box 64Parker CO 80134 USA

Phone: (303)768-8080; FAX: (303)768-8181

Page 3

SummitekInstruments

add the required offsets to the measureddata.

Summitek Instruments, Inc. 1999