Fast and precise measurement of low RF levels

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14 March 2001 Test & Measurement

Roland Minihold, Rohde & Schwarz

Some measurements require very low and highly accurate RF levels (typically -100 to -110 dBm), eg for the BER (bit error rate) of digital communication receivers. This R&S test hint summarises how to calibrate extremely precisely, the levels of signal generators and communication testers used for BER measurements.

Rohde & Schwarz' spectrum analyser features measurement uncertainty of 0,5 dB, minimum sweep time of 2,5 ms and up to 30 measurements per second on the IEC/IEEE bus (GPIB). It comes in four models with upper frequencies 3 GHz, 7 GHz, 13 GHz and 30 GHz

Certain digitally modulated signals, such as IS-95 or WCDMA, exhibit a high crest factor. A power meter with a thermal sensor is best suited for determining their levels. For this purpose, Rohde & Schwarz offers the dual-channel power meter NRVD with thermal power sensor NRV-Z51 or NRV-Z52. Due to the limited sensitivity of thermal power sensors, only relatively high levels can be measured (typ greater than and equal to 3330 dBm).

Diode sensors provide comparable measurement accuracy for RF levels down to about -50 dBm due to higher sensitivity. Care should be taken, however, that they are only used in the square law region. Extra measurement uncertainties can occur otherwise in the case of signals with high crest factor or with harmonics. For the high-sensitivity diode sensor NRV-Z4 such effects are negligible in the range -40 dBm ±10 dB.

Figure 1. Suggested test setup to determine small RF levels

The significantly lower RF levels necessary for BER measurements can only be measured with a selective RF receiver or a spectrum analyser. But their absolute measurement uncertainty - even that of a state-of-the art spectrum analyser like the R&S FSP - is higher (0,5 dB) compared to high-quality power meters.

The point of this test hint is to eliminate the absolute error of the FSP spectrum analyser by calibrating it to the dual-channel power meter NRVD with high-sensitivity diode sensor NRV-Z4 for a reference level of approximately -40 dBm (Figure 1). When measuring tiny levels, only the very small linearity error of the FSP that occurs with digital bandwidths adds to the measurement uncertainty of the power meter (Figure 2). Thus a total measurement uncertainty of 0,26 dB can be achieved when measuring a level of -110 dBm.

Figure 2. Linearity error of the FSP spectrum analyser, measured on eight different units (resolution bandwidth 300 Hz)

If necessary, the measurement range can be extended to approximately -130 dBm by means of a low-noise preamplifier. Using the zero span measurement and integration in the (time domain power RMS) of the FSP, the maximum possible measurement speed can be obtained for a given S/N ratio. Application Note 1MA21 provides a detailed description of this procedure and can be downloaded from the Rohde & Schwarz website.

For further information contact Protea Communication & Measurement, (011) 786 3647.

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Fast and precise measurement of low RF levels

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