ADRF6801 datasheet(23/36 Pages) AD

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ADRF6801

Rev. 0 | Page 23 of 36

EVM MEASUREMENTS

EVM is a measure used to quantify the performance of a digital

radio transmitter or receiver. A signal received by a receiver has

all constellation points at their ideal locations; however, various

imperfections in the implementation (such as magnitude imbalance,

noise floor, and phase imbalance) cause the actual constellation

points to deviate from their ideal locations.

In general, a demodulator exhibits three distinct EVM limitations

vs. received input signal power. As signal power increases, the

distortion components increase. At large enough signal levels,

where the distortion components due to the harmonic non-

linearities in the device are falling in-band, EVM degrades

as signal levels increase. At medium signal levels, where the

demodulator behaves in a linear manner and the signal is well

above any notable noise contributions, the EVM has a tendency to

reach an optimal level determined dominantly by either quadrature

accuracy and I/Q gain match of the demodulator or the precision

of the test equipment. As signal levels decrease, such that the

noise is the major contribution, the EVM performance vs. the

signal level exhibits a decibel-for-decibel degradation with

decreasing signal level. At lower signal levels, where noise proves

to be the dominant limitation, the decibel EVM proves to be

directly proportional to the SNR.

The basic test setup to test EVM for the ADRF6801 consisted

of an Agilent E4438C, which was used as a signal source. The

900 MHz modulated signal was driven single ended into the

RFIN SMA connector of the ADRF6801 evaluation board. The

IQ baseband outputs were taken differentially into two AD8130

difference amplifiers to convert the differential signals into single-

ended signals. A Hewlett Packard 89410A VSA was used to sample

and calculate the EVM of the signal. The ADRF6801 IQ base-

band output pins were presented with a 450 Ω differential load

impedance.

The ADRF6801 shows excellent EVM performance for 16 QAM.

Figure 41 shows the EVM of the ADRF6801 being better than

−40 dB over a RF input range of about +35 dB for the 16 QAM

modulated signal at a 10 MHz symbol rate. The pulse shaping

filter’s roll-off (alpha) was set to 0.35.

–45

–40

–35

–30

–25

–20

–15

–10

–5

–65

–55

–45

–35

–25

–15

–5

INPUT POWER (dBm)

Figure 41. EVM vs. Input Power, EVM Measurements at fRF = 900 MHz;

fIF = 0 MHz (that is, Direct Down Conversion); 16 QAM; Symbol Rate = 10 MHz

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