ADRF6806ACPZ-R7 datasheet(24/36 Pages) AD

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ADRF6806

Data Sheet

Rev. B | Page 24 of 36

EVM MEASUREMENTS

Figure 42 shows that the ADRF6806 exhibited excellent EVM

performance, with the EVM being better than −40 dB over an

RF input range of about +35 dB for a 4 QAM modulated signal

at a 5 MHz symbol rate at a 0 Hz IF. The pulse shaping filter’s

roll-off, or alpha, was set to 0.35. EVM and was tested for both

power modes: lower power mode disabled (LPEN = 0) and low

power mode enabled (LPEN = 1). When low power mode was

enabled, the EVM was better at lower RF input signal levels due

to less noise while running in low power mode. While in normal

power mode (LPEN = 0), the EVM remained undegraded at

higher RF input signal levels.

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 a 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.

–45

–40

–35

–30

–25

–20

–15

–10

–5

RF INPUT POWER (dBm)

–60

–50

–40

–30

–20

–10

LPEN = 0

LPEN = 1

The basic test setup to test EVM for the ADRF6806 consisted of an

Agilent E4438C, which was used as a signal source. The 140 MHz

modulated signal was driven single-ended into the RFIN SMA

connector of the ADRF6806 evaluation board. The IQ baseband

outputs were taken differentially into a pair of AD8130 difference

amplifiers to convert the differential signals to single-ended. The

output impedance driven by the ADRF6806 was set to 450 Ω

differential. The single-ended I and Q signals were then sampled

by an Agilent DSO7104B oscilloscope. The Agilent 89600 VSA

software was used to calculate the EVM of the signal. The signal

source used for the reference input was a Wenzel 100 MHz quartz

oscillator set to an amplitude of 1 V p-p. The reference path was

set to divide-by-four, resulting in a PFD frequency of 25 MHz.

Figure 42. EVM Measurements @ 140 MHz 16 QAM; Symbol Rate = 5 MHz;

BB IF Frequency of 5 MHz

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