ADRF6516ACPZ-R7 datasheet(22/32 Pages) AD

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ADRF6516

Data Sheet

Rev. B | Page 22 of 32

SERIAL PORT CONNECTIONS

The ADRF6516 has a SPI port to control the gain and filter band-

width settings. Data can be written to the internal 8-bit register

and read from the register. It is recommended that low-pass RC

filtering be placed on the SPI lines to filter out any high frequency

glitches. See Figure 58, the evaluation board schematic, for an

example of a low-pass RC filter.

ENABLE/DISABLE FUNCTION

To enable the ADRF6516, the ENBL pin must be pulled high.

Driving the ENBL pin low disables the device, reducing current

consumption to approximately 9 mA at room temperature.

ERROR VECTOR MAGNITUDE (EVM) PERFORMANCE

Error vector magnitude (EVM) is a measure used to quantify

the performance of a digital radio transmitter or receiver by

measuring the fidelity of the digital signal transmitted or

received. Various imperfections in the link, such as magnitude

and phase imbalance, noise, and distortion, cause the

constellation points to deviate from their ideal locations.

In general, a receiver 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 compo-

nents due to the harmonic nonlinearities in the device are

falling in-band, EVM degrades as signal levels increase.

•

At medium signal levels, where the signal chain behaves

in a linear manner and the signal is well above any notable

noise contributions, EVM has a tendency to reach an opti-

mal level determined dominantly by either the quadrature

accuracy and IQ gain match of the signal chain or the

precision of the test equipment.

•

As signal levels decrease, such that noise is a major con-

tributor, EVM performance vs. the signal level exhibits

a decibel-for-decibel degradation with decreasing signal

level. At these lower signal levels, where noise is the

dominant limitation, decibel EVM is directly proportional

to the SNR.

EVM TEST SETUP

The basic setup to test EVM for the ADRF6516 consisted of an

Agilent E4438C used as a signal source and a Hewlett-Packard

89410A vector signal analyzer (VSA) used to sample and calculate

the EVM of the signal. The E4438C IQ baseband differential

outputs drove the ADRF6516 inputs. The I and Q outputs of the

ADRF6516 were loaded with 1 kΩ differential impedances and

connected differentially to two AD8130 differential amplifiers to

convert the signals into single-ended signals. The single-ended

signals were connected to the input channels of the VSA.

EFFECT OF FILTER BANDWIDTH ON EVM

Care should be taken when selecting the filter bandwidth. In

a digital transceiver, the modulated signal is filtered by a pulse

shaping filter (such as a root-raised cosine filter) at both the

transmit and receive ends to guard against intersymbol inter-

ference (ISI). If additional filtering of the modulated signal is

done, the signal must be within the pass band of the filter. When

the corner frequency of the ADRF6516 filter begins to encroach

on the modulated signal, ISI is introduced and degrades EVM,

which can lead to loss of signal lock.

Figure 52 shows that a digitally modulated QAM baseband

signal with a bandwidth at 9.45 MHz has excellent EVM even

at a filter corner frequency of 8 MHz. Further reduction in the

corner frequency leads to complete loss of lock. As RF input

power was swept, the ADRF6516 attained an EVM of less than

−45 dB over an input power range of approximately 20 dB.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

–25

–20

–15

–10

–5

RF INPUT POWER (dBm)

30MHz

15MHz

10MHz

9MHz

8MHz

GAIN VOLTAGE

Figure 52. EVM vs. RF Input Power at Several Filter Corner Settings

(256-QAM, 14 MSPS Signal with α = 0.35; Output Differential Signal Level

Held to 700 mV p-p; OFDS Pulled High)

Figure 53 shows the degradation that a fixed filter corner has

on EVM as the signal bandwidth corner is increased in fine

increments until loss of signal lock occurs.

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

SIGNAL BANDWIDTH CORNER (MHz)

FILTER BANDWIDTH CORNER

Figure 53. EVM vs. Signal Bandwidth Corner at a Filter Corner of 5 MHz

and a 16-QAM Signal with α = 0.35

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