ADRF6510ACPZ-R7 datasheet(21/32 Pages) AD

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Data Sheet

ADRF6510

Rev. B | Page 21 of 32

EVM

The basic setup to test EVM for the ADRF6510 consisted of an

Agilent E4438C used as a RF signal source with an Agilent

InfiniiVision DSO7104B oscilloscope in conjunction with the

Agilent 89600 VSA software to sample the signal and compute

the EVM. The E4438C RF output drove the RF port of the

ADL5380 IQ demodulater, which in turn drove the baseband

differential inputs of the ADRF6510.

The I and Q outputs of the ADRF6510 were taken differentially

into two AD8130 difference amplifiers to convert them into

single-ended signals. The single-ended signals were connected

to the input channels of the oscilloscope, which captured the

modulated waveforms.

An overall baseband EVM performance was measured on the

ADRF6510. A modulation setting of 4 QAM and, unless

otherwise noted, a 5 MHz symbol rate were used, with a pulse

shaping filter alpha of 0.35. The analog gain of the ADRF6510

was adjusted to maintain 1.5 V p-p into a 1 kΩ differential load

impedance. Figure 50 shows EVM vs. input power for three

different IF frequencies. The input power is the integrated input

power over the bandwidth of the modulated signal.

In Figure 50, the ADRF6510 shows excellent EVM of better

than −-35 dB over a 50 dB range at a 0Hz IF. The user can

chose to use a complex IF of 5 MHz to achieve even a better

EVM of at least −40 dB over a 50 dB range.

–80

–10

PIN (dBm)

–5

–10

–15

–20

–25

–30

–35

–40

–45

–50

–60

–70

–40

–30

–20

0Hz IF

2.5MHz IF

5MHz IF

Figure 50. EVM vs. RF Input Power Level; OFDS Pulled Low, COFS = 1 µF

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 ADRF6510 filter begins to encroach

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

which can lead to loss of signal lock.

While low-pass filtering with the ADRF6510 to reject out-of-

band undesired signals (blockers), more rejection of the

undesired signals may be required. If the filter bandwidth is set

to approximately the same as the signal bandwidth, the user

may trade some degradation of EVM for a gain in rejection of

the out-of-band undesired signals, by lowering the low-pass

filter bandwidth corner (for example, by 1 MHz).

Lowering the filter bandwidth to gain more rejection works

progressively better the lower the signal and filter bandwidths

are set to (see Figure 43). A 1 MHz change from 3 MHz filter

bandwidth to 2 MHz filter bandwidth yields about 20 dB more

rejection. Compare that to a 1 MHz change from 29 MHz filter

band-width to 28 MHz filter bandwidth, which will yield about

1 dB more in rejection.

Figure 51 shows that degradation of EVM as signal bandwidth

(positive frequency only) is swept while keeping the filter

bandwidth set to 5 MHz. Three different COFS capacitor values

were used.

SIGNAL BANDWIDTH (MHz)

–5

–10

–15

–20

–25

–30

–35

–40

COFS = 1µF

COFS = 100nF

COFS = 1nF

FILTER BW CORNER

Figure 51. EVM vs. Signal Bandwidth over COFS Values While Maintaining a

Filter Bandwidth of 5 MHz

EFFECT OF OUTPUT VOLTAGE LEVELS ON EVM

Output voltage level can affect EVM greatly when the signal

is compressed. When changing the output voltage levels of

the ADRF6510, take care that the output signal is not in compression, which

causes EVM degradation.

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