ADRF6518ACPZ-R7 datasheet(29/39 Pages) AD

29 / 39 page

Data Sheet

ADRF6518

Rev. A | Page 29 of 39

Figure 77 shows EVM vs. input voltage over various digital gain

settings. There is about a 1 dB spread of EVM over the gain

settings.

Figure 77. EVM vs. Input Voltage over Digital Gain Settings, Filter Corner =

63 MHz, QPSK, 50 MSPS, 1.5 V p-p Differential Output Level Maintained

EVM SYSTEM MEASUREMENT

An overall EVM measurement was completed with the ADL5380

IQ demodulator driving the ADRF6518. The interface between

the two parts was dc-coupled. To achieve this, the VICM/AC

pin was floated to enable dc coupling mode and the VPI pin on

the ADRF6518 was connected to 5 V to accommodate the 3.1 V

output common-mode voltage of the ADL5380. The RF carrier

frequency applied to the RF input of the ADL5380 and the LO

frequency were set to 900 MHz, creating a zero intermediate

frequency (I/F). The alpha of the pulse response filter was set to

0.35. The RF input power to the ADL5380 was swept, and the

analog gains on the ADRF6518 were adjusted to maintain a

target 1.5 V p-p differential signal level on both the I and Q

outputs. The VGA1 analog gain was adjusted to limit its output

to 1.5 V p-p (0.75 V peak on the peak detector output). The

filter corner was set to 63 MHz, and digital gains for VGA1,

VGA2, VGA3, and the postamplifier were set to 15 dB, 21 dB,

21 dB, and 3 dB, respectively. Several signal bandwidths, signal

types, gains, and output levels were tested, in filter mode and in

filter bypass mode.

Figure 78 shows three different symbol rates: 10 MSPS, 50 MSPS,

and 80 MSPS, with the filter enabled. There is a degradation of

EVM with increasing symbol rate, but at 10 MSPS, the system

achieves better than −40 dB of EVM for about 50 dB of the

input power range. The degradation of EVM at the high input

power for Figure 78 to Figure 83 is caused by the ADL5380

compressing. By placing an RF attenuator in front of the

ADL5380, the user can extend the dynamic range of the system.

Figure 78. EVM vs. Input Power Over Symbol Rate; QPSK, Filter Corner =

63 MHz, Gain Code = 0000000, 1.5 V p-p Differential Output Level

Maintained

Figure 79 shows four different symbol rates, with the filter in

bypass mode. EVM generally improves while in filter bypass

mode, especially at the higher symbol rates, due to the absence

of noise, IQ gain mismatch, IQ phase mismatch, raw group

delay, and group delay mismatch, which are some dominant

sources of error that the filter adds when enabled.

Figure 79. EVM vs. Input Power over Symbol Rate; Filter Bypass Mode,

Gain Code = 0000000, 1.5 V p-p Differential Output Level Maintained

Figure 80 shows the EVM for a 50 MSPS signal over several

different digital modulation types while the filter is in bypass

mode. Up to 256 QAM, there is an improvement to EVM, but

this is due to how EVM is calculated, rather than absolute

symbol error being reduced. (EVM is calculated as the ratio of

the rms power of the symbol error vector to the rms average

power of the constellation. A similar and perhaps better metric

is modulation error ratio, or MER, which is defined as the ratio

of the rms power of the ideal symbol to the rms power of the

symbol error vector.) The 1024 QAM signal starts to degrade

due to the noise and distortion components impacting the

closely packed symbols in the constellation.

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

–40

–30

–20

–10

VIN (dBV p-p)

9dB, 12dB, 12dB, 3dB

12dB, 12dB, 12dB, 3dB

15dB, 12dB, 12dB, 3dB

15dB, 15dB, 15dB, 3dB

15dB, 18dB, 18dB, 3dB

15dB, 21dB, 21dB, 3dB

15dB, 21dB, 21dB, 9dB

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

–80

–10

–20

–30

–40

–50

–60

–70

PIN (dBm)

10MSPS

50MSPS

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

–80

–10

–20

–30

–40

–50

–60

–70

PIN (dBm)

10MSPS

50MSPS

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