ADL5906ACPZN-R2 datasheet(22/32 Pages) AD

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ADL5906

Data Sheet

Rev. 0 | Page 22 of 32

Table 5. Recommended Minimum CRMS Values for Various Modulation Schemes

Modulation/Standard

Peak Envelope

Power Ratio (dB)

Carrier

Bandwidth (MHz)

CRMSMIN (nF)

Output Noise

(mV p-p)

Rise/Fall

Time (µs)

QPSK, 5 MSPS (SQR COS Filter, α = 0.35)

3.8

0.2/10

QPSK ,15 MSPS (SQR COS Filter, α = 0.35)

3.8

0.2/10

64 QAM, 1 MSPS (SQR COS Filter, α = 0.35)

7.4

265

3/85

64 QAM, 5 MSPS (SQR COS Filter, α = 0.35)

7.4

380

0.2/10

64 QAM, 13 MSPS (SQR COS Filter, α = 0.35)

7.4

205

0.2/10

W-CDMA, One-Carrier, TM1-64

10.56

3.84

820

0.2/10

W-CDMA Four-Carrier, TM1-64, TM1-32, TM1-16, TM1-8

12.08

18.84

640

0.2/10

LTE, TM1, One-Carrier, 20 MHz (2048 QPSK Subcarriers)

11.58

140

0.2/10

Table 5 shows the recommended minimum values of CRMS for

popular modulation schemes. Using lower capacitor values results

in rms measurement errors. Output response time is also shown.

If the output noise shown in Table 5 is unacceptably high, it can

be reduced by

•

Increasing CRMS

•

Implementing an averaging algorithm after the output voltage

of the ADL5906 has been sampled by an analog-to-digital

converter (ADC)

The values in Table 5 were experimentally determined to be the

minimum capacitance that ensures good rms accuracy for that

particular signal type. This test was carried out by starting out

with a large capacitance value on the CRMS pin (for example,

10 µF). The value of VRMS was noted for a fixed input power level

(for example, −10 dBm). The value of CRMS was then progressively

reduced (this can be done with press-down capacitors) until

the value of VRMS started to deviate from its original value (this

indicates that the accuracy of the rms computation is degrading

and that CRMS is becoming too small).

In general, the minimum required rms averaging capacitance

increases as the peak-to-average ratio of the carrier increases. The

minimum required CRMS also tends to increase as the bandwidth

of the carrier decreases. With narrow-band carriers, the noise

spectrum of the VRMS output tends to have a correspondingly

narrow profile. The relatively narrow spectral profile demands

a larger value of CRMS that reduces the low-pass corner frequency

of the averaging function and ensures a valid rms computation.

OUTPUT VOLTAGE SCALING

The linear output voltage range of the ADL5906 is nominally

0.3 V to 3.7 V. VRMS is clamped to a maximum voltage of ~3.9 V;

this helps improve falling edge settling speeds because the VRMS

output stays closer to the nominal linear-in-dB output range of

0.3 V to 3.7 V. Within the 0 V to 3.9 V maximum output range,

the slope can be adjusted as needed via extra resistors, as shown

in Figure 52.

If only a part of the RF input power range of the ADL5906 is

being used (for example, −10 dBm to −60 dBm), increase the

scaling so that this reduced input range fits into the available

output swing (0 V to 3.9 V) of the ADL5906.

The output swing is reduced by simply adding a voltage divider on

the output pin, as shown in the A side of Figure 52. Reducing the

output scaling can be used when interfacing the ADL5906 to an

ADC with a 0 V to 2.5 V input range.

VSET

VRMS

VSET

R15

VRMS

Figure 52. Decreasing and Increasing Slope

The output voltage swing can be increased using a technique that is

analogous to setting the gain of an op amp in noninverting mode

(see the B side of Figure 52) with the VSET pin being the equivalent

of the inverting input of the op amp.

With VRMS connected to VSET, the nominal transfer function

of the ADL5906 is given by

VRMS = Slope × (PIN − Intercept)

For example at 3.5 GHz, with PIN equal to 0 dBm, the nominal

output voltage is equal to 0.052 V/dB × (0 dBm − (−64 dBm) =

3.328 V.

To scale this voltage downward using a resistor divider, choose a

value for R15 and calculate R1 using the following equation:













−

RMS

R15

(11)

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