ADL5904ACPZN-R7 datasheet(20/28 Pages) AD

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Data Sheet

ADL5904

Rev. 0 | Page 19 of 27

CHOOSING A VALUE FOR CRMS

CRMS provides the averaging function for the internal rms

computation. Using the minimum value for CRMS allows the

quickest response time to a pulsed waveform, but leaves signifi-

cant output noise on the output voltage signal. However, a large

filter capacitor reduces output noise and improves the rms

measurement accuracy but at the expense of the response time.

In applications where the response time is not critical, place a

relatively large capacitor on the CRMS pin. In Figure 44, a value

of 100 nF is used. For most signal modulation schemes, this value

ensures excellent rms measurement accuracy and low residual

output noise. There is no maximum capacitance limit for CRMS.

Figure 45 and Figure 46 show how output noise varies with

CRMS when the ADL5904 is driven by a single-carrier W-CDMA

(Test Model TM1-64, peak envelope power = 10.6 dB, bandwidth

= 3.84 MHz) and by an LTE signal (Test Model TM1-20, peak

envelope power = 11.58 dB, bandwidth = 20 MHz), respectively.

Figure 45 and Figure 46 also show how the value of CRMS affects

the response time. This response time is measured by applying

an RF burst at 2.14 GHz at 0 dBm to the ADL5904. The 10% to

90% rise time and 90% to 10% fall time are then measured.

100

200

300

400

500

600

0.1

1.0

100

10k

100k

0.1

100

1000

CRMS CAPACITANCE (nF)

10% TO 90% RISE TIME (µs)

90% TO 10% FALL TIME (µs)

OUTPUT NOISE (mV p-p)

Figure 45. Output Noise, Rise and Fall Times vs. CRMS Capacitance,

Single-Carrier W-CDMA (Test Model TM1-64) at 900 MHz with PIN = 0 dBm

100

200

300

400

600

700

0.1

1.0

100

500

10k

100k

0.1

100

1000

CRMS CAPACITANCE (nF)

10% TO 90% RISE TIME (µs)

90% TO 10% FALL TIME (µs)

OUTPUT NOISE (mV p-p)

Figure 46. Output Noise, Rise and Fall Times vs. CRMS Capacitance,

Single-Carrier LTE (Test Model TM1-20) at 900 MHz with PIN = 0 dBm

Table 4 shows the recommended minimum values of CRMS for

various modulation schemes. Table 4 also shows the output rise

and fall times and noise performance. Using lower capacitor

values results in faster response times but can result in degraded

rms measurement accuracy. If the output noise shown in Table 4

is unacceptably high, it can be reduced by increasing CRMS or by

implementing an averaging algorithm after the output voltage of

the ADL5904 is sampled by an analog-to-digital converter (ADC).

The values in Table 4 were experimentally determined to be the

minimum capacitance that ensures good rms accuracy for that

particular signal type. This test was initially performed with a

large capacitance value on the CRMS pin (for example, 10 µF).

The value of VRMS was noted for a fixed input level (for example,

−10 dBm). The value of CRMS was then progressively reduced (this

can be accomplished 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 CRMS value required 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 requires a larger

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

averaging function and ensures a valid rms computation.

Table 5. Recommended Minimum CRMS Values for Various Modulation Schemes

Modulation/Standard

Peak Envelope

Power Ratio Ratio

(dB)

Carrier

Bandwidth (MHz)

CRMSMIN (nF)

Output Noise

(mV p-p)

Rise/Fall

Times (µs)

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

3.3

4/25

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

3.3

0.5/6

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

7.4

100

35/276

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

7.4

100

35/276

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

7.4

4/25

W-CDMA, One-Carrier, TM1-64

10.6

3.84

100

35/276

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

15.96

18.84

100

35/276

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

11.58

100

35/276

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