ADL5920

Data Sheet

Rev. B | Page 20 of 26

CHOOSING VALUES FOR CRMSF AND CRMSR

CRMSF and CRMSR provide the averaging function for the

rms computation in the forward path and reverse path rms

detectors, respectively. Using the minimum value for these

capacitances allows the quickest response time to a pulsed

waveform but leaves significant output noise on the output

voltage signal, especially with input signals that are modulated.

Similarly, a large filter capacitor reduces output noise at the

expense of response time.

In applications where response time is not critical, place a

relatively large capacitor on the CRMSF and CRMSR pins. In

Figure 38, a 0.1 μF capacitor was used on these pins. For most

signal modulation schemes, this value ensures excellent rms

measurement compliance and low residual output noise. There

is no maximum capacitance limit for CRMSF and CRMSR.

Figure 41 shows how output noise varies with CRMSF when

the ADL5920 is driven by a single-carrier W-CDMA signal

(Test Model TM1-64, peak envelope power = 10.56 dB,

bandwidth = 3.84 MHz). The response for the reverse path

is identical.

0.1

1

10

100

1000

10000

100000

1000000

0

50

100

150

200

250

300

350

1

10

100

1000

10000

OUTPUT NOISE (mV p-p)

RISE TIME (µs)

FALL TIME (µs)

Figure 41 also shows how the response time is affected by the

value of CRMSF and CRMSR. To measure this response time, an

RF burst at 2.14 GHz at 0 dBm is applied to the ADL5920. The

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

measured.

Table 6 shows the recommended minimum values of CRMSF and

CRMSR 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 6 is too high,

increase the CRMSF and CRMSR values to reduce the noise.

However, increasing the CRMSF and CRMSR values results in

slower rise and fall times.

The values in Table 6 are experimentally determined as the

minimum capacitance that ensures achieving the specified rms

accuracy for that particular signal type. This test is carried out

by starting out with a large capacitance value on the CRMSF pin

(for example, 10 μF). The VRMSF value is noted for a fixed

input power level (for example, 10 dBm). The CRMSF value is

then progressively reduced (with press down capacitors) until

the value of VRMSF starts to deviate from its original value.

This deviation indicates that the accuracy of the rms computation

is degrading and that CRMSF 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 CRMSF and CRMSR values also tend to

increase as the bandwidth of the carrier decreases. With

narrow-band carriers, the noise spectrum of the VRMSF and

VRMSR outputs tend to have a correspondingly narrow profile.

The relatively narrow spectral profile demands larger CRMSF

and CRMSR values to reduce the low-pass corner frequency of

the averaging function and to ensure a valid rms computation.

Table 6. Recommended Minimum Capacitor Values on CRMSF and CRMSR for Various Modulation Schemes

Modulation/Standard

Peak Envelope

Power Ratio (dB)

Carrier

Bandwidth (MHz)

CRMSF and

CRMSR (nF)

Output Noise

(mV p-p)

Rise/Fall

Time (μs)

QPSK, 5 MSPS (SQR COS Filter,

 = 0.35)

3.8

5

1

84

0.2/10

QPSK ,15 MSPS (SQR COS Filter,

 = 0.35)

3.8

15

1

42

0.2/10

64 QAM, 1 MSPS (SQR COS Filter,

 = 0.35)

7.4

1

10

265

3/85

64 QAM, 5 MSPS (SQR COS Filter,

 = 0.35)

7.4

5

1

380

0.2/10

64 QAM, 13 MSPS (SQR COS Filter,

 = 0.35)

7.4

13

1

205

0.2/10

W-CDMA, One Carrier, TM1-64

10.56

3.84

1

820

0.2/10

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

12.08

18.84

1

640

0.2/10

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

11.58

20

1

140

0.2/10