ADL5511ACPZ-R7 datasheet(21/29 Pages) AD

21 / 29 page

ADL5511

Data Sheet

Rev. B | Page 20 of 28

For input frequencies in the 900 MHz range, there will still be

significant carrier content on the envelope output. With the

two filters providing a combined 6 dB roll-off at approximately

900 MHz and with the residual carrier at 1.8 GHz, carrier

filtering of approximately 18 dB can be expected (the two

single-pole filters provide a combined roll-off of 12 dB per

octave.

The internal filtering of the carrier in the envelope detection

path can be augmented by adding additional supply-referenced

capacitance to the FLT2 and FLT3 pins. The required capaci-

tance can be calculated using the following equations:

)

400

(

−

Ω

FLT2

(6)

and

)

250

(

−

Ω

FLT3

(7)

where fLT2 and fLT3 are the desired corner frequencies.

For example, to set the corner frequency to 200 MHz, CFLT2

and CFLT3 should be set to 1.6 pF and 2.4 pF, respectively.

The two corner frequencies should be set so that they are

approximately equal.

Care should be taken not to set the corner frequency of this

carrier suppression filter too low as it will start to degrade

envelope bandwidth. The ADL5511 has an envelope bandwidth

of 130 MHz. Thus, if the capacitors on FLT2 and FLT3 are so

big that the carrier-suppression corner frequency approaches

130 MHz, the carrier filtering effort will directly impact the

envelope bandwidth. Thus, the corner frequency should be set

low enough so that the RF carrier is adequately removed from

the envelope output while still maintaining the desired envelope

bandwidth. An alternative option would be to filter the carrier

at the VENV output using a higher order filter.

CHOOSING A VALUE FOR THE RMS AVERAGING

CAPACITOR (CFLT4)

CFLT4 provides the averaging function for the internal rms

computation, the result of which is available at the VRMS

output. As already noted, the on-chip rms filtering corner is

internally set by a 400 Ω resistor and a 20 pF capacitor, yielding a

corner frequency of approximately 20 MHz. For adequate rms

filtering, connect an external filter capacitor between FLT4

(Pin 14) and VPOS (Pin 15). This capacitance acts on the

internal 400 Ω resistor to yield a new corner frequency for the

rms filter given by the following equation:

)

400

(

−

Ω

FLT

FLT4

(8)

For example, a supply-referenced 0.1 µF capacitor on FLT4

reduces the corner frequency of the rms averaging circuit to

approximately 4 kHz.

The size of the rms filtering capacitor has a direct impact on

the rms accuracy up to a point. For most accurate detection, the

rms filter corner should be low enough to filter out most of the

modulation content. This corresponds to a corner frequency

that is significantly less than the bandwidth of the signal being

measured.

Table 4 shows recommended minimum values of CFLT4 for

popular modulation schemes. Using smaller capacitor values

than these will result in rms measurement errors; using higher

values will not further improve rms accuracy but will reduce the

output noise on VRMS at the expense of increased rise and fall

times. In Table 4, rise and fall times are also shown along with

residual output noise.

The recommended minimum values for CFLT4 were experimen-

tally determined by starting out with a large capacitance value

on the FLT4 pin (for example, 10 µF). The value of VRMS was

noted for a fixed input power level (for example, 0 dBm). The

value of CFLT4 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 CFLT4 is becoming

too small).

The recommended minimum value for CFLT4 is roughly

inversely proportional to the bandwidth of the input signal, that

is, wider bandwidth signals tend to require smaller minimum

filter capacitances. As already noted, the value of CFLT4 sets up

an internal low pass corner frequency, which filters the rms

voltage. As carrier bandwidth increases, a larger proportion

of the residual noise (which has been effectively mixed down

to baseband) is filtered away. This results in smaller capaci-

tances being required as carrier bandwidths increase.

Table 4. Recommended Minimum CFLT4 Values for Various Modulation Schemes (Pin = 0 dBm)

Modulation/Standard

PEP to RMS

Ratio

Signal

Bandwidth

CFLT4

(Min)

Output Noise

Rise/Fall Time (10% to 90%)

W-CDMA, One-Carrier, TM1-64

9.83 dB

3.84 MHz

220 nF

98 mV p-p

82 µs/310 µs

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

TM1-16, TM1-8

12.08 dB

18.84 MHz

100 nF

140 mV p-p

40 µs/140 µs

LTE Test Model E-TM1_1_4MHz

9.83 dB

4 MHz

220 nF

135 mV p-p

82 µs/310 µs

LTE Test Model E-TM1_1_10MHz

11.99 dB

10 MHz

100 nF

89 mV p-p

40 µs/140 µs

LTE Test Model E-TM1_1_20MHz

11.58 dB

20 MHz

47 nF

90 mV p-p

20 µs/70 µs

ADL5511ACPZ-R7 数据表(PDF) 21 Page - Analog Devices

ADL5511ACPZ-R7 数据表(HTML) 21 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

数据表下载