Data Sheet

ADL5506

APPLICATIONS INFORMATION

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Rev. C | 18 of 22

large input resistance. For low frequencies, the option shown in

Figure 46 or Figure 48 is recommended.

In Figure 47, the matching components are drawn as general

reactances. Depending on the frequency, the input impedance at

that frequency and the availability of standard value components,

either a capacitor or an inductor, is used. As in the previous case,

the input impedance at a particular frequency is plotted on a Smith

Chart and matching components are chosen (Shunt or Series L,

or Shunt or Series C) to move the impedance to the center of

the chart. Matching components for specific frequencies can be

calculated using the Smith Chart (see Figure 17). Table 4 outlines

the input impedances for some commonly used frequencies.

The impedance matching characteristics of a reactive matching net-

work provide voltage gain ahead of the ADL5506, which increases

device sensitivity (see Table 4). The voltage gain is calculated by

R1

R2

Gain

Voltage

dB

10

log

20



(12)

where:

R2 is the input impedance of the ADL5506.

R1 is the source impedance to which the ADL5506 is being match-

ed.

Note that this gain is only achieved for a perfect match. Component

tolerances and the use of standard values tend to reduce gain.

Figure 46. Broadband Resistive Method for Input Coupling

Figure 47. Narrow-Band Reactive Method for Input Coupling

Figure 48. Series Attenuation Method for Input Coupling

Figure 48 shows a third method for coupling the input signal into

the ADL5506 in applications where the input signal is larger than

the input range of the log amp. A series resistor, connected to the

RF source, combines with the input impedance of the ADL5506 to

resistively divide the input signal being applied to the input. This

has the advantage of very little power being tapped off in RF power

transmission applications.

Table 4. Input Impedance with 52.3 Ω Shunt for Select Frequency

Frequency (GHz)

S11

Impedance Ω (Series)

Real

Imaginary

0.05

+0.0023

−0.031

50.14 − j3.10

0.1

−0.014

−0.033

48.48 − j3.21

0.9

−0.144

+0.007

37.37 − j0.51

1.9

−0.052

+0.282

38.66 − j23.73

2.2

+0.074

+0.329

45.89 − j34.09

2.5

+0.233

+0.312

61.85 − j45.48

3.0

+0.467

+0.096

131.91 − j32.64

3.5

+0.394

−0.305

81.58 − j66.25

Table 5. Raw Input Impedance for Select Frequency

Frequency (GHz)

S11

Impedance Ω (Series)

Real

Imaginary

0.1

+0.838

−0.251

131.9 − j281.59

0.9

−0.206

+0.714

11.41 − j36.33

1.9

−0.571

+0.397

9.84 + j15.11

2.2

−0.284

+0.639

12.42 + j31.05

2.5

+0.077

+0.699

18.87 − j52.17

3.0

+0.564

+0.407

72.70 + j114.52

3.5

+0.602

−0.099

186.70 − j58.55

Effect of Waveform Type on Intercept

Although specified for input levels in decibels relative to 1 mW

(dBm), the ADL5506 fundamentally responds to voltage and not

to power. A direct consequence of this characteristic is that input

signals of equal rms power but differing crest factors, produce

different results at the output of the log amplifier.

The effect of differing signal waveforms is to shift the effective

value of the intercept upwards or downwards. Graphically, this looks

like a vertical shift in the transfer function of the log amplifier. The

logarithmic slope, however, is not affected. For example, consider

the case of the ADL5506 being alternately fed by an unmodulated

sine wave and by a 64 QAM signal of the same rms power. The

output voltage of the ADL5506 differs by the equivalent of 1.6 dB

(31 mV) over the complete dynamic range of the device (with the

output for a 64 QAM input being lower).

Figure 35 and Figure 38 show the transfer function of the ADL5506

when driven by both an unmodulated sine wave and several

different signal waveforms. For precision operation, calibrate the

ADL5506 for each signal type that is driving it. To measure the rms

power of a 64 QAM input, for example, add the millivolt equivalent