AD8314

–12–

REV. 0

of the AD8314 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 II. Recommended Components for X1 and X2 in

Figure 32b

Frequency

Voltage Gain

(GHz)

X1

X2

(dB)

0.1

Short

52.3

Ω

0.9

33 nH

39 nH

11.8

1.9

10 nH

15 nH

7.8

2.5

1.5 pF

3.9 nH

2.55

Increasing the Logarithmic Slope in Measurement Mode

The nominal logarithmic slope of 21.5 mV/dB (see Figure 7 for

the variation of slope with frequency) can be increased to an

arbitrarily high value by attenuating the signal between V_UP

and VSET as shown in Figure 33. The ratio R1/R2 is set using

the equation

R1/R2 = (New Slope/Original Slope) – 1

In the example shown, two 5 k

Ω resistors combine to change the

slope at 1900 MHz from 20 mV/dB to 40 mV/dB. The slope can

be increased to higher levels. This will, however, reduce the usable

dynamic range of the device.

AD8314

R1

5k

V_UP

VSET

40mV/dB

@ 1900MHz

R2

5k

Figure 33. Increasing the Output Slope

Effect of Waveform Type on Intercept

Although specified for input levels in dBm (dB relative to 1 mW),

the AD8314 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 will produce different

results at the log amp’s output.

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 log amp’s transfer function. The

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

the case of the AD8314 being alternately fed by an unmodulated

sine wave and by a single CDMA channel of the same rms power.

The AD8314’s output voltage will differ by the equivalent of

3.55 dB (70 mV) over the complete dynamic range of the device

(the output for a CDMA input being lower).

Table III shows the correction factors that should be applied to

measure the rms signal strength of a various signal types. A

sine wave input is used as a reference. To measure the rms power

of a square wave, for example, the mV equivalent of the dB value

given in the table (20 mV/dB times 3.01 dB) should be subtracted

from the output voltage of the AD8314.

Table III. Shift in AD8314 Output for Signals with Differing

Crest Factors

Correction Factor

(Add to Measured

Signal Type

Input Level)

Sine Wave

0 dB

Square Wave

–3.01 dB

GSM Channel (All Time Slots On)

0.55 dB

CDMA Channel (Forward Link,

3.55 dB

9 Channels On)

CDMA Channel (Reverse Link)

0.5 dB

PDC Channel (All Time Slots On)

0.58 dB

Mobile Handset Power Control Examples

Figure 34 shows a complete power amplifier control circuit for

a dual mode handset. This circuit is applicable to any dual

mode handset using TDMA or CDMA technologies. The

PF08107B (Hitachi) is driven by a nominal power level of

+3 dBm. Some of the output power from the PA is coupled off

using an LDC15D190A0007A (Murata) directional coupler.

This has a coupling factor of approximately 19 dB for its lower

frequency band (897.5

± 17.5 MHz) and 14 dB for its upper band

(1747.5

± 37.5 MHz) and an insertion loss of 0.38 dB and 0.45 dB

respectively. Because the PF08107B transmits a maximum power

level of +35 dBm, additional attenuation of 15 dB is required

before the coupled signal is applied to the AD8314.

1

2

3

4

ENBL

RFIN

AD8314

8

7

6

5

VSET

FLTR

VPOS

COMM

V UP

2.7V

VSET

0V–1.1V

PF081807B

(HITACHI)

PIN BAND 1

+3dBm

PIN BAND 2

+3dBm

1000pF

0dBm

MAX

ATTN

15dB

V DN

220pF

POUT BAND 2

+32dBm MAX

POUT BAND 1

+35dBm MAX

4.7 F

TO

ANTENNA

49.9

7

8

5

1

4

3

26

LDC15D190A0007A

BAND

SELECT

0V/2V

3.5V

0.1 F

52.3

Figure 34. A Dual Mode Power Amplifier Control Circuit