AD8364ACPZ-R2 datasheet(34/44 Pages) AD

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AD8364

Data Sheet

Rev. C | Page 34 of 44

CHANNEL ISOLATION

Isolation must be considered when using both channels of the

AD8364 at the same time. The two isolation requirements that

must be considered are the isolation from one RF channel input

to the other RF channel input and the isolation from one RF

channel input to the other channel output. When using both

channels of the AD8364, care must be taken in the layout to

isolate the RF inputs from each other. Coupling on the PC

board affects both types of isolation.

In most applications, the designer has the ability to adjust the

power going into the AD8364 through the use of different

valued temperature-stable couplers and accurate temperature-

stable attenuators. When isolation is a concern, it is useful to

adjust the input power so the lowest expected detectable power

is not far from the lowest detectable power of the AD8364 at the

frequency of operation. The AD8364 lowest detectable power

point has little variation from part to part and is not affected by

the balun. This equalizes the signals on both channels at their

lowest possible power level, which reduces the overall isolation

requirements and possibly adds attenuators to the RF inputs of

the device, reducing the RF channel input isolation requirements.

Measuring the RF channel input to the other RF channel input

isolation is straight forward, and the result of such an exercise is

shown in Figure 73. Note that adding an attenuator in series with

the RF signal increases the channel input-to-input isolation by

the value of the attenuator.

The isolation between one RF channel input and the other channel

output is a little more complicated. Do not assume that worst-

case isolation happens when one RF channel has high power

and the other RF channel is set at the lowest detectable power.

Worst-case isolation happens when the low power channel is at a

nominally low power level, as chosen in Figure 74. If the inputs

to both RF channels are at the same frequency, the isola-tion

also depends on the phase shift between the RF signals put into

the AD8364. This can be seen by placing a high power signal on

one RF channel input and another signal (low power) slightly offset

in frequency to the other RF channel. If the output of the low power

channel is observed with an oscilloscope, it has a ripple that looks

similar to a full-wave rectified sine wave with a frequency equal

to the frequency difference between the two channels, that is, a

beat tone. The magnitude of the ripple reflects the isolation at a

specific phase offset (note that two signals of slightly different

freq-uencies act like two signals with a constantly changing

phase), and the frequency of that ripple is directly related to the

frequency offset. The data taken in Figure 74 assumes worst-case

amplitude and phase offset. If the RF signals on Channel A and

Channel B are at significantly different frequencies, the input-to-

output isolation increase, depending on the capacitors placed

on CLP[A, B] and CHP[A, B] and the frequency offset of the two

signals (Figure 75), due to the response roll-off within AD8364.

FREQUENCY (MHz)

10,000

100

1,000

–40

–50

–60

–70

–80

–90

–45

–55

–65

–75

–85

A->B

B->A

Figure 73. RF Channel Input-to-Input Isolation

INTERFERING CHANNEL AMPLITUDE (dBm)

–20

–10

–15

–5

B–>A

450 MHz

A–>B

450 MHz

B–>A 880MHz

A–>B 880MHz

B–>A 1880MHz

A–>B 1880MHz

B–>A 2140MHz

A–>B 2140MHz

B–>A 2500 MHz

A–>B

2500MHZ

PEAK INTERFERENCE (IN dB) TO A –45dBm INPUT SIGNAL

DUE TO AN INTERFERING SIGNAL ON THE OTHER

CHANNEL. A->B = A INTERFERING WITH B, X-AXIS IS

CHANNEL A INPUT B->A = B INTERFERING WITH A, X-AXIS

IS CHANNEL B INPUT FREQUENCY SEPARATION OF THE

TWO CHANNELS = 1kHz. SEE CHARACTERIZATION

DESCRIPTION SECTION FOR MORE INFORMATION.

Figure 74. Apparent Measurement Error Due to Overall Channel-to-Channel

Cross-Coupling

–20

–15

–10

–5

INTERFERING CHANNEL AMPLITUDE (dBm)

B->A

A->B

PEAK INTERFERENCE (IN dB) TO A -45dBm INPUT

SIGNAL DUE TO AN INTERFERING SIGNAL ON THE

OTHER CHANNEL.

A->B = A INTERFERING WITH B, X-AXIS IS CHANNEL A

INPUT Freq chA = 2500 MHz Freq CHB = 1880MHz

B->A = B INTERFERING WITH A, X-AXIS IS

CHANNEL B INPUT Freq CHB = 2500 MHz

Freq CHA = 1880 MHz

FREQUENCY SEPARATION OF THE

TWO CHANNELS = 620 MHz.

SEE CHARCTERIZATION DESCRIPTION

SECTION FOR MORE INFORMATION

Figure 75. Improved Measurement Error with Increased Frequency

Separation

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