AD8364-EVAL-500 Analog Devices, AD8364-EVAL-500 Datasheet - Page 35

• Manufacturer Part Number: AD8364-EVAL-500
• Description: LF to 2.7GHz, Dual 60dB TruPwr™ Detector; Package: EVALUATION BOARDS; No of Pins: -; Temperature Range: Commercial
• Manufacturer: Analog Devices

CHANNEL ISOLATION
Isolation must be considered when using both channels of the
AD8364 at the same time. The two isolation requirements that
should 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 should 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’s 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 75. 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 its lowest detectable
power. Worst-case isolation happens when the low power channel
is at a nominally low power level, as chosen in Figure 76. 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
would have a ripple that would look 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 frequencies 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 76 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 77),
due to the response roll-off within AD8364.
Rev. 0 | Page 35 of 48
Figure 76. Apparent Measurement Error Due to Overall Channel-to-Channel
Figure 77. Improved Measurement Error with Increased Frequency
16
14
12
10
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90
8
6
4
2
0
–20
10
7
6
5
4
3
2
1
0
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.
–20
2500MHZ
A–>B 2140MHz
A–>B
B–>A 2500 MHz
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
A–>B 1880MHz
Figure 75. RF Channel Input-to-Input Isolation
B–>A 2140MHz
–15
B–>A 1880MHz
INTERFERING CHANNEL AMPLITUDE (dBm)
–15
A–>B 880MHz
INTERFERING CHANNEL AMPLITUDE (dBm)
–10
–10
100
B->A
FREQUENCY (MHz)
Cross-Coupling
–5
Separation
–5
B–>A 880MHz
0
0
1,000
A->B
450 MHz
5
A–>B
5
10
450 MHz
10
B–>A
B->A
A->B
AD8364
15
10,000
15