Data Sheet

ADL5391

Rev. A | Page 11 of 15

Matching the Input/Output

The input and output impedances of the ADL5391 change over

frequency, making it difficult to match over a broad frequency

range (see Figure 15 and Figure 16). The evaluation board is

matched for lower frequency operation, and the impedance

change at higher frequencies causes the change in gain seen in

Figure 6. If desired, the user of the ADL5391 can design a

matching network to fit their application.

Wideband Voltage-Controlled Amplifier/Amplitude

Modulator

Most of the data for the ADL5391 was collected by using it as a

fast reacting analog VGA. Either X or Y inputs can be used for

the RF input (and the other as the very fast analog control),

because either input can be used from dc to 2 GHz. There is a

linear relationship between the analog control and the output of

the multiplier in the VGA mode. Figure 6 and Figure 7 show the

dynamic range available in VGA mode (without optimizing the

dc offsets).

The speed of the ADL5391 in VGA mode allows it to be used as

an amplitude modulator. Either or both inputs can have

modulation or CW applied. AM modulation is achieved by

feeding CW into X (or Y) and adding AM modulation to the Y

(or X) input.

Squaring and Frequency Doubling

Amplitude domain squaring of an input signal, E, is achieved

simply by connecting the X and Y inputs in parallel to produce

through a balun (frequency range and dynamic range can be

limited if used single ended).

When the input is a sine wave Esin(ωt), a signal squarer behaves

as a frequency doubler, because

[

]

(

)

(

)

t

E

t

E

2

ω

−

=

ω

2

cos

1

2

)

sin(

2

(3)

Ideally, when used for squaring and frequency doubling, there is

no component of the original signals on the output. Because of

internal offsets, this is not the case. If Equation 3 were rewritten

to include theses offsets, it could separate into three output

terms (Equation 4).

[

]

[

]













+

+

ω

+

ω

=

+

ω

×

+

ω

2

)

sin(

2

)

cos(2

2

)

sin(

)

sin(

2

2

2

E

OFST

OFST

t

E

t

E

OFST

t

E

OFST

t

E

(4)

where:

The input signal bleedthrough is 2Esin(ωt)OFST.

The dc component of the output is related to the square of both

the offset (OFST) and the signal input amplitude (E). The offset

can be found in Figure 4 and is approximately 20 mV. The

second harmonic output grows with the square of the input

amplitude, and the signal bleedthrough grows proportionally

with the input signal. For smaller signal amplitudes, the signal

bleedthrough can be higher than the second harmonic component.

As the input amplitude increases, the second harmonic component

grows much faster than the signal bleedthrough and becomes

the dominant signal at the output. If the X and Y inputs are

driven too hard, third harmonic components will also increase.

For best performance creating harmonics, the ADL5391 should

be driven differentially. Figure 17 shows the performance of the

ADL5391 when used as a harmonic generator (the evaluation

board was used with R9 and R10 removed and R2 = 56.2 Ω). If

dc operation is necessary, the ADL5391 can be driven single ended

(without the dc blocks). The flatness of the response over a broad

frequency range depends on the input/output match. The

fundamental bleed through not only depends on the amount of

power put into the device but also depends on matching the

unused differential input/output to the same impedance as the

used input/output. Figure 18 shows the performance of the

ADL5391 when driven single ended (without ac coupling

capacitors), and Figure 19 shows the schematic of the setup. A

resistive input/output match were used to match the input from

dc to 1 GHz and the output from dc to 2 GHz. Reactive matching

can be used for more narrow frequency ranges. When matching

the input/output of the ADL5391, care needs to be taken not to

load the ADL5391 too heavily; the maximum reference current

available is 50 mA.

–15

–65

–60

–55

–50

–45

–40

–35

–30

–25

–20

10

100

200

300

400

500

600

700

800

900

1000

FREQUENCY (MHz)

SECOND HARMONIC GAIN

BLEEDTHRU GAIN

THIRD HARMONIC GAIN

Figure 17. ADL5391 Used as a Harmonic Generator