AD834

Data Sheet

Rev. F | Page 16 of 20

FREQUENCY DOUBLER

Figure 21 shows another squaring application. In this case, the

output filter has been removed and the wideband differential

output is converted to a single-sided signal using a balun, which

consists of a length of 50 Ω coaxial cable fed through a ferrite

core (Fair-Rite Type 2677006301). No attempt is made to reverse

terminate the output. Higher load power can be achieved by

replacing the 50 Ω load resistors with ferrite bead inductors.

The same precautions should be observed with regard to printed

circuit board (PCB) layout as recommended in the Power

Measurement (Mean-Square and RMS) section. The output

spectrum shown in Figure 22 is for an input power of +10 dBm

at a frequency of 200 MHz. The second harmonic component at

400 MHz has an output power of −15 dBm. Some feedthrough

of the fundamental occurs; it is 15 dB below the main output. A

spurious output at 600 MHz is also present, but it is 30 dB

below the main output. At an input frequency of 100 MHz,

the measured power level at 200 MHz is −16 dBm, while the

fundamental feedthrough is reduced to 25 dB below the main

output; at an output of 600 MHz the power is −11 dBm and the

third harmonic at 900 MHz is 32 dB below the main output.

SMA FROM

HP8656A

GENERATOR

SMA TO

HP8656A

SPECTRUM

ANALYZER

8

7

6

5

1

2

3

4

X2

Y1

Y2

AD834

0.1µF

0.1µF

0.1µF

0.1µF

75Ω

560pF

560pF

BALUN

+5V

10Ω

49.9Ω

49.9Ω

–5V

25Ω 125Ω

125Ω

25Ω

Figure 21. Frequency Doubler Connections

FREQUENCY (MHz)

–10

150

–20

–30

–40

–50

–60

–70

–80

–90

–100

0

200

250

300

350

400

450

500

550

600

650

Figure 22. Output Spectrum for Configuration of Figure 21

WIDEBAND THREE-SIGNAL MULTIPLIER/DIVIDER

Two AD834 devices and a wideband op amp can be connected

to make a versatile multiplier/divider having the transfer

function

(

)(

)

(

)

Z

U2

U1

Y2

Y1

X2

X1

W

+

−

−

−

=

with a denominator range of about 100:1. The denominator

input U = U1 − U2 must be positive and in the range 100 mV

to 10 V; X, Y, and Z inputs may have either polarity. Figure 23

shows a general configuration that may be simplified to suit a

particular application. This circuit accepts full-scale input voltages

of 10 V, and delivers a full-scale output voltage of 10 V. The optional

offset trim at the output of the AD834 improves the accuracy for

small denominator values. It is adjusted by nulling the output

voltage when the X and Y inputs are zero and U = 100 mV.

The op amp is internally compensated to be stable without the

use of any additional HF compensation. As Input U is reduced,

the bandwidth falls because the feedback around the op amp is

proportional to Input U. Note that, this circuit was originally

characterized using the AD840 op amp; some alternative op

amps include the AD818 and the AD8021.

This circuit can be modified in several ways. For example, if the

differential input feature is not needed, the unused input can be

connected to ground through a single resistor, equal to the parallel

sum of the resistors in the attenuator section. The full-scale input

levels on X, Y, and U can be adapted to any full-scale voltage

down to ±1 V by altering the attenuator ratios. Note, however,

that precautions must be taken if the attenuator ratio from the

output of A3 back to the second AD834 (A2) is lowered. First,

the HF compensation limit of the op amp may be exceeded if

the negative feedback factor is too high. Second, if the attenuated

output at the AD834 exceeds its clipping level of ±1.3 V, feedback

control is lost and the output suddenly jumps to the supply rails.

However, with these limitations understood, it is possible to adapt

the circuit to smaller full-scale inputs and/or outputs, for use

with lower supply voltages.