ADL5331

Rev. 0 | Page 13 of 16

For the AGC loop to remain in equilibrium, the AD8319 must

track the envelope of the output signal of the ADL5331 and

provide the necessary voltage levels to the gain control input

of the ADL5331. Figure 21 shows an oscilloscope of the AGC

loop depicted in Figure 19. A 100 MHz sine wave with 50% AM

modulation is applied to the ADL5331. The output signal from the

VGA is a constant envelope sine wave with amplitude corres-

ponding to a setpoint voltage at the AD8319 of 1.3 V. The gain

control response of the AD8319 to the changing input envelope

is also shown in Figure 21.

AD8319 OUTPUT

CH3 250mV Ω

M2.00ms

A CH4

1.80V

1

2

3

T

0.00000s

T

T

ADL5331 OUTPUT

AM MODULATED INPUT

Figure 21. Oscilloscope Showing an AM Modulated Input Signal and the

Response from the AD8319

Figure 22 shows the response of the AGC RF output to a pulse

on VSET. As VSET decreases from 1.5 V to 0.4 V, the AGC loop

responds with an RF burst. In this configuration, the input signal to

the ADL5331 is a 1 GHz sine wave at a power level of −15 dBm.

CH3 200mV Ω

CH2 500mV

M 4.0µs 12.5MS/s 80.0ns/pt

A CH1

150mV

2

t1: 4.48µs

t2: 2.4µs

Δt: –2.08µs

1/Δt: –480.8kHz

MEAN(C1) 440.3mV

CURS2 POS

2.4µs

CURS1 POS

4.48µs

AMPL(C1) 3.36V

AMPL(C2) 900mV

Figure 22. Oscilloscope Showing theResponse Time of the AGC Loop

Response time and the amount of signal integration are con-

trolled by CLPF. This functionality is analogous to the feedback

capacitor around an integrating amplifier. While it is possible

to use large capacitors for CLPF, in most applications, values

under 1 nF provide sufficient filtering.

More information on the use of AD8319 in an AGC application

can be found in the AD8319 data sheet.

CMTS TRANSMIT APPLICATION

Interfacing to AD9789

Because of its broadband operating range, the ADL5331 VGA

can also be used in direct-launch cable modem termination

systems (CMTS) applications in the 50 MHz to 860 MHz cable

band. The ADL5331 makes an excellent choice as a post-DAC

VGA in a CMTS application when used with the Analog

Devices AD9789 wideband DAC. The AD9789 also contains

digital signal processing specifically designed to process

DOCSIS type CMTS signals. A typical AD9789-to-ADL5331

interface is shown in Figure 23.

16mA

16mA

50Ω

55Ω

20Ω

AD9789

DAC

25Ω

70Ω

VGA

5V

SERIES

TERMINATION

Figure 23. Block Diagram of AD9789 interface to ADL5331 in a DOCSIS Type Application