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ADL5331 Datasheet(PDF) 12 Page - Analog Devices |
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ADL5331 Datasheet(HTML) 12 Page - Analog Devices |
12 / 15 page ADL5331 Data Sheet Rev. A | Page 12 of 15 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 CH1 250mV Ω CH2 200mV 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 ∆t: –2.08µs 1/∆t: –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 |
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