DC1076A Analog Devices

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1076 FILTER DIPLEXER DESCRIPTION Demonstration circuit 1076 is a filter diplexer. This cir- hence the demo circuit name. However, a true diplexer cuit facilitates bench evaluation of high speed analog to of equivalent performance requires more parts and odd digital converters in an environment where coaxial cable component values to achieve good crossover perform- length exists between the ADC input and any non- ance. absorptive terminations such as bandpass filters that Design files for this circuit board are available. Call will reflect sampling impulses. Technically DC1076 is an the LTC factory. absorptive filter, not a true diplexer. True diplexers will PowerPath is a trademark of Linear Technology Corporation also work the original development used a true diplexer, Table 1. Performance Summary (T = 25C) A PARAMETER CONDITION VALUE LPF Bandwidth 3dB cutoff f > 150MHz Impedance 50W Out of Band Return Loss f > f > 20dB 3DB OPERATING PRINCIPLES When transmission line length exists between reflective tortion products. The schematic for DC1076 is shown in terminations and the input of a High speed ADC, the dis- Figure 1 and the layout in Figure 2, note that the filter is turbance caused by the sample switches opening and actually < 1.00 inch square. The sampling glitch exhib- closing reflect back and forth between the reflective ter- its very fast edges and therefore most of the frequency mination and the ADC input. In a practical layout, this content is at higher harmonic frequencies. Figure 3 transmission line length is very short and the reflections shows the filter response, S21, and input and output damp out between sample clocks. In a bench evaluation return losses, S11 and S22 respectively of the DC1076 environment, external connectorized filters and cables Filter Diplexer. Figure 4 shows the same parameters for often must be introduced between signal sources and a commercial 70MHz band pass filter, out of band en- the ADC demonstration circuit board. This can easily ergy is completely reflected back to the source. Figure 5 introduce 100x more low loss transmission line length shows the cascaded response of the commercial filter between the reflective termination of the filter and the with the DC1076. The combination provides the pass- ADC input. As a result, the sampling glitch disturbance band attribute of the 70MHz BPF while the return loss does not completely dissipate in the circuit losses be- presented to the ADC (S22) is 20dB in the upper stop tween sampling clock edges. This results in additional band to beyond 1GHz. The source of course will still see distortion products, particularly elevated harmonics in complete reflection of out of band energy as S11 is the ADC output. The DC1076 is an absorptive filter 0dB outside the pass band of the 70MHz BPF. which when placed directly on the input to the ADC aids in attenuating the sample glitches and reducing the dis- 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1076 FILTER DIPLEXER QUICK START PROCEDURE Demonstration circuit 1076 is easy to connect to a Lin- any additional hardware. The filter provides a well- ear Technology High Speed ADC board. The filter is pro- behaved low pass filter response with a wideband VSWR vided with a Male SMA output connector which will di- less than 1.25:1 or 20dB return loss to over 1GHz. It is rectly connect to the input of the ADC demonstration suitable for use with input frequencies up to 150MHz circuit without any adaptors. The input is fitted with a before insertion loss and phase delay begin to compro- Female SMA connector as used on the ADC demonstra- mise the input signal. For frequencies above this range, tion circuits and so it can be connected in line without consult factory. Figure 1. DC1076 Filter Diplexer Schematic 1.000 Inch Figure 2. DC1076 Filter Diplexer Board 2