CN-0268

Circuit Note

Rev. 0 | Page 4 of 6

Figure 6 and Figure 7 shows the resulting antialiasing filter

response with a 1 dB bandwidth of 41 MHz and a 3 dB bandwidth

of 89 MHz, centered at an IF of 203 MHz. Figure 8 shows the FFT

spectrum for the final receiver circuit of Figure 1, where the SNR is

72.5 dBFS, and the SFDR performance approaches 90 dBc.

0

100

200

FREQUENCY (MHz)

300

400

–14.286

–8.571

–2.857

2.857

8.571

14.286

–20.000

20.000

–3.5

–3.0

–2.5

–2.0

–1.5

–1.0

–0.5

0

120

140

160

180

200

220

240

FREQUENCY (MHz)

1dB BW = 41MHz

3dB BW = 89MHz

–130

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0

0

20

40

60

80

100

120

FREQUENCY (MHz)

SNR = 72.5dB

SFDR = –88.9dBc

H2/H3 = –89.1dBc/–88.9dBc

Figure 8. Single Tone FFT Plot, Input = 203 MHz,

Sampling Rate = 245.76 MSPS

Using ADS as a simulation tool, the filter components can

be further tuned to shift the resonant peak to the desired IF.

For example, by changing the parallel 8.2 pF capacitor of the

antialiasing filter to 10 pF shifts the resonance peak lower to

180 MHz. Figure 9 through Figure 11 show the filter profile

and single-tone FFT performance for this condition.

0

100

200

FREQUENCY (MHz)

300

400

–14.286

–8.571

–2.857

2.857

8.571

14.286

–20.000

20.000

–3.5

–3.0

–2.5

–2.0

–1.5

–1.0

–0.5

0

100

120

140

160

180

200

220

240

FREQUENCY (MHz)

1dB BW = 40MHz

3dB BW = 75MHz

1 dB and 3 dB Bandwidth

–130

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0

0

20

40

60

80

100

120

FREQUENCY (MHz)

SNR = 73dB

SFDR = –91dBc

H2/H3 = –94dBc/–91dBc

Figure 11. Single Tone FFT Plot, Input = 183 MHz,

Sampling Rate = 245.76 MSPS