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signalpath.national.com/designer

SIGNAL PATH

designer

1st Nyquist Zone

2nd Nyquist Zone

3rd Nyquist Zone

4th Nyquist Zone

Frequency

Input

Signal

Input

Image

Input

Image

5th Nyquist Zone

Unwanted Input

Signal Spur

Input

Image

Input

Image

Low-Pass Filter at

ADC Input

Wanted Signal

Band

ADC Dynamic

Range

To use the full ADC dynamic range,

ensure that any undesired, out-of-band

signal components are filtered to less

than the ADC Least Significant Bit

(LSB) level. This requires high-order

filters to obtain a sufficiently sharp roll

off if the wanted and unwanted input-

signal components approach too close

One solution is to increase the ADC

sample rate and over-sample the input

signal. This spreads the Nyquist zones

further out in frequency and relaxes

the channel-filter design (Figure 2d).

High-speed baseband sampling is

found in many test and measurement

applications requiring data conversion

from DC to GHz.

An under-sampled system employs an

ADC with a full-power bandwidth

is not unusual to find a 1 GHz-input

bandwidth on a 100 MHz-sampling

ADC. This allows a narrowband in-

under-sampled at a rate much lower

and aliased or “folded” back down to

the 1st Nyquist zone. This is shown in

Figure 3a where signal A is the desired

signal being converted.

At higher input frequencies, the

input stage of the ADC becomes slew-

rate limited. For optimum distortion

performance from the ADC, it is

recommended to keep the center

frequency of the under-sampled

signal to no more than 10% to 30%

of the ADC’s full-power bandwidth depending on the

performance of the ADC.

In an under-sampled system, the channel filter is the

key to ensuring that the desired signal is optimally

recovered at baseband and separated from all the

1st Nyquist Zone

2nd Nyquist Zone

Frequency

Input

Signal

Unwanted Input

Signal Spur

Input

Image

Input

Image

Low-Pass Filter at

ADC Input

Wanted Signal

Band

Input Signal Spur

Attenuated by Filter

ADC Dynamic

Range

1st

Nyquist

Zone

Wanted

Input

Signal

A

2nd

Nyquist

Zone

3rd

Nyquist

Zone

4th

Nyquist

Zone

5th

Nyquist

Zone

6th

Nyquist

Zone

7th

Nyquist

Zone

8th

Nyquist

Zone

9th

Nyquist

Zone

10th

Nyquist

Zone

11th

Nyquist

Zone

12th

Nyquist

Zone

Frequency

Image

of A

Image

of A

Image

of A

Image

of A

Image

of A

Image

of A

Alias

of A

Image

of A

Image

of A

Image

of A

Image

of A

Image

of A

Figure 2d. 1st Nyquist baseband >2x over-sampling with ‘relaxed’

low-pass filter requirement

Figure 3a. Wanted signal A >fs under-sampled from 8th Nyquist zone

back to 1st Nyquist zone

Figure 2c. 1st Nyquist baseband sampling with low-pass filter

other aliased components. A bandpass filter is used

to remove all interfering frequencies and noise from

the ADC input which might otherwise alias back to

baseband with the wanted signal. Figure 3b shows

the effects of a second unwanted signal B folding

back from the 7th Nyquist zone to interfere with

s

s

s

s

1st

s

Frequency

2f

f

3f

4f

Nyquist

Zone

Wanted

Input

Signal

A

5f

2nd

Nyquist

Zone

3rd

Nyquist

Zone

4th

Nyquist

Zone

5th

Nyquist

Zone

6th

Nyquist

Zone

7th

Nyquist

Zone

8th

Nyquist

Zone

9th

Nyquist

Zone

10th

Nyquist

Zone

11th

Nyquist

Zone

12th

Nyquist

Zone

Unwanted

Signal B

Alias of B

Inteferes

with Alias

of A

Image

of B

Image

of B

Image

of B

Image

of B

Image

of B

Image

of B

Image

of B

Image

of B

Image

of B

Figure 3b. Failure to bandpass filter unwanted signal B allows it to alias back

to the 1st Nyquist zone and interfere with the recovery of wanted signal A

SignalPathDesigner.indd 3

SignalPathDesigner.indd 3

9/5/07 3:24:31 PM

9/5/07 3:24:31 PM