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90 Appendix B: Theory of Operation

Instead of computing the magnitude-squared of the DFT to obtain an estimate of the PSD, the

5120A computes the cross-spectrum of two identical measurement systems as shown in Figure B-

6. The ADC noise is highly uncorrelated and averages down, allowing improvement of the

broadband noise floor to better than –170 dBc/Hz.

Figure B-6: 5120A and 5120A-01 with external reference: The PSD is computed as the cross-spectrum of the DFTs

from two identical processing front ends.

The 5120A-01 computes the cross-spectrum of two identical measurement systems as shown in

Figure B-7. The ADC noise and the internal reference oscillator noise are highly uncorrelated and

average down, allowing improvement of the broadband noise floor to fall below –170 dBc/Hz with

only one instrument input.

Figure B-7: 5120A-01 with internal references: The PSD is computed as the cross-spectrum of the DFTs from two

identical processing front ends.

Sample,

Down-convert,

& Transform A

Input 1

Reference 2

–

Multiply

ADC1A

ADC2A

–+–

Sample,

Down-convert,

& Transform B

∗

ADC1B

∗

ADC2B

–+–

Uncorrelated

Sample,

Down-convert,

& Transform A

Input

ADC1A

ADC2A

–+–

Sample,

Down-convert,

& Transform B

∗

ADC1B

∗

ADC2B

–+–

Uncorrelated

Internal

Reference 2

Multiply

Internal

Reference 1

Brand	Symmetricom
Model	5120A
Category	Recording Equipment
Language	English

Symmetricom 5120A User Manual

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Symmetricom 5120A Specifications