Symmetricom 5115A, Operation And Maintenance Manual

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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
S
Φ
1
Φ
2
–
Multiply
Φ
1 A
Φ
2 A
Φ
ADC1A
Φ
ADC2A
– + –
Sample,
Down-convert,
& Transform B
Σ
Σ
Φ∗
1 B
Φ∗
2 B
Φ∗
ADC1B
Φ∗
ADC2B
– + –
Uncorrelated
Sample,
Down-convert,
& Transform A
Input
S
Φ
1
Φ
1 A
Φ
2 A
Φ
ADC1A
Φ
ADC2A
– + –
Sample,
Down-convert,
& Transform B
Σ
Φ∗
1 B
Φ∗
2 B
Φ∗
ADC1B
Φ∗
ADC2B
– + –
Uncorrelated
Internal
Reference 2
2
3
1
Multiply
Internal
Reference 1