Lambda SR1, Operation Manual, Page: 140 / 317

SR1 Operation Manual 140
© 2014 Stanford Research Systems
Residual Spectrum of 25 kHz, 1 Vrm s Sine from Com m ercial Function Generator
As an example, the spectrum above shows the residual spectrum from a 25 kHz 1 Vrms sine wave
generated by a commercial function generator. The THD+N in a 200 kHz bandwidth as measured by the
TDD is -58.6 dB— hardly audio quality, but within the specifications of this function generator. The
residual spectrum reveals much more information than the single THD+N value does. The first thing we
notice is the profile of the notch filter with the residual fundamental poking through bottom at about -144
dBVrms. The THD+N is clearly dominated by discrete frequencies, rather than noise. In this case,
interestingly, the dominant distortion product is not a harmonic, but the 12.5 kHz subharmonic. As it
turns out this function generator generates it's signals at 1/2 the output frequency and employs a
frequency doubler to create the final signal— the subharmonic is the residual of the original pre-doubler
signal.
By using the "FFT Cursors" to display the integrated power in the residual spectrum it is possible to
directly read out the THD+N due to any part of the residual spectrum between the cursors. This
information can be invaluable in deciding which portions of a circuit to optimize.
When using the FFT1 analyzer in combination with the TDD to measure low levels of THD+N
be sure the TDD is set to the Hi-Bandwidth converter. Only the Hi-Bandwidth converter signal
chain employs the analog notch filter and and analog post-filter gain that enables making the
most sensitive THD+N measurements.