Lambda SR1, Operation Manual, Page: 61 / 317

SR1 Operation 61
© 2014 Stanford Research Systems
Unit Description
FFS According to AES17-1998 (r2004), "Full-scale amplitude is the amplitude of a 997 Hz
sinewave whose positive peak value reaches the positive digital full scale leaving the
negative maximum code unused." The FFS (Fraction Full Scale) unit expresses the peak
amplitude of any SR1 generator waveform relative to this definition of full-scale amplitude.
When dither is on, full scale amplitude is reduced symmetrically by 1 bit to allow for
dither. Small values of FFS may be expressed as mFFS (milli-FFS) or uFFS (microFFS).
%FS Same as FFS (above) but expressed as a percentage of full-scale amplitude. 50 %FS = .5
FFS.
Vrms This digital waveform amplitude unit is directly convertible to FFS depending on the value
of the V FS (Volts Full Scale) set in the
references section of the digital generator panel..
If the value of V FS is 2 Vrms, for instance, then an amplitude of 1 Vrms corresponds to
an amplitude of .5 FFS. These units allow digital amplitudes to be expressed as analog
voltages as is often useful when working with D/A converters where the V FS value can be
thought of as the DAC's full scale output voltage.
Vp This unit of digital wave form amplitude is convertible to FFS depending on the value of the
V FS (Volts Full Scale)
reference and the fixed relationship of peak voltage to rms voltage
for a sinewave. If the value of V FS is 1 Vrms, for instance, then an amplitude of 1.414 Vp
would correspond to an amplitude of 1 Vrms which in turn would correspond to an
amplitude of 1 FFS.
Vpp This unit of digital wave form amplitude is convertible to FFS depending on the value of the
V FS (Volts Full Scale)
reference and the fixed relationship of peak voltage to peak-to-
peak voltage for a sinewave. If the value of V FS is 1 Vrms, for instance, then an
amplitude of 2.0 Vpp would correspond to an amplitude of 1 Vrms which in turn would
correspond to an amplitude of 1 FFS.
dec The peak value of the waveform expressed as a decimal code. The conversion of decimal
code to FFS depends on the setting of the Output Resolution in the
Digital I/O panel. For
the default 24-bit output resolution setting 1FFS = 2
23
-1 = 8,388,607 dec.
hex The peak value of the waveform expressed as a hexadecimal code. The conversion of
hexidecimal code to FFS depends on the setting of the Output Resolution in the
Digital I/
O panel. For the default 24-bit output resolution setting 1FFS = 2
23
-1 = 0x7fffff hex.
dBFS Decibels relative to the full scale amplitude definition. (See FFS above) For instance, 0.1
FFS = -20 dBFS. (20*log10(0.1))
dBV Decibels relative to the Vrms value calculated with the V FS (Volts Full Scale) reference
set in the digital generator panel. For example, with a V FS value of 2 Vrms an amplitude
of -20 dBVrms corresponds to .1 Vrms which in turn corresponds to an amplitude of 0.05
FFS.
dBu Decibels relative to 0.7746 Vrms. (Hiistorically the value of 0.7746 Vrms represents the
voltage required to dissipate 1 mW in a 600 load.) If V FS is set to 1 Vrms than an
amplidue of 0 dBu correesponds to 0.7746 which in turn corresponds to 0.7746 FFS.
dBr Decibels relative to the dBr reference set in the references section of the digital generator
panel. The dBr reference is always set in units of FFS. Thus, with the dBr reference set to
.5 FFS, an amplitude of 0 dBr corresponds to .5 FFS.
Bits Computed from dBFS using the relation bits = 1.76 + (dBFS/6.02). For instance, half
scale (-6.02 dBFS) is 0.76 bits.
Digital Generator Frequency Units
The following table describes the units used by SR1 in setting the frequency of analog generator