LAPLACE INSTRUMENTS RF300 Mk II-A, User Manual

Page: 31 / 32

Laplace Instruments Ltd
Page 31
Note 1.
The details of the LLA were given in Annex B of CISPR15. These are now transferred
to Annex C of CISPR16. Most of the content has remained the same, but Table 1
summarises the changes.
Note 2
CISPR15 gave the verification data as a plot of loop current in dBuA vs frequency for
the standard test signal (1V, open circuit voltage with a source impedance of
50ohm). This seems to be a straightforward method, especially as the limits are
quoted in dBuA, so it’s a direct correlation between the calibration loop and the
limits.
CISPR16 is essentially the same information, but presented differently. It specifies
the relationship between the source voltage (1V, as specified above) and the output
current in the loop as measured by the current probe. Note that the current probe
has a transfer characteristic of 1V/A. The relationship between volts and current is
ohms, hence the use of dB(ohms) as the ‘validation factor’.
The result is therefore a conversion factor scaled in dB(
Ω ) to convert current to
voltage,
CISPR16 defines the validation factor dB(
Ω ) = 20*log(Vs/Ii) where Vs is the source
voltage and Ii is the loop current.
Vs = 1V = 1,000,000uV
Under ‘old’ CISPR15, for Ii @ 100KHz = 46dBuA = 200uA
So the new CISPR16 value is 20*log(1000000/200) = 74 dB(
Ω )
and
Old CISPR15 for Ii @ 30MHz = 29dBuA = 29uA
So the new CISPR16 value is 20*log(1000000/29) = 91 dB(
Ω )
These calculations confirm the relationship between the CISPR15 plot and the
CISPR16 validation factor.
The plots in the standards assume a current probe with a 1V/A transfer function.
Such probes are ‘active’ but provide a flat frequency response. The RF300 uses
passive probes which have a non-flat frequency response. This is not important if the
probe is ‘inside’ the calibration loop and has a linear transfer function with amplitude.
These factors hold true for the probe that is used. So the RF300 antenna uses an
antenna factor correction to produce a calibration that agrees with the validation
factor. This antenna factor is supplied with each antenna, and is equivalent to the
correction factors as supplied with all EMC antennas, test cells, LISNs and other types
of transducer.
Using the antenna factor data with the RF300 enables the output to be compared
directly with the limits as specified in EN55015.