2011 EMC Directory & Design Guide - Interference Technology

filters
Figure 5. Notch filter test.
system. The required notch response is 36 dB. According
to Figure 5, the time-domain measurement system shows
a notch response of better then 38 dB.
MULTI-STAGE BROADBAND DOWN-CONVERTER
As illustrated in Figure 2, a multi-stage broadband downconverter
was added to enable measurements above 1.1
M e a s ur e m e n t s a b o v e 1 GHz in Time-Domain
GHz. For measurements from 1.1-6 GHz, the EMI input
signal is down-converted to the range below 1.1 GHz, where
it is sampled by the floating-point ADC. The amplitude
spectrum is displayed.
For measurements from 6-18 GHz, an additional mixer
stage down-converts the input frequency band to the
frequency range from 1.1-6 GHz. Subsequently, it is downconverted
and processed like described above. A basic prototype
of the time-domain EMI measurement system up to
18 GHz was presented in [4]. In comparison, the presented
system is fully compliant to CISPR 16-1-1, enabling full
compliance measurements from 9 kHz-18 GHz.
1.1 - 6 GHz Down-Converter
Because of the nonlinear characteristics of mixers, a large
number of mixing products are generated at its output.
These frequencies f IF
are determined by [8]
f m,±n
IF
= |m · f LO ± n · f RF | , m, n ∈ N, (3)
where f RF
is the RF input frequency and f LO
is the local
oscillator frequency.
If only the fundamental frequencies of f LO
and f RF
are
taken into consideration, i.e. m, n = 1, we obtain two frequency
components f RF
1,2 according to (3)
f RF 1,2 = |f LO ± f IF |. (4)
The frequency conversion yields two sidebands. The image
frequency signal is converted to the same intermediate
frequency as the desired signal.
To avoid this, a two-stage mixer system is used in the
1.1 - 6 GHz down-converter [9]. The block diagram of the
1.1 - 6 GHz down-converter is shown in Figure 6. The input
band is divided into 14 subbands with a bandwidth of 325
MHz each. Each of those bands is sequentially up-converted
to a first high intermediate frequency band which is located
above the input frequency band. A second mixer downconverts
the IF-band to the range below 1.1 GHz, where it is
sampled by the floating-point ADC. A fixed bandpass-filter
is sufficiently suppressing the image band, because the input
band and the image band do not overlap spectrally. This
preselection filter also enhances the spurious-free dynamic
range of the system by preventing the LNA and mixers being
driven into saturation by high-level narrowband and
broadband out-of-band EMI.
6 - 18 GHz Down-Converter
To extend the upper frequency limit of the time-domain-
EMI measurement system to 18 GHz, a third mixer stage is
added. The block diagram of the 6 - 18 GHz down-converter
is shown in Figure 7. For measurements above 6 GHz, the
preselection is dividing the input band into three ultrabroadband
subbands: band 1 from 6 - 9 GHz, band 2 from
9 - 13 GHz and band 3 from 13 - 18 GHz. The switching
between these bands is done via broadband, low-loss, singleinput,
triple-output (SP3T) PIN-diode switches. These ultra-
118 interference technology emc Directory & design guide 2011