Anritsu 37 C Series, Руководство по эксплуатации, Страница: 594 / 603

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Lowpass Mode:
This mode displays a response equivalent to
the classic "TDR" (Time Domain Reflectometer) response of the
device under test. Lowpass response may be displayed in either
the impulse or step mode. This type of processing requires a
sweep over a harmonic series of frequencies and an extrapolated
or user-entered DC value.
Bandpass Mode:
This mode displays a response equivalent to
the time response of the device under test to a band limited
impulse. This type of processing may be used with any arbitrary
frequency sweep range, limited only by the test set range or device
under test response.
Phasor Impulse Mode:
This mode displays a response similar
to the Lowpass impulse response, using data taken over an
arbitrary (band limited) sweep range. Detailed information, similar
to that contained in the lowpass impulse response may be used to
identify the nature of impedance discontinuities in the device under
test. Now, with Phasor Impulse, it is possible to characterize
complex impedances on band-limited devices.
Windowing:
Any one of four window functions may be applied to
the initial frequency data, to counteract the effects of processing
data with a finite bandwidth. These windows provide a range of
trade offs of main lobe width versus sidelobe level (ringing). The
general type of function used is the Blackman-Harris window with
the number of terms being varied from one to four. Typical
performance follows:
Gating:
A selective gating function may be applied to the time
domain data to remove unwanted responses, either in a pass-band
or reject-band (mask). This gating function may be chosen as the
convolution of any of the above window types with a rectangular
gate of user defined position and width. The gate may be specified
by entering start and stop times or center and span. The gated data
may be displayed in the time domain, or converted back to the
frequency domain.
Time Domain Display:
Data processed to time domain may be
displayed as a function of time or as a function of distance,
provided the dielectric constant of the transmission media is
entered correctly. In the case of dispersive media such as
waveguide or microstrip, the true distance to a discontinuity is
displayed in the distance mode. The time display may be set to any
arbitrary range by specifying either the start and stop times or the
center time and span. The unaliased (non-repeating) time range is
given by the formula:
Number of Frequency Data Points
Unaliased Range (ns) =
Frequency Sweep Range (GHz)
The resolution is given by the formula:
Main Lobe Width (null
−
null) in ns =
kW
Freq. Sweep Range (GHz)
where kW is two times the number of window terms
(for example, four for a two-term window)
For a 40 GHz sweep range with 1601 data points, the unaliased
range is 40.025 nanoseconds. For a 65 GHz sweep with 1601 data
points, the unaliased range is 24.646 nanoseconds.
Frequency with Time Gate:
Data that has been converted to
time domain and selected by the application of gating function may
be converted back to the frequency domain. This allows the display
of the frequency response of a single element contained in the
device under test. Frequency response accuracy is a function of
window and gate type, and gate width. For a full reflection,
minimum gate and window accuracy is within 0.2 dB of the
ungated response over a 40 GHz range.
Type of Window
(Number of Terms)
First Side Lobe
Relative to Peak
Rectangle (1) -13 dB
Nominal-Hamming (2) -43 dB
Low Side Lobe,
Blackman-Harris (3)
-67 dB
Minimum Side Lobe,
Blackman-Harris (4)
-92 dB 2.7W
2.1W
1.8W
1.2W
Impulse Width 1
1
W(Bin Width) = 1/2
∆
f sweep width.
Example. When
∆
f = 40 MHz to 40 GHz, W = 12.5 ps
When
∆
f = 40 MHz to 65 GHz, W = 7.7 ps