Tektronix 1L30 Instruction Manual page 25

N O T
Accuracy of this measurement- depends on the DIS­
PERSION RANGE settin gs.
section.
Frequency S tab ility
The Type 1L30 may be used to measure both long and
short term frequency instabilities, when the local oscillator is
phase locked to a stable crystal-controlled reference fre­
quency. See Stability in Characteristics section.
Short term stability measurements apply to fast frequency
changes such as those caused by power supply noise and
ripple, vibration or other random factors.
the random frequency modulation characteristics of a kly­
stron.
Fig. 2-20 Short term stability msasurment.
istic of a klystrom. DISPERSION is 2 kHz/CM and RESOLUTION
is 1 kHz. Oscillator FM is about 6 kHz.
Long term stability measurements require a recorder, a
series of photographs, or the use of a storage oscilloscope to
show frequency drift as a function of time.
compensation can be computed by this process.
Amplitude M o dulation
Modulating frequency or frequencies and modulation per­
centage are the quantities most often desired from an AM
signal measurement. Fig. 2-13 illustrates some amplitude
modulated signals, the methods to measure the modulating
frequency, and modulation percentage.
Over-modulation produces extraneous sidebands resulting
in a spectrum that is very similar re the spectrum of a multi­
frequency modulated carrier.
distinguished from the multi-frequency modulated display
because the spacing between sidebands is equal, while the
sidebands in a multi-frequency spectrum will be arbitrary
unless the modulating frequencies are harmonically related.
The 'over-modulared carrier spectrum is usually symmetrical,
whereas the spectrum of a multi-frequency modulated sig­
nal is asymmetrical in amplitude.
l
See Characteristics sec-
Fig. 2-20 shows
Random FM character­
Temperature
Over-modulation is usually
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Operating Instructions— Type 1L30
Frequency Modulated Spectrum
FM measurements generally determines the modulating fre-
quency, amplitude of the modulating signal or frequency
deviation, and index of modulation. A typical FM spectrum
is shown in Fig. 2-15. The exterior modulation envelope
resembles a cos2 curve, which' is an identifying feature of
the frequency modulated carrier.
Frequency Deviation Measurement
There is no clear relationship between spectral width and
deviation, because in theory the FM spectrum approaches
infinity. In practice, however, the spectral level falls .quite
rapidly. See Fig. 2-15B. Accurate deviation measurements
can be made if the modulating frequency and the modulation
index (where the carrier goes to zero) are known.
Carrier deviation
Modulation Index =
Modulating frequency
Values of modulation index corresponding to zero carrier
amplitudes are listed in Table 2-1.
TABLE 2-1
Values of modulation index for carrier
null points
Order of Carrier Null
1
2
3
4
n (n > 4 )
Accurate carrier null is essential for accurate measurement.
Analysis and Measurement from the
Spectrum of a Pulse Modulated Signal
An examination of the spectrum from a pulse modulated
device such as a radar transmitter, provides a variety of in­
formation about the system. The amount of frequency shift
(long term or short term) in the display indicates the stability
of the transmitter oscillator. The absence of deep or prom­
inent lobe minima's adjacent to the main lobe is an indica­
tion of frequency modulation, provided the resolving power
of the analyzer is sufficient. See Fig. 2-18C. Double peaks
in the main lobe indicate that the oscillator is operating
two or more modes, which could be caused by some ex­
ternal load such as mismatched transmission lines or fluctu­
ating supply voltages. A visual indication is provided to
tune the transmitting system so that most of the output power
is within the frequency bandwidth of the receiving system.
The following measurements may be performed from the
spectrum of a pulse modulated display.
Pulse Width: The theoretical pulse width for a square
wave is the reciprocal of the spectral side lobe frequency
width. The main frequency lobe or its side lobes can there­
'
Modulation Index
2.4
5.52
8.65
11.79
11.79 + (n - 4 )
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