Tektronix 1L30 Instruction Manual page 22

Operating Instructions— Type 1L30
Fig. 2-16. Formation of a pulse modulated signal spectrum.
envelope however, can be plotted with pulses. If the an­
alyzer swept frequency is slow, it will plot a series of pips
or lines, the locus of which represents the relative energy
distribution of the swept spectrum. The number or density
of these pips for a given PRF will depend on the sweep speed,
or Time/Cm selection of the plug-in oscilloscope. It is pos­
sible, by sweeping very slowly, to obtain the spectrum of
a very low PRF signal. This display simulates a pulsed spec­
trum and contains the same information for analysis.
This spectrum may now be resolved, since the resolution
bandwidth of the analyzer need only be less than the side
lobe .frequency width, or the reciprocal of the modulating
pulse width. Fig. 2-18 illustrates the effect of frequency
modulation on the pulse modulated display.
The peak amplitude of the main lobe of a pulse modu­
lated RF spectrum represents only a portion of the total
energy contained in the lobe. The main lobe is less than
the amplitude of an equal peak value C W signal by an
amount which is approximately 3 /2
width), where tp (pulse width) is measured in seconds and
bandwidth is the selected resolution bandwidth of the an­
alyzer in hertz. Spectrum
ments should therefore be made only with a CW signal
applied, as indicated in the Performance and Calibration
checks.
(tP ) resolution band­
analyzer sensitivity measure­
Signal Identification and Frequency
Measurement
Spectrum Analyzers that have no preselection prior to the
first mixer will display signals which do not conform to the
indicated frequency reading of the dial.
referred to as spurii (many or plural) or spur (singular)
which are colloquial terms used to relate to spurious re­
sponses. (See definitions of spectrum analyzer terms). They
are the products of the following:
1. IF feedthrough: In the Type 1L30, the IF passband is
150 to 250 M Hz. Frequencies within this passband
appear as non-tunabie or IF feedthrough signals on a 100
M H z dispersion screen.
2. Signal images: The dial scales of the Type 1L30 Spec­
trum Analyzer are calibrated below' the frequency of the
tunable first local oscillator. The response to an input sig­
nal whose frequency is above the local oscillator frequency
by a difference of the IF, is called an image response. The
input signal that is the IF below Hie oscillator frequency
is the true response. For example: the analyzer will re­
ceive a 700 M Hz signal at a dial reading of 700 M Hz
(oscillator frequency of 900 MHz) and at a dial reading
of 300 M Hz (oscillator frequency of 500 MHz). At the image
response point, the local oscillator frequency is 2 0 0 M Hz
(IF) below the input frequency instead of 200 M Hz above the
input frequency. Note that the difference between these
two response points is 400 M Hz or twice the iF.
The dial is also calibrated for signal frequencies that
will mix with harmonics (2nd through the 4th) of the local
oscillator.
3. Higher order modulation (Harmonic conversion and
intermodulation) -— signals:
differences) of the frequency multiples from the local oscil­
lator and the signal, plus the myriad of combinations of
more than one input signal that produce a frequency within
the IF passband. The combination of the 2nd harmonic of
an input signal mixing w ith the fundam ental of another
input signal to produce the IF (3rd order] is the most se­
vere.
The possible combinations can be expressed mathemat­
ically as; nfS ig ± rnf|0 — IF; where n and m are intergers
including 0 and indicate the harmonic order of the signal
or local oscillator frequency.
lator (dial calibrated) frequency of 500 MHz could mix with
frequencies of 300/v\Hz, 700 MHz,
1300 MHz, 1700 MHz, etc., to produce a 200 M Hz IF.
4. Video detection: See Spectrum Analyzer Terms. These
spurious responses are usually no problem if the input signal
strength is below — 30 dBm.
5. Internal: These spurious signals are norm ally below 2 X
the noise level for the Type IL30.
Most spurious responses are easily identified as follows:
IF feedthrough signals wi 1 1 not tune across the display.
Image signals tune aaoss the display in. the direction that is
opposite to that of the t r s i g n a l response. In the Type 1 L30
the true signal response tunes from the left side of the display
to the right as the dial frequency is increased or the RF CEN­
TER FREQ control is turned clockwise. Their movement across
the dispersion window is coincident with the frequency
These signals are
may
All the products (sums and
For example: A local oscil­
800 MHz, 1200M Hz,