the second harmonic of 900 MHz (1800 MHz] will mix with
Identification and Frequency Measurement of
1600 MHz and 2000 MHz.
Displayed Signals
These response are identified and read as follows:
Bandpass and dispersion characteristics of spectrum an-
1. Tune the RF Center Frequency across a dispersion win-
alyzers require very limited preelection ahead of the first
dow and observe the signal movement.
mixer. Signals with frequencies different than that indicated
by the dial will therefore appear on the display. These
2. True responses move across the dispersion window from
signals are called spurious responses. See spurious responses
left to right, on the Type 491, as the RF CENTER FREQUENCY
under Spectrum Analyzer Terms at the beginning of this sec-
is increased, or in the same direction as the tuning knob
tion.
is turned.
3. Images move across the dispersion window opposite to
The dial scales for the Type 491 indicate frequencies that
the direction of the true response.
are below the local oscillator frequency by the IF (200 MHz).
For example: a dial reading of 700 MHz means the local
4. IF feedthrough signals are not tunable and remain fixed
oscillator frequency of 900 MHz (700 MHz + 200 MHz). This
in position as the RF CENTER FREQUENCY is tuned.
local oscillator frequency will mix with 700 MHz and 1100
MHz to produce the IF of 200 MHz. The 1100 MHz response
5. Signal frequency shifts across the dispersion window that
are not coincident with the RF CENTER FREQUENCY change
is called the image. Note that the image response is twice
are spurious. Some of these spurii are mixing with higher
the IF away from the true, or dial calibrated response. Har-
harmonics of the local oscillator. The upper dial scales of
monics of the local oscillator fundamental frequency also
the Type 491 are calibrated to harmonics of the local os-
convert incoming signals to the IF response. For example,
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