Followings is the sequence of blocks in spectrum Analyser : A. Detector & video Amplifier B. Fixed Frequency Active filter C. Attenuator (Adjustable) D. Mixer & Tunable Detector Choose the correct answer from the options given below :
C$\longrightarrow$D$\longrightarrow$B$\longrightarrow$A
What a spectrum analyser is. It is a swept superheterodyne receiver that displays amplitude against frequency instead of against time. The signal therefore passes through the same chain as any superhet, and the block order follows directly from that.
C — Adjustable attenuator (first). The input must be conditioned before anything else: a step attenuator scales a large signal down so that the mixer is not overdriven. Overdriving the mixer creates spurious intermodulation products that would appear on screen as signals that do not exist — which is why the attenuator has to come first.
D — Mixer and tunable local oscillator (second). A voltage-swept local oscillator beats with the input, translating each input frequency in turn down to the fixed intermediate frequency. The same ramp voltage that sweeps the oscillator also drives the display's horizontal axis, so screen position corresponds directly to frequency.
B — Fixed-frequency (IF) filter (third). This narrow filter is what actually selects one frequency slice at a time; its bandwidth is the analyser's resolution bandwidth. A narrower RBW resolves closer-spaced signals and lowers the noise floor, but forces a slower sweep.
A — Detector and video amplifier (last). The IF signal is rectified into a d.c. level proportional to its amplitude, smoothed by the video filter, and applied to the vertical axis of the display.
So the chain is C → D → B → A: attenuate, translate, filter, detect.
The reasoning shortcut. Any receiver-type instrument runs input conditioning → frequency conversion → selectivity → detection → display. Fixing the attenuator at the front and the detector at the back eliminates every option except the correct one.
Hence, the correct sequence is C → D → B → A.
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