Envelope Detector

Envelope detector6
Spectrum analyzers typically convert the IF signal to video7 with an envelope
detector. In its simplest form, an envelope detector consists of a diode,
resistive load and low-pass filter, as shown in Figure 2-15. The output of the
IF chain in this example, an amplitude modulated sine wave, is applied to
the detector. The response of the detector follows the changes in the envelope
of the IF signal, but not the instantaneous value of the IF sine wave itself.

Figure 2-15. Envelope detector

For most measurements, we choose a resolution bandwidth narrow enough
to resolve the individual spectral components of the input signal. If we fix
the frequency of the LO so that our analyzer is tuned to one of the spectral
components of the signal, the output of the IF is a steady sine wave with a
constant peak value. The output of the envelope detector will then be a
constant (dc) voltage, and there is no variation for the detector to follow.

However, there are times when we deliberately choose a resolution bandwidth
wide enough to include two or more spectral components. At other times,
we have no choice. The spectral components are closer in frequency than
our narrowest bandwidth. Assuming only two spectral components within
the passband, we hav two sine waves interacting to create a beat note, and
the envelope of the IF signal varies, as shown in Figure 2-16, as the phase
between the two sine waves varies.

Figure 2-16. Output of the envelope detector follows the peaks of the
IF signal

6. The envelope detector should not be confused with
the display detectors. See Detector types later
in this chapter. Additional information on envelope
detectors can be found in Agilent Application
Note 1303, Spectrum Analyzer Measurements and
Noise, literature number 5966-4008E.
7. A signal whose frequency range extends from zero
(dc) to some upper frequency determined by the
circuit elements. Historically, spectrum analyzers
with analog displays used this signal to drive the
vertical deflection plates of the CRT directly. Hence
it was known as the video signal.