The instability is indicated by the growing modulation envelope
. The amplitude keeps increasing as it reaches x = -100 m, x = -50 m, and x = -30 m until it reaches the maximum peaking at x = 0.
For a true reproduction of the modulation envelope
, the amplifier must be operating in its linear region.
Since the group delay is derived from the modulation envelope
and not the carrier frequency, the technique can be applied to measure frequency-converting networks.
A wideband peak power sensor is a diode sensor with sufficient bandwidth that it can follow a high-speed modulation envelope
For example, a spectrum analyzer monitoring forward power will record the top of the modulation envelope
. However, this is not the proper amplitude to record and must be modified for the depth of modulation.
The main advantage this technique has over the direct phase implementation is that since the group delay is derived from the modulation envelope
and not the carrier frequency, the technique can be applied to measure frequency converting networks.
Also, systems that pass the bandedge emissions with CCK modulation at 11Mb/s may fail at the same power level with OFDM at 54 Mb/s because of the modulation envelope
differences between OFDM and CCK.
An envelope detector essentially provides an output that is the modulation envelope
of an IF signal.
Though European standards, such as the global system for mobile communications (GSM), use constant modulation envelope
formats, those being considered for use in North America are quadrature phase-shift keying (QPSK) formats.
From the phase shift of the modulation envelope
through the device, the group delay can be calculated.
The instrument also includes built-in digital AM, FM and pulse modulation envelope
generators with digital and analog sweeps of both frequency and output power.
If the electric field zero crossings under the optical pulse modulation envelope
are uniformly spaced in time, the time-bandwidth product should be approximately 0.5.