Agilent Technologies E8257D PSG, E8267D PSG manual Mixer, Effects of Reverse Power on ALC

				Troubleshooting
				RF Output Power Problems
Figure 12-1	Effects of Reverse Power on ALC
	SIGNAL GENERATOR
	OUTPUT CONTROL
		ALC LEVEL	RF OUTPUT	MIXER
		= - 8 dBm	= - 8 dBm
	RF LEVEL			LO
	CONTROL
	DETECTOR	DETECTOR	LO FEEDTHRU	LO LEVEL
	MEASURES	MEASURES	= - 5 dBm	= +10 dBm
	- 8 dBm	- 5 dBm
	ALC LEVEL	REVERSE		IF
		POWER

The internally leveled signal generator RF output (and ALC level) is - 8 dBm. The mixer is driven with an LO of +10 dBm and has an LO- to- RF isolation of 15 dB. The resulting LO feedthrough of - 5 dBm enters the signal generator’s RF output connector and arrives at the internal detector.

Depending on frequency, it is possible for most of this LO feedthrough energy to enter the detector. Since the detector responds to its total input power regardless of frequency, this excess energy causes the ALC to reduce the RF output of the signal generator. In this example, the reverse power across the detector is actually greater than the ALC level, which may result in loss of signal at the RF output.

Figure 12- 2 on page 246 shows a similar configuration with the addition of a 10 dB attenuator connected between the RF output of the signal generator and the input of the mixer. The signal generator’s ALC level is increased to +2 dBm and transmitted through a 10 dB attenuator to achieve the required - 8 dBm amplitude at the mixer input.

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