Internal Leveling; Coupled Mode - HP 8340A Operating Manual

Operating Information
The preceding explanations of the power control and modulation functions are sufficient
for the majority of applications; however, to extract the utmost performance from this
instrument the following special information might be helpful.

INTERNAL LEVELING

The RF Output power is controlled by the automatic level control (ALC) circuit, otherwise
referred to as the leveling loop. Figure 3-29 shows a simplified diagram of this system. The
leveling loop is a feedback control system, in which the output power is measured and
compared to the desired level. If the two are not equal the loop changes the output until they
are equal.
The two inputs labeled "ALC INPUTS" convey the desired power level. One of these is a
voltage derived from the power value shown in the ENTRY DISPLAY. In the absence of
modulation this voltage is used to set the output power level. The amplitude modulation
(AM)
input causes the output to increase or decrease relative to this level. (The pulse
modulation input is essentially on ON/OFF switch, not an ALC input)
The RF power level from the level control circuits is referred to as the "ALC level," and is
measured by a crystal detector. The DC output from this detector is fed back to the level
control circuits for comparison with the ALC inputs. Since crystal detectors lose sensitivity
at low power levels, the detector provides an accurate power indication for ALC levels down
to -10 dBm, and is acceptable (+l dB) down to -20 dBm. The maximum amount of power
available from the level control circuits varies with RF frequency, from + 1 dBm specified at
26.5 GHz to typically +21 dBm at 4.5 GHz; therefore, the level control circuits can provide
continuous control of ALC levels over a maximum span of approximately -20 dBm to +20
dBm.

Coupled Mode

Since many applications require power levels less than -20 dBm, a step attenuator is
provided that has a range of 0 to -90 dB in 10 dB steps. Thus, power levels down to -110
dBm is achieved when the attenuator and ALC work in conjunction. Because of the
attenuator, the ALC will normally be used over only a portion of its 40 dB range:
accuracy suffers below -lOdBm and at some frequencies only + 1 dBm is available, the
ALC is normally set between -10 dBm and 0 dBm. The restricted range of the ALC reduces
the primary power span to 0 to -100 dBm. To get power less than -100 dBm, the attenuator
is left at -90dB, and the ALC used from -10 to -20 dBm; however, the ALC accuracy and
noise performance is degraded at this level, and is the reason that some specifications apply
only down to -100 dBm. At frequencies where power levels above 0 dBm are desired, the
attenuator is left at 0 dB and the ALC used from 0 to +20 dBm (or whatever power is
available at the RF frequency in use). The proper combination of ALC and attenuator is
decided by the internal microprocessor:
ENTRY DISPLAY via the
3-94
USING THE 8340 POWER CONTROL
AND MODULATION SYSTEMS.
[POWER LEVEL]
Scans by HB9HCA and HB9FSX
the user need only set the desired power in the
key.
Model 8340A
Since