HP 8340B Operating Instructions Manual page 110

Referring to Figure 3-29, it is seen that when externally leveled, the POWER dBm display is still
connected to the internal detector of the HP 8340B/41B. Thus, it always tells the true output power,
reqardless of external leveling setup. This is a useful aid to determining if the external hardware is
functioning properly. In the above example, if the POWER dBm display shows -40 dBm when the
amplifier ouput is leveled to —10 dBm, the user knows the gain is 30 dB. As explained in the internal
leveling section above, the POWER dBm display is only accurate down to ALC levels of about -22
dBm below which it is intentionally blanked, therefore, that display is only useful if the ALC level is
forced to be greater than -22 dBm. This is done by inserting step attenuation until the display lights
up again. As still more attenuation is added, the POWER dBm reading remains essentially constant as
the external leveling loop holds the output level constant. With each additional 10 dB step of attenua¬
tion, the ALC level jumps 10 dB, until finally maximum available ALC level is reached and the
UNLEVELED annunciator lights. For best display accuracy and minimum noise, the ALC level should
be greater than -10 dBm. This is achieved by using attenuation equal to the tens digit of output
power. Example: output power = —43 dBm; use ATTN. = —40 dB, ALC = — 3 dBm.
When sweeping while externally leveled, the output power changes with frequency in order to level at
a distant point. The POWER dBm display tries to follow this change. Due to its heavy filtering, at fast
sweep speeds the meter displays an average power level.
When using mixers, or in other situations where reverse power may be encountered, caution is
necessary. See the decoupled mode and unleveled mode sections above. Reverse power into the HP
8340B/41B when externally leveled makes the POWER dBm display read incorrectly; unwanted RF
energy entering the external detector causes the loop to level at the wrong power level.
Bandwidth
When externally leveled, the leveling loop bandwith differs from what it is when internally leveled. If
AM is on or when sweeping with sweep time <5 seconds, the nominal loop bandwidth is:
Internal: 150 kHz
External, crystal: 80 kHz with square law detector, 40 kHz with linear detector.
External, power meter: 0.7 Hz
The significance of loop bandwidth is that it generally equals AM bandwith and influences the fastest
useable sweep speed. The best way to set sweep time is to increase it until measured flatness no
lonqer changes. For the sake of understanding — observe a plot of unleveled flatness across the
band of interest. The number of ripples divided by sweep time gives the number of "ripples per
second'' which the leveling loop must remove. The loop bandwidth must be several times this number
in order to do so. From the above it is apparent that the useable sweep time will depend on frequency
ranqe and RF hardware in use. When using long cables there will be many ripples, and their amplitude
is dependent on the match at each end of the cable. Inserting 10 dB of step attenuation greatly
improves the source match when externally leveled, thereby reducing the ripple amplitude.
For best stability of the leveling loop, the video bandwidth of the external detector should be 100 kHz
or more when XTAL leveling is selected. The capacity of coaxial cables reduces the video bandwidth
of crystal detectors. A typical point-contact detector (HP 420 series 423A, 8470A),w'Lwt7Wn J
4 feet of 500 coax on its output. Typical HP low barrier schottky detectors (HP 423B, 8470B, 3330
series) will drive up to 40 feet of 50 O cable.
The above paragraph applies to square law detectors with no dc offset (Vout = 0 for Pin - 0).
In METER mode, the external leveling bandwidth is reduced to 0.7 Hz in order to allow operation with
power meters. Power meters have a very slow response due to thermal delay time and heavy elec¬
trical filtering. The 0.7 Hz bandwidth assures proper operation with any HP power meter on any range.
When turning on the RF power, either when unblanking or using the RF key, the slow response of the
power meter can cause serious power overshoot in some external leveling systems. This overshoot
may damage active devices or some power sensors. To prevent this problem in the HP 8340B/41B, a
slow turn on circuit is employed when METER leveling is selected. This circuit raises the power slowly
over a period of 2 seconds. The sweep generator automatically waits for this settling time to end
before beginning a sweep.
HP 8340B/41B
Operating Information
3-107