Power Dbm Display Used With The Attenuator; Bandwidth - HP 8340A Operating Manual

Model 8340A

POWER dBm Display used with the Attenuator

Some external leveling applications require low output power from the HP 8340A, for
example, leveling the output of a 30 dB amplifier to a level of-10 dBm. In this application,
the output of the HP 8340A is around -40 dBm when leveled At some frequencies this level
is beyond the range of the ALC modulator alone. If so, the OVERMOD annunciator lights.
Inserting 40 dB of step attenuation results in an ALC level of 0 dBm, which is well within
the range of the ALC. At 26.5 GHz, where only+ 1 dBm is available, 30 dB attenuation is a
better choice as it results in an ALC level of -10 dBm. This gives a margin for AM or other
functions that vary the power level.
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 8340A. Thus, it always tells the true output
power, regardless 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 output 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 attenuation, 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:
use ATTN.= -40 dB, ALC =
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 8340A 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 bandwidth 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:
External, power meter:
The significance of loop . bandwidth is that it generally equals AM bandwidth and
influences the fastest useable sweep speed. The best way to set sweep speed is to reduce it
until measured flatness no longer 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
-3 dBm.
30 kHz with square law detector, 15 kHz with linear detector.
0.7 Hz
Scans by HB9HCA and HB9FSX
Operating Information
output power = -43 dBm;
3-103