HP 8340B Operating Instructions Manual page 108

The reference voltage may be set over a range of -66 dBV (.0005V) to +6 dBV (2.00V). This wide
ranqe accommodates a variety of detectors and leveling situations. The input accepts either positive
or negative voltages automatically. For example, with REF = 20 dBV, the loop will level with an input
of either +0.1V or —0.1V. The input will accept overloads of ±25 volts with no damage. The input
resistance is 1 M£2. Figure 3-33 shows the input power versus output voltage characteristics for
typical HP crystal detectors. From the chart the leveled power at the crystal detector input resulting
from any reference setting may be determined. The range of power adjustment is approximately —30
dBm to +18 dBm.
Detector Characteristics
As shown in Figure 3-33, crystal detectors may be characterized by three operating regions. In the
square law region (Pin <-20 dBm) the output voltage is proportional to the square of the input
voltaqe, in other words, proportional to input power. In the linear region (Pin >+5 dBm) the output
voltaqe'is directly proportional to the input voltage. Because of this, when leveling in the linear region
a 1 dB reference change causes a 1 dB power change, while in square law a 1 dB reference change
causes a 1/2 dB power change. This should be understood when using power sweep or AM. The
power sweep function will sweep the reference by up to 40 dB, but if a square law detector is being
used the power will only sweep 20 dB. The power sweep will only be linear if the operation is entirely
in square law or entirely in linear. In the transition region, the power sweep will be non-linear. As may
be seen in Fiqure 3-33 by drawing a line between —10 dBV and —50 dBV, the worst deviation from a
straiqht line is 3 dB of reference voltage, or 2 dB of RF power. The amplitude modulation system is
designed to be linear with a square law detector. With a linear detector, the modulation depth will be
more than expected, and there will be significant distortion.
HP power meters have a rear panel output ("recorder" output) which responds linearly with power.
The output is +1.00V for full scale on whatever range is selected +.50V for 3 dB below full scale,
+ 10V
for
10
dB below full scale. These numbers may be seen directly on the 0
-1
"WATTS scale on
an analog power meter (e.g., HP 432,435). This response is the same as a square law detector, so all
the comments above for such detectors apply to power meters.
Settiriq the desired power with a non-autoranging meter (HP 432,435) is straightforward. Assume +3
dBm is desired at the power sensor. Set the power meter on the + 5 dBm range, so the desired power
is 2 dB below full scale. Since the RF power changes 1/2 dB for each 1 dB reference change, set the
reference for —4 dBV.
Auto ranqinq meters (e.g., HP 436) must be used in their range hold modes to prevent range change
during blanking or other RF-off intervals. To lock the meter to the desired range, internal leveling must
be used Adjust the HP 8340B/41B output power until the meter is on the desired power range, then
press range hold. As an example, consider the HP 436; The HP 436 changes ranges every 10 dB, so if
—8 dBm is desired, the reference must be set for 8 dB below full scale (REF — 16 dBV) with the HP
436 locked on the —10 to 0 dBm range. (Caution: the HP 436 range change circuits have intentional
hysteresis. Setting the power to 0 dBm may place the meter on either the -10 to 0 range or the 0 to
+10 range. For no ambiguity, force the meter to the middle of the range (-5 dBm, +5 dBm, etc.), then
press range hold.)
POWER dBm Display used with the Attenuator
Some external leveling applications require low output power from the HP 8340B/41B for e*'1/nP1®'
leveling the output of a 30 dB amplifier to a level of —10 dBm. In this application, the output of the HP
8340B/41B 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 (8340B), 30 dB attenuation is a better choice as it results in an ALC level of
— 10dBm. This gives a margin for AM or other functions that vary the power level.
HP 8340B/41B
Operating Information
3-105