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Amplitude modulation is possible when power meter leveled. The system provides linear AM and is dc
coupled. For rates below 0.7 Hz, the leveling is done via the power meter sensor and behaves just like
internal leveling. Above 0.7 Hz, feedback from the power meter rolls off and is replaced by feedback
from the HP 8340B/41B's internal detector. Thus, high frequency envelopes are under control of the
internal detector, while the power meter controls the power level. The modulation depth is still accu¬
rate and the bandwidth is nominally 80 kHz. The power sensor is not following the modulation; hence,
it will average the power. When modulating with a sinusoid symmetric about 0V, the average output
power increases (by 50% for 100% AM). The power sensor senses this increase and reduces the HP
8340B/41B's output until the average equals what the power was with no modulation. As long as the
modulation depth remains constant across the frequency band (use moderate rates and depths), this
system will level the power while providing amplitude modulation.
EXTERNAL SOURCE MODULE LEVELING
The HP 8340B/41B provides an external leveling mode, for use with compatible Hewlett-Packard
instruments, which provides leveling at a remote location along with power calibration and flatness
compensation. Compatible instruments are the HP 83550 series millimeter-wave source modules
when driven by an HP 8349B Microwave Amplifier. The HP 8349B may also be used in a stand-alone
amplifier configuration. Refer to an appropriate HP 83550 mm-wave source-system guide for inter¬
connect instructions.
Once the HP 8340B/41B - HP 8349B - HP83550 series instruments are hooked up, press [SHIFT]
[XTAL] on the HP 8340B/41B. This causes "EXT MODULE POWER: XX.XX dBm" to be displayed in
the ENTRY DISPLAY, which will accept power level changes via the [KNOB] [STEP] keys or ENTRY
keyboard. The HP 8340B/41B will accept power level requests from —20 to +20 dBm, however, this
range exceeds the operational range of existing mm-wave source modules. Refer to the mm-wave
source module manual for more information.
The SHIFT XTAL mode is unlike external leveling in that the POWER dBm display indicates the output
power of the remote module, not the HP 8340B/41B RF output. This mode provides all the modulation
features of an internally leveled HP 8340B/41B, with performance limited by the dynamic range and
leveling bandwidth of the individual source modules.
AMPLITUDE MODULATION
The HP 8340B/41B provides linear, dc coupled amplitude modulation when internally leveled or exter¬
nally leveled with a square law detector or power meter. The input resistance is 6000 whether the AM
function is on or off. The sensitivity is 100% per volt ±5%. This means that +1.0 volt doubles the
output voltage (+6 dB), while —1.0 volt shuts the output completely off. The input accepts ±15 V dc
with no damage, and is resistor-diode clamped to protect against higher voltage transients. Most sine
wave generators are calibrated in terms of RMS voltage, so 0.707 V RMS equals 1.00 V peak. The
generator's output meter is accurate only if the load impedance equals the source impedance.
POWER
dBm
Display used with AM
The POWER dBm display on the HP 8340B/41B always tells actual output power. A dc input to the AM
jack causes the power level to shift, and the display reflects this: +1.0 volt causes the display to
increase 6 dB. If that much power is not available, the UNLEVELED annunciator lights and the display
shows the actual output. Inputs which reduce the ALC level below -22 dBm will blank the display.
Inputs of —1.0 volt or more negative shut off the output and light the OVERMOD annunciator.
The POWER dBm display is filtered so that it will not flicker for AM rates above 20 Hz. The filtering
creates an average of RF voltage, which is then displayed as power in a 50Q system (dBm, 50
Cl).
3-108
Operating Information
HP 8340B/41B
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