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When using decoupled mode to set low ALC levels, some caution is necessary. At ALC = —20 dBm,
the amplitude accuracy degrades by ± 1 dB. This results from temperature induced drift in the
detection circuitry. Some spurious signals, such as ac power line related sidebands are worse at —20
dBm and may exceed specifications (which do not apply at ALC = —20 dBm). Despite its level
uncertainty, the output power at ALC = -20 dBm is more stable than a normal power meter. In
checking the output, care must be taken to zero the meter with the HP 8340B/41B's RF turned off.
Pressing the "sensor zero" button on a power meter does not assure that it is zeroed, since the
display on some meters is blank for inputs below —30 dBm when in the "dBm" mode. Selecting the
"WATT" mode keeps the display alive, and the user can then see if the meter is really zeroed (press
the zero button several times if necessary).
When the HP 8340B/41B's heterodyne band (.01 to 2.3 GHz) is in use, approximately —30 dBm of
broadband noise is generated along with the desired signal. If the desired signal is —20 dBm and the
result is measured on a power meter, the reading will be about 0.5 dB high. To accurately measure the
signal, the power meter must be zeroed in the presence of the noise: Connect the power meter, then
press [RF] to shut off any RF output; although the RF is off, the noise is still present and the power
meter can now be zeroed. When going to frequencies above 2.3 GHz, the meter must be re-zeroed.
The broadband noise is attenuated by the step attenuator, along with the desired signal. Noise makes
a 0.05 dB contribution at ALC = —10 dBm.
UNLEVELED MODE
The HP 8340B/41B has a power control mode in which the leveling feedback loop is opened. The ALC
inputs are used to directly control the RF modulator. Pressing [SHIFT] [METER]activates this mode.
The annunciators on the leveling mode keys are extinguished, the UNLEVELED annunciator is
lighted, and the ENTRY DISPLAY shows: ATTN:_dB, MOD:_._dB. As with the
decoupled mode, the attenuator is set via the [STEP] keys, and the modulator entry is made with
keypad or [KNOB], The entry range is 0 to -100 dB. The modulator entry is an approximately cali¬
brated relative indication, because the modulator's gain and maximum output change with frequency.
See Figure 3-31.
AM works in this mode with unspecified distortion. Pulse modulation works. Power sweep works with
linearity as depicted in Figure 3-31. The POWER dBm display still indicates actual output power. As
with other leveling modes, it indicates the sum of "ALC level" and attenuation, with useful accuracy
down to ALC levels of — 20dBm. Sweeps will, of course, be unleveled. When in the unleveled mode,
there is no feedback stabilization of power, and its stability versus time and temperature is unspec¬
ified.
This mode is useful for signal tracing while troubleshooting the HP 8340B/41B. it is also useful in
some pulse modulation applications, as explained in that section. It can also be used to output in the
presence of large reverse power (a problem described under "decoupled mode"). To do so, the
reverse power's effect on the POWER dBm display must be eliminated by shutting that power off, or
temporarily setting the HP 8340B/41B's attenuator to a high value. Then, in the unleveled mode
(SHIFT METER), use the knob to set the desired ALC level via the power dBm display (remember to
mentally compensate for any attenuation in use). Then remove attenuation or turn on the reverse
power. In the presence of reverse power, the POWER dBm display will change to an incorrect value,
but the output power will be as previously set.
3-102
Operating Information
HP 8340B/41B
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