Model 8340A - Service
generated in pulse modulation mode. When
[SHIFT] [RF]
is engaged, there is no limit on the
minimum pulse repetition frequency.
The following message will be displayed in the
ENTRY DISPLAY: "POWER SEARCH:
X.XX
dB"
(where X.XX is the last-entered value).
To set the power level, place the HP 8340A in CW mode, or in pulse modulation mode with
pulses wider than
2
usec, and use the
[STEP]
keys, the RPG, or the data entry keypad with the
[dBm]
terminator key to enter the power level desired. The
POWER dBm
display shows the
actual power when the instrument is in CW of pulse modulation mode. The accuracy of the
POWER dBm
display is typically the same as when the instrument is leveled. Table 1-1 contains
this information under
PULSE MODULATION
specifications,
ACCURACY OF INTERNALLY
LEVELED RF PULSE Vp (relative to CW mode level).
The actual power changes very little as
the pulse width is narrowed, even though the
POWER dBm
reading drops. Therefore, at very
narrow pulse widths ignore this reading.
[SHIFT] [RF]
can also be used as a diagnostic function of the ALC circuits (refer to the ALC loop
description/troubleshooting in the beginning of the RF Section for more information).
[SHIFT] [METER]
(HP-IB: SHA3) bypasses the ALC (automatic leveling control) to allow
direct control of the linear modulator circuit, which is useful when very narrow pulses are being
generated in pulse modulation mode. In this mode there is no limit on the minimum pulse
repetition frequency: Press
[SHIFT] [METER],
which will cause "ATN: X-xx dB, MOD: x.x
dB" (where x is the last-entered value) to appear in the ENTRY DISPLAY. To set the power,
place the HP 8340A in CW mode, or in pulse modulation mode with pulses wider than
2
µsec,
and use the
[STEP]
keys to set the ATN (attenuator), and the rotary
[KNOB]
or numerical keys
with
[dB(m)]
terminator key to set the MOD (linear modulator), as follows: Set MOD entry at 0
dB, increment ATN until the POWER dBm follows: Set MOD entry at 0 dB, increment ATN
until the POWER dBm display shows a level 5 dB to 15 dB higher than the desired output power,
then reduce the power to the desired level by changing the MOD value. The POWER dBm
display shows actual power when the HP 8340A is in CW or wide-pulse pulses modulation
modes; this actual power changes very little as the pulse width is narrowed, even though the
POWER dBm reading drops. Therefore, at this point reduce the pulse width to the desired value
and ignore the POWER dBm display. The A TN and MOD values in the ENTRY DISPLAY also
have a limitation: Although the ATN displayed value is always accurate, the MOD becomes
. saturated in the top
10
dB (approximately) of its range at which point no change occurs in the true
power; consequently, rely on the POWER dBm display for the true power level instead of the
MOD value.
[SHIFT] [METER]
can also be used as a diagnostic function of the ALC circuits.
8-40
[SHIFT] [PEAK]
(HP-IB: SHRP) is a more extensive version of peaking:
[PEAK],
which
requires a fraction of a second to implement, aligns the output filter with a single CW frequency,
while
[SHIFT] [PEAK]
aligns all of the YTM tracking calibration constants and requires 5-10
seconds to implement. Use
[SHIFT] [PEAK]
to enhance the power output and spectral purity of
swept modes, and to improve tracking performance (especially in harsh environments having
wide temperature variations). Press
[SHIFT] [PEAK],
which will cause "AUTO TRACKING"
to appear in the ENTRY DISPLAY; "AUTO TRACKING" will disappear after 5-10 seconds
when the calibration has been completed.
[SHIFT] [PWR SWEEP]
(HP-IB: SHPS) decouples the attenuator (ATN) from the automatic
leveling control (ALC), as explained in the preceding function. Recouple the ATN and ALC by
pressing
[POWER LEVEL].
[SHIFT] [SLOPE]
(HP-IB: SHSL) allows front panel control of the mechanical attenuator
(ATN). Press
[SHIFT] [SLOPE],
which will cause "A'.fN: xdB" (where x is the last-entered
value) to appear in the ENTRY DISPLAY. Use the
[STEP]
keys, or the numerical keys with any
terminator key to change the attenuator value within the range 0 dB to - 90 dB in 10 dB steps.
Keyboard entries are automatically rounded to the nearest 10 dB. The clicking sound heard after
each attenuator change is the attenuator pad being mechanically switched into the RF output
path.
Scans by HB9HCA and HB9FSX
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