Decoupled Mode - HP 8340A Operating Manual

Operatillg Information
When internally leveled, UNLEVELED annunciator off, and AM off the POWER dBm
display simply repeats the value shown in the ENTRY DISPLAY. When externally leveled,
or the UNLEVELED annunciator is on, or AM modulation is in effect the POWER dBm
display indicates the ALC level and reflects the true output power. This might not agree with
the ENTRY DISPLAY value, which shows the desired level in the absence of modulation.
The above should be understood when using power sweep. When the POWER dBm display
is repeating the ENTRY DISPLAY, it indicates start power. If the POWER dBm display is
indicating ALC level, it shows the average power over the sweep. (The circuit is heavily
filtered above 5 Hz.) Since the start and stop dwell times are unequal, this average reading
has little meaning.
In a variety of situations it is possible to drive the ALC level below -20dBm, such as putting
DC into the AM input, or when externally leveled. Since the internal detector is inaccurate
at these levels, the POWER dBm display would be very misleading. For that reason, and as
a warning, the POWER dBm display blanks at ALC levels below -22.0 dBm. This may
occur at an output power of -22 dBm, -32 dBm, -42 dBm, etc., depending on the
attenuator setting.

Decoupled Mode

In some applications it is advantageous to control the ALC and attenuator separately,
achieving combinations of settings which are not available in the coupled mode. Press
[SHIFn [PWR SWP]
ALC:
via keypad or
[KNOB],
the POWER dBm display indicates the true, composite output power. (Caution: If an entry
is made in decoupled mode, and subsequently
reverts to coupled mode. It will set itself to the same power level, but the attenuator and ALC
settings may be different.)
One use of decoupled operation is power sweep, where the output power linearly tracks the
sweep voltage ramp. The HP 8340A can generate power sweeps of up to 40 dB, depending on
frequency. The power at the start of the sweep is set via
operation) or
[SHIFn [PWR SWP]
amount the power increases during the sweep) is entered by pressing
by the desired dB sweep value. If the sweep range entered exceeds the ALC range (stop
power greater than maximum available power) the UNLEVELED annunciator will light at
the end of sweep. No warning is given at the time of entry. If the start power is entered via
the
[POWER LEVEL]
sweep range to 30 dB at 4.5GHi, or 11 dB at 26.5 GHz. Using decoupled mode and setting
the ALC to -20 dBm gives an additional 10 dB of sweep range (although at -20 dBm, start
power uncertainty is degraded by
Decoupled mode is also useful when working with mixers. Figure 3-30A shows a
hypothetical setup where an HP 8340A is providing the small signal to a mixer. The HP
8340A output is -8 dBm, which in coupled mode results in ALC = -8 dBm, ATTN= 0 dB.
The mixer is driven with an LO of+ 10 dBm, and has LO to RF isolation of 15 dB. The
resulting LO feedthrough of-5 dBm enters the HP 8340A's OUTPUT port, goes through the
attenuator with no loss, and arrives at the internal detector. Depending on frequency, it is
possible for most of this energy to enter the detector. Since the detector responds to its total
input power regardless of frequency, this excess energy causes the leveling circuit to reduce
its output. In this example the reverse power is actually larger than the ALC level, which
may result in the HP 8340A output being shut off.
3-96
to activate decoupled mode, which causes "ATTN:
dBM" to show in the ENTRY DISPLAY. The ALC setting is entered
while the attenuator is incremented with the
(decoupled operation), and the sweep range (the
key, the ALC is set no lower than -10 dBm, limiting available power
1 dB).
+
Scans by HB9HCA and HB9FSX
[STEP]
is pressed, the HP 8340A
[POWER LEVEL]
[POWER LEVEL]
[PWR SWP]
Model 8340A
dB,
keys. As before,
(coupled
followed