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Therefore, modulation inputs with no dc component do not cause the POWER dBm display to shift,
and this displayed number represents the power of the unmodulated carrier. A power meter measure¬
ment of output power changes with modulation present, increasing 1.76 dB with 100% deep,
sinusoidal, no dc component modulation. Attempted deep modulation at high rates causes the
POWER dBm display to shift, because the AM system canot keep up with the input and the resultant
high distortion causes a shift in average power.
Dynamic Range
As mentioned previously with reference to Figure 3-29, the AM input is an ALC input which does not
differ in its effect from a power entry input. Therefore, the AM system is limited by where it is operating
within the ALC range. In the normal "coupled" operating mode, the ALC will likely be set between —10
dBm and 0 dBm (see the preceeding pertinent section). Depending on frequency, the maximum
available power is between +1 dBm (HP 8340B) and +21 dBm. The ALC is reasonably accurate down
to —20 dBm, and typically is well behaved to about —30 dBm. Expressing the desired modulation
depth in dB's will let the user determine the range over which the ALC loop is being exercised. Thirty
percent AM creates excursions of +2.3 dB to —3.1 dB, relative to the quiescent level; 50%: +3.5 dB
to -6.0 dB; 90%: +5.6 dB to -20.0 dB. (The above assumes a modulation waveform symmetric
about 0 V.)
For example: Output power = —19.0 dBm. In coupled mode this results in ATTN = —10 dB, ALC =
—9.0 dBm. Ninety percent modulation depth results in an ALC range of —9.0 dBm + 5.6 dB = —3.4
dBm maximum, — 9.0 dBm —20 dB = — 29.0 dBm minimum. This is within the ALC limits at any
frequency but the distortion may suffer due to operation below —20 dBm. Using decoupled operation
the desired output power may be set with ATTN = — 20DB, ALC = +1.0 dBm. Then 90% depth
swings the ALC from +6.6 dBm to -19.0 dBm. The distortion will probably be better under these
conditions if +6.6 dBm is available at the frequency of interest. At some frequencies +20 dBm is
available, and setting ATTN = -30 dB, ALC = +11.0 dBm may give a further improvement,
especially if attempting greater than 90% depth. For minimum distortion the ALC should be used
between —15 dBm and +15 dBm, but not within 2 dB of maximum available power.
On HP 8340B's operating above 23 GHz the available output power is limited: +1 dBm specified, +3
dbm typical. If the ALC is set close to 0 dBm, the headroom available for modulation is limited. Three
dB excess power allows a maximum of 40% peak modulation. The available depth is not affected.
Decoupled mode may be used to advantage here. For example, in coupled mode an output power of
—10 dBm results in ATTN = 10 dB, ALC = 0.0 dBm. Using decoupled mode, setting ATTN = 0 dB,
ALC = —10.0 dBm gives plenty of headroom with enough depth available for 90% symmetric AM.
Available power can be maximized at a CW frequency by using the PEAK function.
Bandwidth for AM Applicaitons
The small signal AM bandwidth extends from dc to a -3 dB frequency of at least 100 kHz. The actual
upper limit is a direct function of the loop gain of the ALC loop. The primary variable in the loop gain is
the gain of the modulator, which varies with both power level and frequency. In general, the modulator
gain deviates the most from nominal at power levels just below maximum, although at some frequen¬
cies the gain will deviate at lower power levels because of non-optimum YTM bias. The latter is only a
problem above 7.0 GHz. The small signal (30% depth) bandwidth may be expected to vary between
100 kHz and 300 kHz as power and/or frequency is changed. The bandwidth for greater depths is
less. At 90% depth expect about 1/2 the 30% bandwidth.
The above bandwidths are for internal leveling. When external leveling with a crystal detector in its
square law region the bandwidth is 1/2 of the internally leveled bandwidth at the same frequency and
output power level. With a crystal detector in its linear region, the bandwidth is 1/4.
The external leveling system is designed to provide linear AM when using a square law detector. The
RF output follows this law:
Vout = Vo
X
(Vin + 1).
HP 8340B/41B
Operating Information
3-109
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