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The amount of accuracy degradation as the pulses are narrowed varies with frequency, temperature,
and power level. The variation with frequency and temperature is due to detector characteristics and
RF envelope shape. The detector has a finite rise time determined by its output resistance and shunt
capacitance. At some frequencies there is a slight amount of overshoot on the RF envelope, which
tends to charge the shunt capacity faster, resulting in better narrow pulse leveling accuracy. A much
more pronounced effect is due to the use of a different detector for frequencies below 2.3 GHz. The
low band detector has a higher shunt capacity in order to make it function properly at low frequencies.
For operation below 400 MHz, a large amount of additional capacity is switched in, enabling detector
operation down to 10 MHz. Trace 3 in Figure 3-34 is representative of operation above 2.3 GHz, where
pulse accuracy is within 1.5 dB at 100 nsec. From 0.4 to 2.3 GHz, the slower rise time gives a 1.5 dB
specification at 200 nsec width. Operation below 0.4 GHz is not specified, but typically is within 1.5 dB
at 2 jisec width.
The detector's rise time depends on its output resistance, which drops with increasing temperature.
Therefore, the narrow pulse leveling accuracy improves at higher operating temperatures.
Narrow pulse accuracy is also power level dependent. Very high ALC levels reduce the detector's
output resistance, improving rise time and therefore accuracy. The rise time of a log amp is dependent
on signal level, degrading with small signals. In low band (<2.3 GHz) the log amp is faster than the
detector at any ALC level above —10 dBm, so there is no degradation due to the log amp in any
coupled mode operation. In high band, the log amp rise time at ALC = —10 dBm is slow enough to be
comparable to the detector rise time. Therefore, as power is decreased, the leveling accuracy slightly
degrades (narrow pulse amplitude grows relative to CW).
The leveling specifications apply to coupled operation, with no AM; in other words, ALC> —10 dBm.
Using the decoupled mode or AM, the ALC level can be driven down to —20 dBm or lower. At —20
dBm, the log amp slows down enough that high band accuracy is typically 1.5 dB at 150 nsec, 3.0 dB at
100 nsec. Decoupled mode can also be used to operate the ALC at high levels and achieve better
narrow pulse accuracy.
The above discussion applies to internal leveling only. Externally leveled pulse performance will, of
course, depend on the detector, but even with a perfect detector the external leveling circuitry is not
as fast as internal. It typically will level pulses wider than 2 psec.
HP 8340B/41B
Operating Information
3-111
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