HP 8340A Operating Manual page 171

Model 8340A
Figure 3-34 is a block diagram of the detector circuitry, with waveforms. Trace 1 is the pulse
modulation input signal to the HP 8340A. It controls a fast RF modulator which is either
full on or full off The amplitude when on is controlled by the linear modulator used for CW
leveling and AM. Trace 2 is the resultant RF pulse, which is the HP 8340A's output. This
pulse is detected by the crystal detector. It trails the pulse input by 55 nsec, representing
propagation delays in the pulse modulator and its drive circuits.
The output of the crystal detector is amplified by a logarithmic amplifier (log amp). The log
amp is used for several reasons, one of which is its high gain for small signals, reducing the
effects of sample and hold errors. Trace 3 is the output of the log amp. The delay and
relatively slow rise time are caused by the finite bandwidths of the detector and log amp.
The pedestal (arrow) represents the RF amplitude. This level is captured for further
processing by the sample and hold circuit (S/H), represented by the switch-capacitor
combination. Trace 4 shows the signal controlling the switch, which is closed when trace 4 is
high.
Trace 4 is timed to coincide with the pedestal of trace 3. This timing is done by circuitry
associated with the pulse modulator and is factory adjusted for best coincidence. Since the
S/H switch is closed only during trace 3's pedestal, the capacitor charges to a constant de
voltage. This voltage is the same as what comes out of the log amp during CW operation at
the same power level. The capacitor is isolated by a buffer to prevent the following circuits
from discharging it between pulses. The output of the buff er is compared to the ALC inputs
in the same manner as with CW operation.
Figure 3-34 shows a 200 nsec pulse. If the pulse were narrowed to 100 nsec, trace 3 would not
quite reach its pedestal before it begins to fall. The result is a de output from the S/H that is
smaller than it would be in CW. The ALC circuits respond by raising the RF output until
that voltage is what it should be. This is the reason for poor leveling accuracy with narrow
pulses. As the pulses are made narrower, their amplitude grows.
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 µ,sec 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
=
Therefore, as power is decreased, the leveling accuracy slightly degrades (narrow pulse
amplitude grows relative to CW).
-10 dBm is slow enough to be comparable to the detector rise time.
Scans by HB9HCA and HB9FSX
Operating Information
3-107