HP 70427A User Manual page 454

Block Diagram
600 MHz Reference Loop (A7A2)
The amplifier's output is filtered by a 200 MHz bandpass filter. The filter is a
single resonator design, with a input capacitor to match the input impedance
up to the resonator impedance, and an output inductor to match the resonator
impedance down to 50Ω. The amplifier's 22 pF output capacitor is combined
with the filter's 3.6 pF input capacitor to produce a single component of 3.9
pF. Also, the filter's 168 nH output inductor is combined with the 3×
multiplier's 82 nH input inductor to produce a single inductor of 270 nH.
The 200 MHz interstage amplifier drives the 3× multiplier through the 200
MHz output filter. The 3× multiplier is based on the harmonic content of a
square wave. Its theoretical efficiency is as follows:
efficiency (dB) = 20 log (1/n)
where n = the desired harmonic
For example, if n = 3, then efficiency = –9.5 dB.
The 3× multiplier works by first impedance transforming the 200 MHz input
signal to about 300Ω. This is done so that the input voltage is large enough to
switch the 20 V Schottky diodes. The impedance transformer output
capacitor is also a low impedance for the desired third harmonic. The diodes
are arranged as a full wave bridge rectifier, with an inductor across the "+
and – outputs". One of the "AC inputs" is connected to the input impedance
transformer output. The other "AC input" is connected to the parallel
resonator to ground and the series resonator output.
During the positive part of the 200 MHz cycle, current flows from the
impedance transformer, through D1, through the inductor across the bridge
output, through D3, and then through the inductor in the parallel resonator to
ground. The inductor in the parallel resonator has been chosen to be a low
impedance at 200 MHz.
During the negative part of the 200 MHz cycle, current flows from ground,
through the inductor in the parallel resonator, through D2, through the
inductor across the bridge output, through D4, and then into the impedance
transformer.
It is important to note the current in the inductor across the bridge has
current flow in the same direction in both cases. Since an inductor will not
allow its current flow to change instantaneously, the inductor supplies the
voltage necessary to switch the diodes fast. The fast switching of the diodes
produces a square wave very rich in odd harmonics. The third harmonic
signal flows through the capacitor at the output of the impedance matching
network, through the bridge rectifier squaring circuit and out the series
resonant circuit at the multiplier output. The parallel resonant circuit to
ground is resonant at the third harmonic so it does not short the 600 MHz
signal to ground. The multiplier output level is about +3 dBm.
The 600 MHz signal is amplified by the 600 MHz multiplier output
amplifier. This 19 dB gain amplifier is designed to remain low noise when
HP 70427A/HP 70428A User's Guide 13-19