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HEWLETT-PACKARD
JOURNAL
T E C H N I C A L I N F O R M A T I O N F R O M T H E - d p - L A B O R A T O R I E S
Vol. 16, No. 3
PUBLISHED BY THE HEWLETT-PACKARD COMPANY, 1501 PAGE MILL ROAD, PALO ALTO, CALIFORNIA NOVEMBER, 1964
Absorption Modulators for Simple or Complex
Microwave Modulation
High-performance diode modulators permit microwave
modulation by signals ranging from fast waveforms
to dc control voltages without frequency reaction
AMPLITUDE and pulse modulation of micro
wave oscillators have always presented cer
tain problems to the microwave engineer
because of the undesirable 'moding' and in
cidental FM that can occur when modulating
signals are applied to an element in the
microwave tube.
Because of these problems, much interest
has been shown in modulation techniques
which allow the microwave oscillator itself
to operate unmodulated while a diode switch
gates the power on the output transmission
line. These systems, however, have required
isolators or pads to prevent the frequency
'pulling' that is caused in tightly-coupled
oscillators by reflections from the switch.
These limitations are circumvented by
Fig. 1. Waveguide model of P-I-N Modulator can be used on modulation methods which absorb power on
output line beyond coax-to-waveguide adapters or other reflec
tion-producing fittings, and can modulate microwave power the transmission path. This technique not
with sine waves, complex waveforms, or pulsed signals with only allows the oscillator to operate con
extremely fast response. Modulator absorbs microwave power
in accordance with instantaneous bias applied to p-i-n diodes tinuously with fixed electrode voltages, but
and is used here to level microwave power in waveguide system. it also reduces reflections below levels that
cause frequency pulling. In recent years, this
kind of performance has been achieved by
ferrite modulators. The response time of
these devices, however, has been limited by
the switching time of the magnetic field that
controls power absorption.
More recently, p-i-n diodes have demon
strated' a capability for the kind of perform-
1 J. K. Hunton and A. G. Ryals, "Microwave Variable Attenuators and
Fig. by Effect of modulation on klystron operation is shown by Modulators Using P-I-N Diodes," IRE PGMTT Transactions, Vol 10 No 4
July, 1962.
spectrum of RF in klystron cavity. In photo at left, klystron
output is modulated by P-I-N Modulator, which shows no
visible sidebands. In photo at right, klystron output is modu
lated by conventional diode switch; sidebands here show fre SEE ALSO:
quency pulling. (Modulator pulse rate: 10 kc; spectrum width: Microwave signal sources, p. 8
10 kc/cm; vertical scale: linear.)
P R I N T E D I N U . S . A .