GENERAL RADIO COMPANY 1000-P6 Operating Instructions

OPERATING INSTRUCTIONS
TYPE 1000·P6
CRYSTAL DIODE MODULATOR
1 INTRODUCTION.
1.1 PURPOSE. The Type 1000-P6 Crys-
stal Diode Modulatot (Fig. 1) is a simple
device for amplitude modulation of the
output of an r-f generator over the carrier
range of 20 to 1000 Me.
It is especially
useful for wide-band modulation (such as
is required for television) or for radio re-
ceiver tests where incidental fm must be
negligible.
1.2 DESCRIPTION. The modulator con-
sists of a crystal diode between input and
output terminals, an output filter to pre-
vent appreciable modulating voltage from
appearing in the output, and a means of
isolating and applying modulating and
bias voltages.
Figure
1.
Type
lOOO-P6
Crystal Diode Modulator
2 PRINCIPLES OF OPERATION. (See Fig. 3.) The crystal-diode resistance, as a func-
tion of the voltage across the diode, can be modulated by a varying voltage. The unit is
therefore equivalent to an impedance that can be modulated and that can be placed in se-
ries with an r-f generator and its load, thereby affording a means of producing amplitude
modulation.
3 OPERATION.
3.1 GENERAL. The resistance of the r-f source should be 50 ohms or less from the car-
rier frequency down to dc, so that the bias and modulation will be properly applied to the
crystal.
If necessary, insert a 50-ohm, 10- or 20-db pad between the r-f source and the
modulator. The load presented to the output should be 50 ohms for a good modulation
characteristic. Higher resistance loads ·should· be shunted by suitable resistances, or a
50-ohm, 10- or 20-db pad can be inserted between output and load if the additional attenu-
ation can be tolerated. Type 874-G10 (10 db) and Type 874-G20 (20 db) Attenuators are
recommended. The applied r-f input should not exceed 50 millivolts.
3.2 BIAS. A convenient bias supply for sine-wave modulation is a 1.5-v battery connect-
ed across the bias terminals with polarity as indicated on the nameplate. This reduces
the crystal resistance from the high unbiased condition to a value about in the center of
an average crystal characteristic. Increasing the bias increases the output. A variable
bias supply (e.g. a 3-v battery in series with a 10,000-ohm rheostat) may improve the
modulation characteristic. With an asymmetrical modulating signal an adjustable bias -
not to exceed 4 volts - is recommended
if
means are available for observing the modula-
tion characteristic. Since de can be applied through the modulation terminals, the bias