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OPERATING INSTRUCTIONS
~c-Y'I
TYPE 1000·P6
CRYSTAL DIODE MODULATOR
1 INTRODUCTION.
1.1 P U RPOSE. The Type 1000-P6 Crys • stal Diode Modulator (Fig. 1) is a simple device for amplitude modulation of the output of a n r-f generator over the carrier range of 2 0 to 1000 Me. It is especially useful for w ide-band modulation (such as is required for television) or for radio receiver tests where incidental fm must be negli g ibl e .
1.2 DESCRIPTION. The modulator consists of a crystal diode between input and output terminals, an output filter to prevent appreciable modulating voltage from appearin g in the output, and a means of isolating and applying modulating and bias voltages.
Figure 1. Type 1000-P6
Crystal Diode Modulator
2 PRINCIPLES OF OPERATION. (See Fi g . 3.) The crystal-diode resistance, as a function of the voltage across the diode, can be modula te d by a varying vol ta ge. The unit is therefore equivalent to an impedance that can be mod ulate d and that ca n be placed in series with an r-f generator and its load, thereby affordi n g 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 carrier frequency down to de, so that the bias and mod ulation 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 go od modul ation 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 attenuation can be tolerated. Type 874 GlO (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 -w ave modulation is a 1.5-v battery connected 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 adj ustable bias not to exceed 4 volts - is recommended if means ar e available for observin g the modulation characteristic. Since de can be applied through the modulation terminals, the bias
GENERAL RADIO COMPANY
can be included as part of the modulating voltage instead of being applied at the bias terminals. If bias is applied in this way, negative bias is required a t the high modulation terminal. With no bias applied, the modulator will function on the n ega ti ve cycles of the modulating voltage, producing a chopped output. Of course, no bias is necessary if t he applied modulation consists only of ne ga tive pulses .
3.3 MODULATION. Modula tion from 0 to 5 Me can be applied wi th co nnection of a Type
874 Cable or Type 274 M Plug at the modulation terminals. Neg ative modulation increases r-f output. The voltage required for a given percent mo d ulation depends on the
1N21-B diode used and on the bias applied. For sine wave mod ul a tion un de r specified terminal conditions and 1.5-v bias, the average crystal r eq uir es ab out 0 .
2 v rm s f o r 3~ percent modulation. The crystal supplied with the Type 10 00 P6 was s elec te d so that required modulating voltage is within 20% of the stated value and the envelope distortion is lo w at 30% modulation.
Every modulator, with a fixed value of d-e bias and fixed va lu es of ge nerator and loa d impedance, can be calibrated at various carrier frequencies in terms of appl ie d modulating voltage. There are two ways to determine perce nt modulation in t erms of applied modulating voltage and to determine the op timu m bias . In the conventional a -c me tho d, an oscilloscope is connec t ed to the i f output of a rece iver s upplied with a modulated signal from the Type 1000-P6. (It is unlikely that an osci llosco pe will have either the sensitivity or the frequency range to work di rectl y with the Type 1000-P6.) The r-f input voltage to the receiver should be kept at a minimum to p rev e nt receiver nonline a rity. The modulating frequency should be kept low enough so th at th ere is n o possibi lity of si de band cutting in the receiver. A trapezoidal display of the modulation envelope, obtained by use of the modulating voltage for horizont al deflect ion, is best for observing modulation linearity, for de terminin g optimum bias, and for measuri n g pe rc en t modulation.
The second method is to determine the static mod ul a tion characte ri stic of th e Type
1000-P6 by application of various values of positive and ne ga tive d -e vol t age applied to the modulation terminals, with the variation of r-f plo tte d agai n s t d -e vol ta ge. Peak ac required for a given pe rcent modulation is easy to de termine from this p lot. To measure r-f output of the Type 1000-P6, use either a receiver with a ca rrier meter, a d -e vtvm across the diode load resistor, or a microamme t er in se rie s wi th the lo ad resistor. To eliminate errors due to receiver nonlinearity, keep the receiver output indicator a t a fixe d reference level at each value of d-e modulating voltage by adjusting the signal-generator output. Since the change in signal-generator output wi th a fixe d modulator is inversely proportional to the change in modulator output w ith a fixe d r-f input , the pe rcent modulation, linearity, and effect of bias changes can be de termined from plots of si g nalge nerator output vs applied d-e modulating volta ge .
For t-v picture modulation (Fig. 2), the synchroniz in g pulses of the com p osite video
UNIT
OSCILLATOR
Figure 2. Int erconnection Diagram of Unit Oscillator and
Video M odula tor Used as o Television Signal Generator
2
TEST OUTPUT .
TYPE 1000·P6 CRYSTAL DIODE MODULATOR
signal applied must be negative for standard modulation. A suitable source of video signals for test purposes is a t-v receiver tuned to a local station. The output can be taken from the plate circuit of the last video amplifier through a large coupling capacitor and a suitable compensated voltage divider. The video-signal polarity at this point is correct for application to the modulator, and a large voltage division is possible, permitting a minimum disturbance of the receiver circuit conditions. The exact modulating and bias voltages required are best determined experimentally. A 60-to-1 voltage divider at the t-v receiver output has been found satisfactory.
3.4 CRYSTAL REPLACEMENT. To remove the crystal diode : Turn clockwise the threaded ring bearing the words "General Radio Co. Type 874" at the input connector until the ring falls free. Then lift off the input connector and invert the modulator, allowing the crystal cartridge to fall out. If means are available for observing the modulation envelope, select replacement crystals that give 30% modulation with 1.5-v bias and 0.2-v rms modulation applied at a carrier frequency of 100 Me. (R2 may be shunted or replaced to give the required characteristics, if desired.) As a rough guide, crystals should have a d-e forward resistance of from 200 to 300 ohms with 0.25 v applied, and a back resistance of over 10,000 ohms with 1.5 v applied.
3.5 CONNECTORS. A complete line of Type 874 coaxial elements is available, including tees, ells, rigid lines, patch cords, attenuators, filters, etc. Write for information or consult the General Radio Catalog.
+
Figure 3.
Schematic Diagram of
Type 1 OOO-P6 Crystal
Diode Modulator
C1
C2
Dl
L1
L2
R1
R2
R3
R4
PARTS LIST
CAPACITOR, 47 J.lJ.l
f ± 10%
CAPACITOR, 300 J.lJ.l
f ± 10 %
(ERIE)
DIODE
CHOKE, 7.6 J.l
h
CHOKE, 0 .5 f.lh
180 D ±5%,
Y2 w
600-1500 D(I ab adj) ±1 0 %,
510D ±5%, Y2w
560D ±5%, }'2w
Y2w
N750-A
COU 8-2
1N21-B
ZCHA-19
CHA-18
REC-20BF
REC-20BF
REC-20BF
REC-20BF
3
GENERAL RADIO COMPANY
SP
ECI FICA TIOHS
Carrier Frequency Range: 20-1000 Me. Insertion loss increases approx 10 db at 10-Mc carrier frequency due to output filter.
Modulating Frequency Range: 0-5 Me. Response approx 2 db down at 5 Me, with gradual roll-off to prevent serious phase distortion of video signals.
Impedance: Looking into either input or output terminals, impedance is a function of bias and modulating voltages. Unit designed for use with a 50-ohm source and 50-ohm load.
Impedance at modulation terminals approx 600 ohms.
Modulation: With no more than 50 mv r-f input, 30% amplitude modulation is available at carrier frequencies from 20-1000 Me. For optimum sine-wave modulation, an _ average crystal requires 1.5 v a t the bias terminal. Insertion loss under these conditions is approx
12 db, and approx 0.2 v rms at modulation terminals will produce 30% modulation. Max percent modulation is a function of carrier frequency, and at 1000 Me is limited to about
30%. Peak modulation voltage with respect to ground should not exceed 4 v.
Terminals: Type 874 Coaxial Connectors used for r-f and modulation terminals. Modulation terminals accept 874 Connector or Type 274-M . Plug.
Accessories Supplied: One Type 274-MB Plug. ies Required: Proper connectors; 1.5-v battery for fixed bias or 3-v battery and stat for adjustable bias.
1mensions: Width 5, height 4, depth 1-1/6 inches ( 13 .
0 by 105 by 30 mm), over-all.
Weight : 1 lb (0 .5 kg).
, U. S. Patent No. 2,548,457
/0
GENERAL RADIO COMPANY
WEST CONCORD, MASSACHUSETTS
PHILADELPHIA ' llSO York Road
Abington, P e nnsylvan ia
WASHINGTON, BOSS 13th Stre e t
Silv er Spring, Maryland
NEW YORK: Broad Av e nu e at linden
Ridg e field , New Jers ey
CHICAGO: 6605 West North Av enue
Oak Pork , Illinois
CANADA: 99 Floral Parkway
Toronto 15, Ontar io l OS ANGELES: 1000 North Seward Stre et
Los Angel es 38 , California
SAN FRANCISCO: 1186 Los Altos Av e nue
Los Altos , California
REPAIR
SER
VICES
NEW YORK
General Radio Company
Service Department
Broad Avenue at linden
Ridgefield, New Jersey
EAST COAST
G ene ral Rc.dio Company
Service D epar tment
22 Bak e r Avenue
W es t Concord , Massachusetts
WEST COAST
G eneral Rad io Company
S ervice Department
1 000 North Seward Street
Los Angeles 38, California
CANADA
Bayly Eng i n eeri ng , ltd.
First Str eet
Ajax, Ontari · o
MIDWEST
G e neral Radio Company
Service Department
6605 West North Avenue
Oak Park, Illinois
Form 1000-0110-F
May, 1961
Printed in U.S.A.
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