US2360764A - Phase modulated carrier receiver - Google Patents

Description

Oct. 17, 1944. M. G. CROSBY 2,360,764

PHASE MODULTED CARRIER RECEIVER Filed Dec. 27, 1941 2 Sheets-Sheet 1 INVENTOR f'z/foy ATTORNEY Oct. 17, 1944. M. G. CROSBY PHASE MODULATED CARRIER RECEIVER Filed Deo. 2'?, 1941 2 Sheets-Sheet 2 INVENTOR I arl/'ag C'y ATTORNEY Patented Oct. 17, 1944 PHASE MODULATED CARRIER RECEIVER Murray G. Crosby, Riverhead, N. Y., assgnor to Radio Corporation of America, a corporation of Delaware Application December 27, 1941, Serial No. 424,583

2 Claims. (Cl. Z50-20) My present invention relates in general to phase modulated carrier wave (PM) receivers, and more particularly to a PM receiveremploying a simplified stable high frequency local oscillator network.

In the prior art of phase modulation reception it has been necessary to utilize automatic frequency control (AFC) to maintain the receiver in tune with the desired carrier frequency. AFC was necessitated by virtue of the fact that the PM receiver included a crystal lter, or required synchronizing with a local oscillator. For example, such AFC systems are shown in Figs. 8 and 9 of my U. S. Patent No, 2,085,008, granted June 29. 1937. Despite the fact that the AFC acts to keepv the receiver in tune, a disadvantage is encountered. .An interfering signal may wrest the control from the desired signal, and thus force the receiver out of tune. For instance, in the case of diathermy interference, which drifts in frequency. the interfering waves are at times stronger than the desired signal and Will actually cause the AFC to detune the receiver. Again, in the case of amateur radio reception the desired signal may be weaker than the undesired interfering signal.

Hence, -it may be said that one of the primary objects of my present invention is to provide a phase modulation receiver of the crystal discriminator type, vand in which the requirement of using AFC is displaced by the use of a stable high frequency oscillator consisting of a crystalcontrolled ultra-high frequency oscillator heterodyned by a variable frequency oscillator operating at a, relatively low frequency.

Another important object of this invention is to provide in a PM receiver of the off-neutralized crystal lter type a stabilized oscillator system to maintain tuning; the tuning being dependent upon the stability of all of the oscillators in the systeminstead of upon an automatic correction network which corrects for the instabilities of the various oscillators.

Still other objects of my invention are to improve the efficiency and simplicity of angular velocity-modulated carrier wave receivers, and more y especially to provide a PM receiver which is reliable and economical.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the invention itself, however, as to both its organizationand method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings: Fig. 1 schematically shows a PM receiver employing the invention,

Fig, la shows the discriminator characteristic ofthe receiver,

Fig. 2 illustrates a specific embodiment of the stabilized oscillator network,

Fig. 3 shows a modification of the network of Fig. 2. o

Referring now to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, in Fig. 1 is shown in schematic manner the various networks of a PM receiver of the off-neutralized crystal filter type. Such a receiver is described in my aforesaid patent. In that patent it is shown how the signals may be collected at ultra-high frequency, and amplified prior to conversion. The usual superheterodyne local oscillator is coupled to the converter, or first detector, and feeds the heterodyne oscillations thereto. The first detector output energy, at intermediate frequency (I. F.), is passed through an I. F. amplifier of the band pass type. The I. F. energy is then transmitted through an amplitude limiter which functions to eliminate the carrier. amplitude variations from the waves reaching'the crystal discriminator. For some typesv of reception, such as that in which selective fading ispresent, it may be desirable to eliminate the limiter unit.

The discriminator comprises the I. F. transformer I whose primary and secondary circuits 2 and 3 are each tuned to the Fc (carrier frequency) value of the PM waves. The ,Fc value is, of course, the operating I. F. value. As is fully explained in my aforesaid patent, a PM wave differs from an amplitude modulated (AM) carrier wave in that in the latter the carrier, at any instant, may be portrayed as a vector at right angles to opposed vectors representing the upper and lower modulation sidebands. The PM wave is represented by the carrier vector being in phase with one of the modulation sidebands. Hence, to convert a PM wave to an AM wave it is necessary to rotate the phase of the carrier through ninety degrees into the aforesaid quadrature relation. Since `this is fully discussed in my aforesaid patent, it is not believed necessary to discuss the vector relations any further.

The PM discriminator, functioning to shift the phase of the carrier degrees relative to the sidebands, comprises the simple off-neutralizedcrystal lter 4 arranged in the path between the high potential side of input circuit 3 and the control grid 5 of coupling tube 6. The cathode resistor bypassed for carrier currents, provides cathode bias for grid 5. Resistor 8 provides a direct current return circuit for the grid. There is developed across the tuned output circuit 9 of screen grid coupling tube 6 the AM Wave energy whose carrier frequency is of the I. F. Value. The AM wave energy may now be transmitted to any AM detector by the following tuned circuit I0. The detected voltage may be utilized at audio frequency in the well known manner.

The characteristic of the unneutralized, or oilneutralized, crystal filter is shown in Fig. la. With this characteristic the phase of the carrier frequency, or the maximum output pointis unshifted, but both the upper and lower side- A bands are shifted 90 degrees in the same direction since the energy at frequencies on either side of the resonant frequency would have a leading phase. Further explanation of the theoretical background of the crystal filter will be found in my aforesaid patent. As shown therein, an AFC system is provided to hold the receiver in tune with the crystal filter, since the selectivity of the crystal filter is very high compared with the degrees of frequency stability obtainable. Not only is the AFC system complex, but on strong interfering signals there may actually occur a wresting of control of the local oscillator from the weak desired signal. l

Hence, the usual local oscillator of the superheterodyne is replaced by a net work comprising a highly stable oscillator. By way of illustration, the oscillator may be a crystal-controlled or linecontrolled oscillator, operating in the 10 megacycle (mc.) range. Such highly stable oscillators are usually diicult to vary in frequency. I

Accordingly, to obtain a variable frequency output at a frequency approximately the same as that of the stable oscillator, the output of the oscillator II is heterodyned in detector I2 by the output of the variable low frequency oscillator I3. The latter may be adjusted by variable tank circuit I4 over a range, for example, of 100 to 300 kilocycles (kc.).' Since oscillator I3 Vis of a low frequency, relative to oscillator it is relatively stable. Its percentage stability may not be high, but the actual change in cycles is small because of its low frequency. The detector output could then comprise a heterodyne, or beat, frequency which is the sum or difference of the frequencies of oscillators and I3. One of these beat frequencies would .be employed to heterodyne the incoming PM wave at the converter to the I. F.Value. The unit, or net work, |3-||-| 2 functions as a highly stable source of local oscillations and readily replaces the AFC system. Further, a strong interfering signal will have no effect on the network |3-I |-|2, since the latter is not responsive to received signals,

In Fig. 2 I have shown a specific form of stabilized oscillator network that may be used. Thus, tube 'I0 is employed in the stable oscillator II; tube 90 is used in the detector I2; and tube 80 is utilized at the variable low frequency oscillator I3. The tube 'I0 may be a pentode having a resonant tank circuit 'Il tuned to the same frequency as crystal 12. The latter is shunted by grid leak resistor 13, and it will be seen that the crystal-controlled oscillator is of a well-known type. Similarly, the adjustable oscillator I3 has its variably-tunable tank circuit |4 connected to triode 80 in conventional manner. In the grid circuit of the tube is located a bypassed leak resistor having its grid end grounded.

The oscillatory output of oscillator tube I0 is applied to grid 9| of detector tube 90. The output of oscillator 80 is applied to grid 92 of tube 90. The tube may be of the pentagrid type. The usual bypassed self-biasing resistor 93 is located in the cathode circuit of tube 90. Grid bias is applied to grids 9| and 92 over grid-return resistors 9|' and 92 respectively. The resonant circuit 94 is arranged in the plate circuit of tube 90. The adjustable tuning condenser 95 is varied concurrently with the adjustable condenser I4 so that there will be developed across circuit 94 the voltage of proper frequency. Assume oscillator II is operating at 10 mc., and oscillator I3 is adjustable over a range of to 300 kc. If the sum heterodyne is employed across circuit 94, the condenser 95 would be adjustable to tune the latter over a range of 10,000 plus 100 to 300 kc., or between 10,100 and 10,300 kc. Of course, the range could be 9700 to 9900 kc. A common unicontrol mechanism may be used for adjusting the condensers I4', 95 and the tuning condenser in the converter input circuit. In this way there would be produced a constant I. F. value in the converter output circuit; the I. F. value could be 455 kc., by way of example.

In general then, in Fig. 2 each oscillator and I3 feeds its output to a respective grid of the multi-grid detector tube, and the heterodyne output of the detector is taken from the plate circuit thereof and fed to the converter. By biasing the grids 9| and 92 of the detector so that they operate on the non-linear portions of their characteristics, the resonant circuit 94 maybe tuned to an harmonic of the beat frequency as well as to the fundamental beat frequency. This is true since the non-linearity would cause a heterodyning action between the harmonics of the two oscillators, or between the harmonic of one oscillator and the fundamental frequency of the other. In this Way the crystal-controlled oscillator would operate at 10 mc., and the heterodyne output of the detector could be 20 mc. plus or minus 100 to 300 kc.

In the arrangement of Fig. 3 there is utilized a balanced detector circuit in place of the single tube 90 to balance out the high frequency oscillations from the resonant output circuit of the detector I2. The stable oscillator consists of a pentagrid tube |00. The injector grid |0| is used as a plate, and, hence, is connected through resistor |02 to the source of plate voltage plus B. The control grid |03 is connected to the grounded cathode through the grid leak resistor |04. A plurality of crystals |05, |06 and |01 are arranged for selective connection between the upper end of resistor |02 and the adjustable tap |08. In this way a desired crystal can be connected between the anode electrode IOI and the .control grid |03.

In this type of oscillator the crystals are tuned to different frequencies so that a wider band of frequencies may be covered. For example, a band of 10 to 20 mc. could be covered in this way. 'I'he range of frequency covered by this system is limited to the sum and difference between the lowand high-frequency oscillations. In order to cover a Wide range of frequencies, the crystal frequency would have to be changed'when one of these limits was reached. Also, for the particular range of frequencies close to the frequency of the stable oscillator, the low-frequency oscillator has such a low frequency that the heterodyne output is difficult to separate from the crystal oscillator frequency. Hence, `to cover the range in the immediate vicinity of the crystal oscillator frequency, another crystal must be switched in.

The output of oscillator tube is taken from the resonant circuit connected to the plate |09. The plate circuit itself comprises a coil I I I which is shunted by a pair of series arranged condensers. The condensers I|2 and II3 provide a balanced tuning condenser, and the junction of the condensers is connected to ground. The condenser II4 in shunt with condenser II3 is provided to compensate for the fact that one side of the balanced push-pull input circuit does not have the plate to ground capacity of oscillator tube |00 across it. The split plate coil I I I is connected to the plus B supply through a choke coil III', the lower end of the latter being by-passed to ground.

The variable low frequency oscillator I3 comprises the tube |30 whose plate and grid circuits include the variable tunable circuit I4 Since this circuit is purely conventional it is not necessary to describe it in any further detail, except to point out that the oscillatory output thereof is adjusted in frequency by variation of the tuning condenser i4. The oscillations from the low frequency oscillator are fed in-phase to the injector grids of the multi-grid detector tubes IIO and |20.. Tube IIO has its inner control grid |15 coupled to one end of coil I I I, while tube |20 has its inner control grid I2I coupled to the lower end of coil III. The grids IIS and I2I are connected to ground through separate grid leak resistors, and the common cathode connection of tubes I I0 and |20 is connected to ground through the usual bypassed biasing resistor.

The injector grid II6 of tube IIO is connected in common to the injector grid |22 of tube |20, and the common connection is made through a coupling condenser |3I to the grid of oscillator tube |30. The grounded grid-return resistor |32 provides a biasing path for grids IIB and |22. The heterodyne output is taken from the common detector plate circuit. The plates of tubes IIO and |20 are connected in common. The common connection is in turn arranged for selective connection to various taps on a tapped coil |40. Adjustment of the adjustable tap I4! permits a selected portion of coil |40 to be connected in the plus B supply to the plates of the detector tubes. In this way frequency variation of the output is facilitated. The adjustable condenser |42 is connected across the selected portion of coil |40, and its adjustment will be similar to that of condenser 95 in Fig. 2.

In the case of the system of Fig. 3 it will be clear that the stable oscillator Output could be balanced out of the detector output circuit equally well by feeding the oscillator voltage from oscillator II to the detector grids in-phase, and combining the detector outputs in push-pull. This feature of balancing out the stable oscillator frequency from the detector output circuit prevents the stable oscillator from acting as a heterodyne oscillator which might allow interfering signals to be received. Of course, the output developed across circuit I42-I40 would be applied to the converter shown in Fig. 1.

modulated carrier waves of the type comprising a converter to reduce received waves to an intermediate frequency, and a discriminator of the crystal filter type adapted to convert the phase modulated waves to amplitude modulated waves;

y the improvement which comprises a source of `stabilized local oscillations for said converter, said source comprising a stable n frequency oscillator operating at a relatively high frequency close to the value of said modulated carrier wave frequency, a second oscillator of a frequency which is a small fraction of said stable oscillator frequency, said second oscillator frequency having such a low value of cyclic change as to insure stabilization of said source of local oscillations, means to cornbine the output of said two oscillators to provide a highly stable heterodyne frequency which is unaffected by a strong interfering signal thereby to maintain the receiver in tune With a desired carrier frequency, and means for applying said heterodyne to said converter, said combining` means comprising a multi-grid detector tube, and

means for applying said two oscillator` outputs to spaced control grids of the detector tube.

2. In a superheterodyne receiver of phase modulated carrier waves of the type comprising a converter to reduce received Waves to an intermediate frequency, and a discriminator of the crystal lter type adapted to convert the phase modulated waves to amplitude modulated waves; the improvement which comprises a source of stabilized local oscillations for said converter, said source comprising a stable frequency oscillator operating at a relatively high frequency close to the value of said modulated carrier wave frequency, a second oscillator of a frequency which is a small fraction of said stable oscillator frequency, said second oscillator frequency having such a low value of cyclic change as t0 insure stabilization of said source of local oscillations, means to combine the output of said two oscillators to provide a highly stable heterodyne frequency which is unaffected by a strong interfering signal thereby to maintain the receiver in tune with a desired carrier frequency, and means for applying said heterodyne to said converter, said combining means comprising a pair of multigrid tubes, means applying the output of said stable oscillator to like grids of said pair of tubes in a desired phase relation, means applying the output of the second oscillator to a different pair of like grids of said tubes in a phase relation opposite torsaid first phase relation, and means for derivng said heterodyne frequency from the output of said pair of tubes free of the energy of said stable oscillator.

MURRAY G. CROSBY.

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