US3437931A - Shunt fed pi-l output network - Google Patents

Description

Ap 8, 1969 I I P. F. SCHULTZ, JR 3,

I I SHUNT FED PI-L oum u'r NETWORK File d Dec. 16, 1965- r Sheet #br 2- Fi 1' Fe/02 fl27 I v INVENTOR.

: Pawn SZHUITZJRQ I E 1',- ZTTORNEYS P. F. SCHULTZ, JR 3,437,931

SHUNT FED PI -L OUTPUT NETWORK April 8, 1969 Sheet 3- of2 Filed Dec. 16, 1965 INVENTUR.

PORTER F. SCHU/TZ ,JR-

W y ATTORNL'I s United States Patent 3,437,931 SHUNT FED PI-L OUTPUT NETWORK Porter F. Schultz, Jr., Liberty, 11]., assignor to Gates Radio Company, Quincy, Ill., a corporation of Illinois Filed Dec. 16, 1965, Ser. No. 514,325 Int. Cl. H04b 1/66, 1/04; 1103f 1/00 US. Cl. 325-172 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a high frequency transmitter and in particular to a transmitter having a shunt fed Pi-L network for coupling the plate of a power amplifier to a low impedance antenna feed line.

Recent developments in high frequency radio transmitters have led to the widespread acceptance of a Pi configured network for coupling the plate circuit of the RF power amplifier to the antenna feed line. The Pi networks have been found to attenuate to a high degree all spurious and harmonic signals above its resonant frequency and to aid in acquiring maximum power transfer to the antenna.

However, the introduction of the Pi and Pi-L networks as amplifier coupling devices has required applying the plate supply voltage at a point in the circuit that is at a high RF potential. The application of the plate supply voltage at such a point has heretofore been found not to be entirely satisfactory.

In the Pi or Pi-L networks the plate supply voltage can be connected either at the plate end or at the antenna end of the circuit. Both connections, however, present drawbacks to the circuit designer.

Since the supply voltage is to be connected at a point of high RF potential, an adequate plate choke must be used to assure a high impedance path at radio frequencies. However, for the plate end connection the use of the plate choke presents a problem of introducing a series resonant point into the circuit. Should a series resonant point occur in the operating frequency, a low impedance path would be created to ground resulting in the destruction of the choke.

Series resonance in the operating range of 2-30 mc. can be substantially avoided by decreasing the inductance of the choke as a means of increasing the resonant frequency. But decreasing the choke inductance then offers a lower impedance path to the RF signal which greatly increases the choke current and requires forced air cooling or the like.

In contrast, connection of the plate supply at the antenna end of the network reduces the series resonance problem because of the lower circuit impedance at that point. However, the antenna connection has other drawbacks. Principally, connection at the antenna end requires greatly increased ratings for the tuning capacitor and for the high voltage blocking capacitor. The problem of developing capacitors to meet the ratings required under such conditions has assured that the antenna end connection has seldom been attempted.

Accordingly, it is a principal object of this invention to provide a more etficient means of applying a DC voltage supply to the plate of a power amplifier using a Pi or Pi-L coupling network.

It is also an object of this invention to provide a means for feeding the plate circuit of a Pi-L network, while eliminating the series resonance problem encountered at the plate connection.

It is another object of this invention to provide a more effective means for feeding the plate of a radio frequency power amplifier, while eliminating the series resonance problem encountered at the plate connection of a Pi-L network and simultaneously reducing the voltage and current ratings of the tuning and blocking capacitors.

It is a further object of this invention to provide a Pi-L amplifier network for a high frequency, high power radio transmitter which retains the advantages of a plate connected and antenna connected power supply but which eliminates the disadvantages of those connections.

It is also an object of this invention to provide a Pi-L network for a power amplifier of a high frequency transmitter wherein the Voltage power supply is connected at the plate of the amplifier through a coupling inductor which forms a nearly ideal transformer with the tuning inductor of the Pi network.

It is an additional object of this invention to provide a Pi-L coupling network for a high frequency transmitter wherein the tuning coil of the Pi network has a groove formed along the helical periphery thereof, and wherein a DC voltage supply is applied directly to the plate of the power amplifier through a coupling inductor which is embedded within the groove of the tuning coil, and wherein the amplifier end of the power lead is dressed through a metallic tubing which forms the amplifier lead of the tuning coil.

These and other objects, features and advantages of the present invention will be understood in greater detail from the following description and the associated drawings wherein reference numerals are utilized in designating a preferred embodiment and wherein:

FIGURE 1 is a schematic of a prior art circuit showing a Pi network and a plate connected voltage supply;

FIGURE 2 is a view similar to the schematic of FIG- URE 1 showing the antenna connection of the voltage power supply;

FIGURE 3 is also a schematic of a prior art circuit showing a widely used means for connecting the plate voltage power supply in a Pi-L network;

FIGURE 4 is a diagrammatic view of a Pi-L network as used in a high frequency transmitter of this invention showing the transformer feeding of the voltage supply directly to the plate of the power amplifier tube, and

FIGURE 5 is a sectional view of a tuning coil and a coupling coil as employed in the transformer of FIG- URE 4.

Generally, this invention provides a means for overcoming the disadvantages associated with the circuit connections of the three prior art networks shown in FIG- URES l, 2 and 3.

The circuit shown in FIGURE 1 is commonly referred to as a Pi network due to the configuration of the filter circuit employed. In FIGURE 1 a common triode amplifier 10 is shown to have a cathode 11 which is grounded at a circuit point 12 and a plate 13' which is connected to the Pi network and ultimately to the transmitter antenna feed line. The triode 10 is a power amplifier tube and has a grid 14 which may be energized from prior oscillator or frequency multiplier stages.

In FIGURE 1 the circuit elements commonly referred to as constituting a Pi network comprise a tuning inductor 15 and first and second variable capacitors 1'6 and 17. The capacitor 16 is connected from the plate lead wire 18 of the tuning inductor 15 to ground potential at point 19. Similarly, the capacitor 17 is connected from the antenna end lead wire 20 to ground potential at a point 21. The capacitor 16 is utilized as a tuning capacitor in conjunction with the tuning coil to determine the output frequency response of the circuit. The capacitor 17, however while also affecting the tuning of the elements 15 and 16 is used essentially as a loading capacitor to shunt to ground potential all resonant harmonic frequencies developed in the oscillator circuit. The capacitor 17, therefore, keeps undesirable signals from influencing the low impedance antenna feed line 22 which is connected to the lead wire of the tuning inductor 15. In this way, only the desired modulated carrier signal reaches the antenna 23 of the transmitter.

While the Pi configured network shown in FIGURE 1 serves the desirable function of greatly attenuating unwanted signals ahead of the antenna 23, the connection of the capacitors 16 and 17 maintains both the plate end lead wire 18 and the antenna end lead wire 20* of the tuning inductor 15 at a high radio frequency potential. Since, however, a DC power supply must be applied to one side of the tuning inductor 15 in order to provide the power amplification source for the plate 13 of the amplifier tube 10 and to energize the tank circuit consisting of the inductor 15 and the capacitor 16, means must be provided to assure that a low impedance path is not provided through the DC source in the frequency levels of the operating band.

One possible connection for the DC power supply is shown in FIGURE 1. The power supply itself is shown schematically as a battery 24. It is understood, however, that in practice the power supply will generally be the output of a full wave rectifier and filter network.

The power supply 24 is connected to the plate 13 of the power amplifier tube 10' through a plate choke 25 at a circuit junction 26. The plate choke 25 is a source of high impedance at radio frequency potentials and, therefore, both protects the power supply 24 and preserves the RF signal. The power supply 24 is further protected from high frequency components by a by-pass capacitor 27 connected from the lead 28 of the choke 25 to ground potential at the circuit point 29. The introduction of the plate supply voltage at the circuit point 26 requires the use of a blocking capacitor 30 between the connection point 26 and the plate end lead wire 18 of the tuning inductor 15. The blocking capacitor 30 keeps the DC voltage from influencing the low impedance antenna feed line 22.

The introduction, however, of the plate supply voltage directly to the plate 13 as at the connection point 26 introduces undesirable consequences for the high power transmitter. In particular, the very use of the choke to obtain a high impedance path at radio frequencies tends to develop a series resonance condition in the operating band. The existence of such a resonance point within the frequency of the highest potential could easily result in the destruction of the choke. The point of series resonance can be moved above the operating band by significantly reducing the inductance of the choke 25. However, reduction of the inductance of the choke 25 reduces the principal function of that choke, namely to provide a high impedance path at radio frequency levels. The result of reducing the choke inductance is to greatly increase the choke current which results in power losses for the circuit and which may require forced air cooling of the choke.

The circuit of FIGURE 2 is essentially equivalent to the circuit of FIGURE 1 and, therefore, reference numerals have been carried from FIGURE 1 to FIGURE 2. However, FIGURE 2 shows an alternate possibility for the connection of the plate voltage supply to the Pi network. In particular, the plate supply Voltage 24 is connected through the plate choke 25 to a junction point 31 which is the equivalent of the antenna end lead wire 20 of the tuning inductor 15. Also, the blocking capacitor 30 is connected between the point 31 and the low impedance antenna lead 22.

The connection of the plate supply source to the connection point 31 in FIGURE 2 greatly attenuates the problem of series resonance associated with the connection illustrated in FIGURE 1. This is due to the fact that a much lower circuit impedance is exhibited at the point 31 than at the point 2 6 in FIGURE 1. Therefore, the series resonance point as shown in FIGURE 2 does not constitute a problem. However, the circuit connection of FIGURE 2 has circuit problems of its own which more than negate the series resonance advantages of that connection.

The circuit connection of FIGURE 2 presents two important problems to the circuit designer. First, it should be noted that the blocking capacitor 30 has been moved from a point between the plate 13 and the antenna end lead Wire 18 to a point in FIGURE 2- which is at the antenna side of the power supply connection of point 31. Therefore, the blocking capacitor 30 has been moved from a point which keeps the DC supply olf the tuning capacitor 16 to a point which no longer performs that function. The result is that the load on the tuning capacitor 16 has been essentially increased by the amount of the plate supply voltage 24. Due to the low resistance loading of the plate 13 this amounts effectively to doubling the voltage requirement of the tuning capacitor 16. That is, where the tuning capacitor 16 formerly was required to sustain a plate voltage of e the tuning capacitor of FIGURE 2 is required to sustain a combined voltage of approximately arlb- The second problem associated with the application of the DC supply voltage as in FIGURE 2 is the increased demands on the blocking capacitor 30'. First, the capacitor 30 must have a lower reactance in the operating frequency range than capacitor 30 shown in FIG- URE 1 as compared with the reactance of the tuning coil 15. Failure to observe this would cause the reactance of tuning coil 15 to be unnecessarily high. Also, the capacitor 30 now must be sufficiently large to sustain the high currents of the tank circuit consisting of the elements 15, 16 and 17 which was not required in FIGURE 1. For these reasons, the connection of FIGURE 2 is an unsatisfactory solution of the series resonance problem of FIGURE 1.

The circuit of FIGURE 3 is similar in many respects to the circuits of FIGURE 1 and FIGURE 2 and, therefore, common reference numerals have been used in FIGURE 3 also. However, in FIGURE 3 an additional variable inductor 32 has been added between the output of the Pi network and the low impedance lead line 22 of the antenna 23. The inductor 32 may be referred to as a loading inductor and functions in conjuction with the loading capacitor 17 to attenuate harmonic frequency and spurious signals which would otherwise reach the antenna 23. As is well understood, the Pi-L network of FIGURE 3 has a higher plate circuit inductance, and, therefore, the blocking capacitor 30 need not have as low a rectance as the capacitor 30 in FIGURE 2. However, in FIGURE 3 the tuning capacitor 16 must nevertheless sustain the additional plate voltage, Ebb and also the blocking capacitor 30 must be capable of sustaining the circulating current of the tank circuit. Therefore, while the circuit of FIGURE 3 has an advantage over the connection of FIGURE 2, it is not an entirely satisfactory solution to the problem posed by the circuits of FIGURES 1 and 2.

In contrast, the circuit of this invention is illustrated in the embodiment of FIGURE 4 retains the advantages of both the connections of FIGURES 1 and 2 while eliminating the disadvantages associated with those connectrons.

In FIGURE 4, the power supply 24 is connected through the plate choke 25 and ultimately through a lead line 33 directly to the plate 13 of the amplifier tube 10 at a circuit junction 34. However, the connectron of the plate supply voltage to the point 34 is accomplished through a coupling inductor 35 which is wound in proximity with the tuning inductor of the plate tank circuit.

The coupling inductor 35 offers no interference to the application of the DC power supply to the plate 13 of the amplifier tube 10. :In this respect the connection of the power supply to the point 34 in FIGURE 4 is substantially equivalent in result to the connection of i the power supply at the point 26 in FIGURE 1. Therefore, a blocking capacitor 36 can be introduced between the point 34 and the antenna feed line 22 which is also between the point 34 and the tuning capacitor 16. [In this way the rating of the tuning capacitor 16 is reduced from E +e to simple e While the circuit of FIGURE 4 has the advantage of the low tuning capacitor rating of FIGURE 1, it does not have the disadvantage of the series resonance problem as described above. In particular, a series resonance problem does not exist from the point 34 through the inductor 35, as the line 33 is maintained, at the same RF potential as the coil 15 such that RF current is opposed from the point 34 to the point 28. Therefore, and RF current in the choke must be associated with the output connections at point 38 through the shunting capacitor 37 rather than with the input connection at point 34. The line 33 including the coupling inductor may be considered as a parallel line for carrying the RF signal to the antenna feed line 22. Essentially, for the purpose of series resonance considerations, the plate voltage supply 24 may be considerd to be connected directly to the point 38 at the antenna end of the tuning inductor 15.

Therefore, the connection of FIGURE 4 not only reduces the voltage rating of the tuning capacitor 16 but also eliminates the series resonance problem of FIG- UR E 1, as well as maintaining the blocking capacitor 36 away from the tank circulating current of FIGURES 2 and 3. a

The construction of the transformer formed by the tuning coil 15 and the coupling coil 35 is shown in FIG- URE S. In FIGURE 5, the turning coil 15 is shown wound about a supporting shaft or hub 39 which in turn is supported by end walls 40 and 41. The coil 15 has a rectangular cross section and has an adjustable inductance, as illustrated in FIGURE 4, via a provision for a rotatable rack 42 and a roller 43 which is mounted to slide on carriage rod 44 of the rack 42. The roller 43 has a bearing surface 45 and circular side or end flanges 46 and 47. Adequate electrical contact is maintained between the roller 43 and the winding 15 as well as between the roller and the carriage rod 44 of the rack 42. Also, the rack 42 has bearing surfaces 48 and 49 for making electrical contact with the hub or shaft 39. Therefore, if the point 50 on the tuning coil 15 is connected to the plate lead wire 18, and the shaft 39 is connected to the antenna end lead 20, the inductance of the tuning coil 15 can be varied by rotating the rack 42 about the shaft 39 and causing the contact roller 43 to be carried along the inner surface of the coil. It is to be noted that the circular flanges 46 and 47 in conjunction with the bearing surface 45 provide a guide for receiving the winding 15 such that rotation of the rack 42 will cause the roller 43 to be moved axially along the carriage rod 44 in response to the tracing of the helical winding 15. The shaft 39 is connected to the antenna end lead wire of the coil 15 by a conection 51 to a sleeve 52 which is mounted at the end support 41 and which makes an electrical contact with the shaft 39. It is apparent that the end supports 40 and 41 are formed of an electrically insulating material for the purpose of insulating the high voltage shaft 39 from the transmitter chassis or cabinet to which the assembly is mounted.

To insure that all flux lines generated by the coil 15 also encircle the coupling coil 35, a slot 53 is formed along the helical periphery of the coil 15. The slot shown in FIGURE 5 is rectangular in cross section and the winding 35 is disposed in the slot 53 such that the conductor 54 of the winding 35 is substantially recessed at the interior of the winding 15. Suitable insulating material such as a Teflon coating 55 is provided about the conductor 53 to insure electrical isolation between the conductors 15 and 35. The Teflon coating shown in FIGURE 5 is particularly advantageous as it enables the use of a comparatively smaller diameter wire for the winding 35. The comparative sizes of the conductors used to form the tuning coil 15 and the coupling coil 35 are shown in FIGURE 5. The conductor 54 need only supply the DC. voltage to the plate circuit, while the tuning inductor 15 must sustain the high circulating current which is ultimately the source of the electromagnetic radiation at the antenna 23.

The exceptional tight coupling of the coils 15 and 35, as shown in FIGURE 5, insures that little leakage inductance is present in either winding and that all flux lines encircling one winding also encircle the other. In so doing, a near perfect transformer is formed, free of internal parasitic resonances. On the winding of such a bifilar transformer, each point on one winding is at the same RF potential as a corresponding point on the other. This being the case, the insulation between such windings need only be suflicient to stand off the DC. plate voltage applied. Therefore, the use of the Teflon coating as indicated is preferable.

To further insure that the tight coupling between the windings 15 and 35 is sustained, and to eliminate the possibility of the development of harmonic and spurious high frequency signals between the coils 15 and 35, the plate end lead wire 56 of the coupling coil 35 is dressed through a conductive tubing 57 which in turn is electrically secured along the length of the plate end lead 'wire 18 of the tuning coil 15. The tubing 57 may be welded or the like along the length of the lead wire 18 as at the point 58.

Through the circuit structure illustrated in FIGURE 4, and through the use of the transformer shown in FIG- URE 5, the advantages of the three prior art circuits are achieved, while the disadvantages are discarded, and this is accomplished without deleterious effects on the output signal.

It will be understood that various modifications and combinations of the above disclosed embodiments may be accomplished by those skilled in the art, but I desire to claim all such modifications and combinations as properly come within the scope of my invention.

I claim as my invention:

1. In a radio frequency transmitter an amplifier output circuit comprising:

an amplifier device having an input and an output terminal,

a filter network connected to said output terminal for attenuating undesirable frequencies from the output signal,

said filter network including a first network inductor,

a first circuit means having a relatively high impedance at RF frequencies,

a power supply circuit coupled to said first circuit means,

a second circuit means having a relatively low impedance at RF frequencies,

said second circuit means being coupled between said first circuit means and the output of said filter network,

an electrical branch coupled from a point intermediate said first and second circuit means to an output of said amplifier device, and

said electrical branch including a second inductor coupled in a transformer relationship to said first inductor.

2. An amplifier output circuit as described in claim 1, wherein said first inductor is deployed in proximity with said second inductor to form a substantial ideal transformer and wherein said second circuit means comprise a capacitance means.

3. An amplifier output circuit as described in claim 1 wherein the wound conductor forming said first network inductor has a longitudinal slot formed therein and wherein the conductor forming said second inductor is disposed within said slot for forming a substantial ideal transformer.

4. In a radio frequency transmitter, a shunt fed Pi-L network comprising:

a power amplifier device having an input and an output terminal,

a Pi-shaped low pass filter connected to said output terminal,

said Pi-shaped low pass filter having a tuning inductor and first and second capacitors connected at opposite terminals thereof for maintaining said tuning inductor at a substantially high RF potential relative to ground,

a first circuit means having a relatively high impedance at RF frequencies, a power supply circuit coupled to said first circuit means, a second circuit means having a relatively low impedance at RF frequencies,

said second circuit means being coupled between said first circuit means and the output of said low pass filter,

an electrical branch coupled from a point intermediate said first and second circuit means to an output of said amplifier device, and

said electrical branch including a coupling inductor wound in proximity with said tuning inductor and having a substantially zero RF voltage drop from said coupling inductor to adjacent points on said tuning inductor.

5. A shunt fed Pi-L network as described in claim 4 wherein a blocking capacitor is connected between said low pass filter and the point of connection of said electrical branch at said output of said power amplifier, and wherein said second circuit means includes a shunt capacitor.

6. A shunt fed Pi-L network as described in claim 4 wherein the wire forming said coupling inductor is physically embedded within and insulated from the wire forming and tuning inductor, whereby substantially 100% of the magnetic field generated by said tuning inductor envelops said coupling inductor.

7. A shunt fed Pi-L network as described in claim 4 wherein a slot is formed along the helical periphery of said tuning inductor and wherein the wire forming said coupling inductor is disposed within said slot for being enveloped by substantially 100% of the magnetic flux generated by said tuning inductor, a metallic tubing conductively secured along the length of the amplifier end lead wire of said tuning inductor, and the amplifier end lead wire of said coupling inductor being dressed through said metallic tubing.

References Cited UNITED STATES PATENTS 1,917,204 7/1933 Horle 330196 X 1,755,386 4/1930 Chireix 325-172 X 1,837,413 12/1931 Dobson 336-195 2,253,381 8/1941 Lee 325124 2,855,508 10/1958 Barlow et al 325172 X ROBERT GRIFFIN, Primary Examiner.

B. V. SAFOUREK, Assistant Examiner.

US. Cl. X.R.

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