Spurious frequency suppressor

Abstract: Spurious frequences are suppressed in a transmitter (7) by an RF pulse fall time controller circuit (10) having a quadrature hybrid matched attenuator (23) located between a predriver (8) and the first stage of class "C" driver amplifiers (11). The controller circuit (10) linearly attenuates RF pulses and slows pulse fall time. Operation of the circuit (10) is initiated when the rising edge of a pretrigger input to a pulse shaper (12) starts a pretrigger generator (14) which then triggers the application of bias current (I BIAS ) by the pulse shaper (12) to the attenuator circuit (23). After a controlled period of time determined by a monostable (16) in the pretrigger generator (14), the pulse shaper (12) decreases the forward current bias of PIN diodes (34,38) in the attenuator (23) thereby increasing the attenuation of the RF pulses. This creates a slow pulse fall time thereby preventing the occurence of spurious emissions which would otherwise be caused by parasitic ringing of the cascaded Class C common base transistor amplifiers (11) in the transmitter (7).

Description: This invention relates to apparatus for producing amplified radio frequency pulses, the apparatus comprising: means for providing RF pulses; means for Class-C amplifying said RF pulses; and means coupled between said RF pulses providing means and said RF amplifying means for suppressing the generation of spurious frequencies. The Class C common base amplifier is the most widely used configuration for power stages of radio frequency (RF) signals, particularly from UHF to S band. It is more efficient than other modes of operation (Class A, Class AB, etc.) and provides higher gain and better stability than its common emitter counterpart. An illustrative Class C common base transistor amplifier comprises an NPN transistor having its emitter electrode as an input terminal, its collector electrode as an output terminal, and its base electrode coupled to a reference voltage, typically ground. In this configuration, the collector electrode is biased positively with respect to the emitter electrode. The operation of this amplifier can be described as follows: during the negative half cycle of the input signal, the base-emitter junction is forward biased. After the negative voltage surpasses the base-emitter voltage drop, the transistor turns on and draws collector current. This current forms the negative half cycle of the output signal. During the positive half cycle of the input signal, the base-emitter junction is reverse biased, and the transistor is turned off. During this time, energy stored in the transistor collector circuit is dissipated in the load to form the positive half cycle of the output signal. Because the transistor is conducting for less than 180 degrees of the RF cycle, this is a very efficient mode of operation. Unfortunately, it is also the mode that produces the worst output spectrum. During normal operation of a Class C common base amplifier, the current flowing out of the emitter will have a small dc component due to the self bias of the base-emitter junction. This dc component flows through the emitter bias return to ground, whenever the RF signal is present at the transistor input. Because the emitter bias return has a certain amount of inherent inductance, energy will be stored during the RF pulse. At the end of the RF pulse, this energy discharges through the transistor input terminals. This inductive discharge injects holes into the transistor base region, and thereby keeps the transistor on for a period determined by the time constant of the inductance and the input parasitics of the transistor. Because the transistor is biased on during this transient period, any positive feedback path that exists is allowed to produce spurious oscillations or ringing. These oscillations appear in the output spectrum as spurious frequency components. These frequency components appear as relatively wide peaks in the skirt of the output spectrum profile, as may be seen in FIG. 5. As shown, the amplitudes of many of these components may exceed acceptable levels as typically specified for use in high power radar transmitters. As an example, most high power radar systems in use today require all spurious components to be from 70 to 90 decibels below the carrier frequency amplitude (dBc) in a one kilohertz bandwidth. The suppression of spurious frequency components in Class C common base transistor amplifiers is described in U.S. Patent No. 4 928 072, issued May 22, 1990, to John C. Scott and assigned to the present assignee. The circuit described in that patent includes a capacitor and PIN diode in series between an emitter and ground of the Class C common base transistor amplifier. The PIN diode is biased on just before the RF pulse ends and this slows the pulse fall time and the generation of spurious frequencies resulting from parasitic ringing is prevented by the locking of the frequency of operation of the transistor to the carrier frequency. In an amplifier having three Class C stages in cascade, the first stage being a Class C common emitter amplifier and the second and third stages being Class C common base amplifiers, only the third stage is provided with capacitor and PIN diode circuitry for preventing the generation of spurious frequencies. A solid state transmitter in high power radar systems uses hundreds or thousands of Class C transistor amplifiers in the output stages, thereby requiring hundreds or thousands of Scott's spurious frequency suppressor circuits. This results in considerable additional electronics affecting size, weight, power, cost and reliability of the radar system. US-A-4 320 399 describes a radar transmitter having an apparatus of the kind defined hereinbefore in which the suppressing means, which shapes each substantially rectangular pulse of radio frequency signal supplied thereto, is a Class C common base NPN transistor amplifier adapted to operate as a modulator by a pulse shaper which applies to the collector of the transistor a voltage pulse having continuous, as opposed to discontinuous, rising and falling edges. The purpose of this is to cause the radio frequency pulses produced by the amplifying means to have an envelope substantially free of discontinuities in the rising and falling edges, instead of very sharp edged rising and falling edges, so that the spectral side lobe energy being transmitted is reduced. However, a degenerate parametric oscillation is sometimes found in the output power RF signals, the oscillation having a frequency which is approximately half the radio frequency carrier signal frequency. Consequently, further measures are necessary to suppress this spurious frequency, and there is accordingly described an LC resonator which is added to the collector output of the modulator circuitry. The inductive and capacitive values chosen for the resonator result in a short circuit at the spurious frequency and an open circuit at the radio frequency carrier signal frequency. Also, a series capacitor is included to provide a path for the radio frequency carrier signal, and a resistor is connected in parallel with the series capacitor to provide a resistive load for the spurious frequency. In EP-A-0 405 349 (corresponding to CA-A-2 018 854) there is described a controlled attenuator circuit for a radio transceiver. The controlled attenuator circuit is inserted between the antenna and the output of the transceiver circuitry, and includes a directional coupler having a first port arranged as an input for radio frequency from the transceiver circuitry, a second port arranged as an output port for supplying radio frequency signals to the antenna, and third and fourth ports coupled to a controlled attenuation network. The directional coupler is a 3dB, 90° hybrid coupler, and the attenuation network includes PIN diodes that are controlled to be either fully conductive or substantially in their high resistance state by current from the collector of a bipolar transistor that is itself controlled by the timed application of square voltage pulses. When the voltage pulse is present, the transistor is biased off, the PIN diodes are held in their high resistance state, and the directional coupler operates to apply 50 ohm resistors to attenuate radio frequency signals to some extent. When the voltage pulse is not present, the directional coupler allows radio frequency signals to pass substantially unattenuated between the transceiver circuitry and the antenna. The voltage pulse is timed to coincide with a change in transmitting frequency if the transceiver is being used in a frequency hopping mode, or with a switch from transmission to reception. Spurious signals are generated by the transceiver circuitry during frequency changes and switching between transmission and reception, and are said to be in the frequency range below the intended transmission and reception range. However, this known controlled attenuator circuit is not sufficient by itself, and therefore a high-pass filter is inserted between the second port (output port) of the directional coupler and the antenna. According to one aspect of the present invention, apparatus of the kind defined hereinbefore at the beginning is characterised in that the suppressing means comprising: a directional coupler having four ports, a first port being an input port and a second port being an output port; said input port of said directional coupler being provided with said RF pulses from said providing means; means coupled to a third port and a fourth port of said directional coupler for attenuating said RF pulses in accordance with a control signal; and means coupled to said attenuating means for generating said control signal, in that said attenuating means comprises a PIN diode network means for controlling the amount of attenuation of said RF pulses in accordance with said control signal, said PIN diode network means changing the fall time of said RF pulses in accordance with said control signal, and in that said control signal generating means comprises pulse shaping means for generating a bias current for said PIN diode network means, said pulse shaping means comprising: means for adjusting the pulse width of a trigger signal in response to a pretrigger input signal; means coupled to said adjusting means for generating a voltage ramp signal; and means coupled to said voltage ramp signal generating means for generating said bias current for said PIN diode network means. According to another aspect of the invention there is provided a method of suppressing spurious frequencies in a transmitter comprising the steps of: providing a source of RF pulses; amplifying said RF pulses with a plurality of Class-C amplifier stages; and suppressing the generation of spurious frequencies by coupling a suppressing means between said source of RF pulses and said amplifier stages, characterised in that the suppressing step further comprises the steps of: providing said RF pulses to an input first port of a four port directional coupler with a second port being an output port; attenuating said RF pulses with means coupled to a third port and a fourth port of said directional coupler in accordance with a control signal; and generating said control signal with means coupled to said attenuating means, in that said step of attenuating said RF pulses comprises the step of providing a PIN diode network means for controlling the amount of attenuation of said RF pulses in accordance with said control signal, said step of providing said PIN diode network means for controlling the amount of attenuation further comprising the step of changing the fall time of said RF input pulses in accordance with said control signal, and in that said step of generating said control signal comprises the step of using pulse shaping means for generating a bias current for said PIN diode network means, said step of generating said bias current with said pulse shaping means comprising the steps of: adjusting the pulse width of a trigger signal in response to a pretrigger input signal; generating a voltage ramp signal in response to said trigger signal; and generating said bias current for said PIN diode network means with current source means coupled to said voltage ramp signal generating means. Preferred embodiments of the invention may be used to suppress the generation of spurious frequencies in amplifier means of a transmitting device such as a radar system transmitter; to suppress spurious frequencies of a transmitter using minimal additional electronic hardware. In a preferred embodiment, the directional coupler comprises a 3 dB 90° hybrid. Brief Description of the Drawings Other and further features and advantages of the invention will become apparent in connection with the accompanying drawings wherein: FIG. 1 is a simplified block diagram of a transmitter including a spurious frequency suppressor of the present invention; FIG. 2 is a combination block diagram and schematic of spurious frequency suppressor circuitry of the present invention; FIG. 3 is a schematic diagram of the voltage controlled current source shown in FIG. 2; FIG. 4 is a timing diagram useful in understanding the present invention; FIG. 5 illustrates the frequency spectrum of an RF output from a radar transmitter using Class C common base amplification without a spurious frequency suppressor circuit; and FIG. 6 illustrates the frequency spectrum of a radar transmitter RF o output shown in FIG. 1 when the transmitter uses the present invention of a spurious frequency suppressor circuit. Description of the Preferred Embodiment Referring to FIG. 1, a simplified block diagram is shown of a transmitter 7 comprising a predriver 8, a spurious frequency suppressor 10 and driver amplifier stages 11. A radio frequency (RF i ) signal is fed to the predriver 8 for amplifying RF i and the predriver output, RF IN, is fed to the spurious frequency suppressor 10 for generating a controlled RF OUT signal. The RF OUT signal is fed to the driver amplifier stages 11. However, the spurious frequency suppressor 10 located in the front end of the transmitter 7 controls the RF waveform to prevent generation of spurious frequencies in the driver amplifier stages 11. Referring to FIG. 2, a diagram of the spurious frequency suppressor 10 is shown which suppresses or controls the spurious frequencies typically generated in Class C amplifiers of a solid state transmitter 7, for example, in a radar system. Suppression of the spurious frequencies is accomplished by controlling the fall time of the RF pulse (RF OUT) that is fed to the driver amplifier stages 11. The spurious frequency suppressor 10 comprises a pulse shaper 12 for generating a PIN diode bias current (I BIAS ). The PIN diode bias current (I BIAS ) is fed to PIN diodes 34, 36 in the RF section 23 circuitry. Controlling the bias current through the PIN diodes 34, 36 controls the RF attenuation; hence, the RF pulse shape is controlled directly by the PIN diode bias current (I BIAS ). The RF section 23 is a quadrature hybrid matched attenuator with shunt connected PIN diodes 34, 36. It includes a 3 dB, 90° hybrid 24 comprising ports 1, 2, 3 and 4. Ports 3 and 4 are coupled to the shunt connected PIN diodes D1 34 and D2 36 via DC blocking capacitors C1 26 and C2 28 respectively. Series inductors L1 30 and L2 32 isolate the RF from the bias signal input (I BIAS ) and they are connected across the junction of C1 26 and D1 34 and C2 28 and D2 36 respectively. The PIN diode bias current (I BIAS ) is fed to the RF section 23 at the junction of L1 and L2. A capacitor C3 38 is connected to the L1 and L2 I BIAS junction point for filtering out RF signals on the DC line. Load network Z 1 42 is connected in series with DC blocking capacitor C4 40 which is connected to D1 34, and load network Z 2 46 is connected in series with DC blocking capacitor C5 44 which is connected to D2 36. Still referring to FIG. 2, an RF IN signal enters the 3 dB, 90° Hybrid 24 at port 1. The amplitude of the signal is split equally and exits at port 3 and port 4 with the phase of the signal at port 4 lagging by 90°. If the diodes D1 34 and D2 36 are heavily forward biased, they have a very low series resistance,Rs, which is approximately zero ohms, and behave as a short circuit. In this short circuit state all of the RF signal from the 3 dB 90° hybrid 24 is reflected off the diodes and applied back to ports 3 and 4 of the 3 dB 90° hybrid 24. These two reflected signals add in phase and exit at port 2 as the RF OUT signal. At port 1 such reflected signals are 180° out of phase and fully cancel out. Under ideal conditions of a lossless, reciprocal system, the signal power at the RF OUT port would equal the power at the RF IN port. If the diodes D1 34 and D2 36 are biased with very low current, e.g. R s > 1000 ohms, they emulate an open circuit. Under this condition all of the RF signal is dissipated in the loads Z 1 42 and Z 2 46. If the bias to the diodes is varied and thus Rs is varied, the diodes will be operating in between the ideal short circuit state and the ideal open circuit state. Such operation allows a percentage of the RF signal to be reflected while the remaining signal power is dissipated in the loads Z 1 42 and Z 2 46. Therefore, the amplitude of the reflected signal and the RF OUT signal is lower than the RF IN signal. By selecting the appropriate PIN diode bias current (I BIAS ) to the diodes D1 and D2, a wide range of attenuation levels is possible. Controlling the shape of the bias current (I BIAS ) allows control of the RF output signal (RF OUT) amplitude resulting in pulse shaping by the method of RF signal attenuation. Still referring to FIG. 2, the pulse shaper 12 comprises a pretrigger generator 14, a voltage ramp generator 20 and a voltage controlled current source 22 which generates the PIN diode bias current (I BIAS ). A pretrigger input signal to the pretrigger generator causes a PT signal to be generated, the rising edge of which starts the voltage ramp generator 20. The V RAMP signal from the voltage ramp generator 20 is coupled to the voltage controlled current source 22 which generates the I BIAS signal for the PIN diodes 34, 36. The monostable 16 in the pretrigger generator 14 controls the current bias (I BIAS ) pulse width to the PIN diodes 34, 36. The falling edge of the I BIAS results in an increase in the series resistance (R S ) across the PIN diodes 34, 36, which increases the attenuation and controls the fall time of the RF pulse (RF OUT). The RF section 23 of the spurious frequency suppressor 10 utilizes the current controlled resistance characteristics of the PIN diodes 34, 36 at their low loss state (i.e. R S being very small) and at in between finite resistance values. The attenuation (A) as a function of PIN diodes 34, 36 series resistance (R S ) is determined by the expression A = 20 log (1 + 2R S /Z o ) -1 dB where Z 0 = Z 1 = Z 2 . Table 1 shows the relationship between the series resistance (R S ), forward current (I BIAS ), (V f ), forward voltage and attenuation (A) of a shunt connected PIN diode such as Model No. MA4P604-131 manufactured by M/A-COM of Burlington, Massachusetts, used for PIN diodes 34, 36 in the present embodiment RF section 23. This M/A-COM PIN diode is a silicon chip diode with a maximum junction capacitance of 0.30 pF at 100V. The thermal resistance of such diode is 15 °C/W. Rs (Ω) I BIAS (mA) V f (V) A (dB) 0.5 100 0.92 0.2 5 3 0.68 1.58 10 1 0.62 2.92 50 0.30 0.52 9.54 100 0.10 0.48 13.97 500 0.02 0.40 26.44 1000 0.01 0.36 32.25 The pulse shaper 12 decreases the forward current diode bias from 100 ma to 0.3 ma over a 2 microsecond nominal window at the end of the RF pulse to provide an attenuation slope of 0 dB to 10 dB. This effectively slows the fall time of the RF pulse to 2 microseconds nominal and suppresses the spurious frequencies at the transmitter 7 output (RF o ). Referring now to FIG. 3, a schematic diagram of the voltage controlled current source 22 is shown comprising an amplifier 50 coupled to the base of transistor 52 forming a current source for generating the voltage controlled I BIAS . This circuit known to one skilled in the art is capable of driving a grounded load, and its output is independent of load impedance provided R2 = R4, R1 = R3 and R2 ≥ 100 R5. Under these conditions I BIAS = V RAMP {R2/(R1 x R5)} In the present embodiment the voltage rate of I BIAS is 50.27 ma per volt. When the output of the voltage ramp generator 20 is 4.3 volts, the total PIN diode bias current (I BIAS ) is typically 216.2 ma or 108.1 ma per diode. Referring now to FIG. 4, a timing diagram is shown which is useful in understanding the invention. In the transmitter 7 of the present embodiment shown in FIG. 1, RF o output pulse widths of both one microsecond and 100 microsecond are transmitted based on a particular radar system application. However, other pulse widths are equally feasible depending on radar system requirements. The pretrigger input to the pretrigger generator 14 of the pulse shaper 12 occurs 9.5 µsec before the RF i input pulses to the transmitter 7 to set up current bias I BIAS timing in the pulse shaper 12. The RF i input pulses provided to the transmitter 7 are 3 microseconds and 102 microseconds respectively as shown in FIG. 4. The RF i pulse widths input to the transmitter 7 are two microseconds longer than the required RF o output pulse widths so that the spurious frequency suppressor 10 may create a 2 microsecond fall time on the RF OUT pulses. This slow fall time eliminates the spurious spectra at the RF o output of the transmitter 7. The PIN diode bias current I BIAS , from the pulse shaper 12 is activated by the start of the pretrigger input which generates the PT signal. I BIAS ramps up to full amplitude for a controlled pulse width. The bias current, I BIAS , then decreases linearly to zero milliamps in 2 microseconds. The pulse width and slope of the current ramp are controlled by the pretrigger generator 14, voltage ramp generator 20, and voltage controlled current source 22 of the pulse shaper 12. Both pulse width and slope of the current ramp are adjustable so that the spurious frequency suppressor 10 may be used with many different RF pulse widths and in applications with other system parameters. The RF OUT pulse from the spurious frequency suppressor 10 shows the one microsecond and 100 microsecond pulses along with a slow fall time of 2 microseconds. Note that when the PIN diode bias current, I BIAS , is decreased, the attenuation is increased, thus creating the slow fall time. This results in uncorrupted RF o output pulses from the transmitter 7 after several amplifier stages 11 of Class C amplification. The RF o output pulses are 1 microsecond and 100 microseconds long with fast fall times. Note that the RF o output pulses from the transmitter 7 are delayed by a period of time, which in this embodiment is 0.5 microseconds as shown in FIG 4. This delay occurs because each Class "C" driver amplifier stage 11 turns on in succession and correspondingly the pulse widths are shortened and the rise and fall times become faster as is the case of the Scott invention (U.S. Patent No. 4,928,072). In other transmitter application embodiments there may be many more cascaded amplification stages whereby it may be desirable to add another spurious frequency suppressor 10 within the cascaded chain of such amplification stages. Referring now to FIG. 5, the frequency spectrum of a radar transmitter output, RF o , is shown without the use of a spurious frequency suppressor 10. Spurious oscillations are observed in the frequency spectrum at 60 dB below the carrier frequency amplitude. As noted previously hereinbefore, most high power radar systems in use today require all spurious components to be from 70 to 90 decibels below the carrier frequency amplitude (dBc) in a one kilohertz bandwidth. Referring to FIG. 6, the frequency spectrum of the radar transmitter output, RF o , is shown for a transmitter 7 embodying the present invention of a spurious frequency suppressor 10. Comparing the spectrum profile therein with the spectrum profile of FIG. 5, there is seen the significant degree of improvement by employing the present invention in which all off carrier frequency components are at least 80 dB below the level of the carrier. This concludes the description of the preferred embodiment. However, many modifications and alterations will be obvious to one of ordinary skill in the art. For example, in some radar system applications more than one spurious frequency suppressor 10 may be employed in a cascaded chain of amplifier stages.

Claims: Apparatus for producing amplified radio frequency pulses, the apparatus comprising: means (8) for providing RF pulses; means (11) for Class-C amplifying said RF pulses; and means (10) coupled between said RF pulses providing means and said RF amplifying means for suppressing the generation of spurious frequencies, characterised in that the suppressing means (10) comprising: a directional coupler (24) having four ports, a first port (1) being an input port and a second port (2) being an output port; said input port (1) of said directional coupler being provided with said RF pulses from said providing means (8); means coupled to a third port (3) and a fourth port (4) of said directional coupler (24) for attenuating said RF pulses in accordance with a control signal; and means (12) coupled to said attenuating means for generating said control signal, in that said attenuating means comprises a PIN diode network means (26,28,30,32,34,36,40,42,44,46) for controlling the amount of attenuation of said RF pulses in accordance with said control signal, said PIN diode network means changing the fall time of said RF pulses in accordance with said control signal, and in that said control signal generating means comprises pulse shaping means (12) for generating a bias current for said PIN diode network means, said pulse shaping means comprising: means (14) for adjusting the pulse width of a trigger signal in response to a pretrigger input signal; means (20) coupled to said adjusting means (14) for generating a voltage ramp signal; and means (22) coupled to said voltage ramp signal generating means (20) for generating said bias current for said PIN diode network means. Apparatus according to claim 1, characterised in that said directional coupler (24) comprises a 3 dB 90° hybrid. A method of suppressing spurious frequencies in a transmitter comprising the steps of: providing a source of RF pulses; amplifying said RF pulses with a plurality of Class-C amplifier stages; and suppressing the generation of spurious frequencies by coupling a suppressing means between said source of RF pulses and said amplifier stages, characterised in that the suppressing step further comprises the steps of: providing said RF pulses to an input first port of a four port directional coupler with a second port being an output port; attenuating said RF pulses with means coupled to a third port and a fourth port of said directional coupler in accordance with a control signal; and generating said control signal with means coupled to said attenuating means, in that said step of attenuating said RF pulses comprises the step of providing a PIN diode network means for controlling the amount of attenuation of said RF pulses in accordance with said control signal, said step of providing said PIN diode network means for controlling the amount of attenuation further comprising the step of changing the fall time of said RF input pulses in accordance with said control signal, and in that said step of generating said control signal comprises the step of using pulse shaping means for generating a bias current for said PIN diode network means, said step of generating said bias current with said pulse shaping means comprising the steps of: adjusting the pulse width of a trigger signal in response to a pretrigger input signal; generating a voltage ramp signal in response to said trigger signal; and generating said bias current for said PIN diode network means with current source means coupled to said voltage ramp signal generating means.

Inventors: DRAKE PETER R, MERRILL PHILIP R, O'LEARY THOMAS M, DRAKE, PETER R., MERRILL, PHILIP R., O'LEARY, THOMAS M.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1992
Main classification: G01S7
IPC codes: H03F1, G01S7, H04B1, G01S13, H03F3