Switched impedance FSK modulator.

<u>Background of the Invention</u>

This invention relates to transmitters which include frequency shift key modulation.

FSK (Frequency Shift Key) modulators are employed in optical data transmission to produce different levels of frequencies corresponding to a binary data input stream. Most existing modulators employ a varactor diode to vary the capacitance of an oscillator circuit and, therefore, the output frequency. The problem with such a technique is that the capacitance vs. voltage characteristic of the varactor tends to vary from part to part and over temperature. Alternatively, a modulator may employ switching between two crystal oscillators, but this tends to produce discontinuous phase transitions in the output.

Recently, it has been proposed in an article identification system to utilize an FSK modulator which switches a capacitor or inductor into and out of an oscillator circuit in order to vary the frequency output (see U.S. Patent 4,918,296). The switches employed were bipolar transistors. Use of such transistors would not be advantageous at the high frequencies required for the present application due to the relatively high and non-linear rf resistance the transistor would exhibit in the saturated state. Also, the oscillator signal amplitude would need to be small to avoid conduction of the collector-base junction of the transistor in the off-state.

A further problem with present modulators used in optical transmission involves producing a square wave output signal with a fixed duty cycle (usually higher than 50 percent) suitable for driving an optical device. High speed comparators could be used, but would tend to draw excessive power. Recent proposals have included the use of an inverter to produce the square wave, but generally without any means for fixing the duty cycle in spite of changes in the inverter threshold (see, e.g., U.S. Patent 4,716,383, U.S. Patent 4,174,534, and U.S. Patent 4,603,301).

<u>Summary of the Invention</u>

The invention, in one aspect, is a transmitter including a Frequency Shift Key modulator comprising an oscillator circuit adapted to produce a specific output level frequency. A capacitor is coupled in parallel with the oscillator circuit, and means are provided for switching the capacitor into and out of the parallel connection with the oscillator circuit in order to change the output level frequency. The switching means is responsive to a binary data input and comprises a PIN diode and a switchable current source coupled between the diode and the binary data input.

<u>Brief Description of the Drawing</u>

These and other features are delineated in detail in the following description. In the drawing: <ul id="ul0001" list-style="none"><li>FIG. 1 is a block diagram illustrating portions of an optical transmitter in accordance with an embodiment of the invention;</li><li>FIG. 2 is a block diagram illustrating portions of an FSK modulator in accordance with the same embodiment;</li><li>FIG. 3 is a circuit diagram of a portion of the modulator in accordance with the same embodiment; and</li><li>FIG. 4 is a circuit diagram of a further portion of the modulator in accordance with the same embodiment.</li></ul>

<u>Detailed Description</u>

FIG. 1 illustrates portions of a typical optical transmitter which utilizes an FSK modulator. Binary data is received by the modulator illustrated as block 10. The modulator converts the binary data into a duty cycle controlled square wave output signal having two levels of frequency. The high frequency typically corresponds to a "1" and the low frequency to a "0" of the binary data input. This output is coupled to a laser driver indicated by block 11 which creates the drive signal needed for the laser. The output of the laser driver is applied to a standard laser illustrated by block 12 to produce the optical signal which also comprises two levels of frequency corresponding to the original data input. Typically, the optical frequencies are at 11.85 MHz and 13.09 MHz.

FIG. 2 illustrates in a block diagram further details of the modulator 10. The data input is coupled to a PIN switch network, 13, to be described, which operates on a tuned circuit oscillator 14. In a manner to be described, the oscillator is adapted to produce two sinusoidal output frequency levels depending upon the state of the switch network 13. The resulting signal is coupled to a buffer circuit 15 which prevents loading of the tuned oscillator circuit by other circuitry coupled thereto. The buffer also provides a feedback signal to an automatic gain control circuit, 17, applied to the oscillator 14. The output of the buffer is applied to a duty cycle control circuit, 16, to be described, which squares up the sinusoidal signal and adjusts the duty cycle to be compatible with the laser to be driven.

FIG. 3 illustrates in more detail the portions of the modulator including the PIN switch network and oscillator circuit. The binary data input stream is coupled to the base of a bipolar transistor Q₁ through a pair of inverters, 30 and 31, and a voltage divider formed by resistors R₁ and R₂₀. A dc voltage of approximately +5 volts, in this example, is applied at terminal 32 to bias the emitter of transistor Q₁ through resistor R₂. The collector of Q₁ is coupled to the anode of PIN diode, D₁, whose cathode is coupled to ground potential. A dc voltage of approximately -5 volts, in this example, is applied at terminal 33 and is also coupled to the anode of diode D₁ though inductor L₁ and resistor R₃.

One plate of capacitor C₁ is also coupled to the anode of PIN diode D₁, and the other plate of capacitor C₁ is coupled to the drain of field effect transistor (FET) T₁. Also coupled to the drain of T₁ is one plate of capacitor C₂, while its other plate is coupled to ground potential. Feedback for oscillation is provided from the drain of T₁ to the gate of T₂ though capacitor C₆.

Transistor T₁ has its drain biased by a constant dc potential at terminal 34, which in this example is +5 volts. The bias is supplied through inductor L₂. The source of the transistor is grounded. The gate of the transistor T₁ is coupled to the drain of field effect transistor T₂ through a capacitor C₃, and also to the automatic gain control circuit (17 of FIG. 2) through a resistor R₄. A diode, D₄, is coupled between the gate of T₁ and ground.

Transistor T₂ has its drain biased by a constant dc voltage applied at terminal 35, which in this example is approximately +5 volts. The bias is supplied through a resistor R₅. The source of transistor T₂ is grounded. The gate of T₂ is coupled through a capacitor C₆ to the feedback from the drain of T₁. Diode D₅ and resistor R₂₁ are coupled between the gate of T₂ and ground.

In operation, it will be recognized that the combination, including T₁, T₂, L₂, C₂, comprises a tuned oscillator circuit producing a specified frequency at the output to the buffer, which in this particular example was 13.09 MHz and constitutes the high level frequency for the modulator. That is, T₂ will invert and amplify the signal from the drain of T₁, and re-apply it to the gate of T₁, which also inverts and amplifies the signal, and the circuit will oscillate at a frequency according to the value of L₂ and C₂. When C₁ is coupled into the circuit parallel to C₂, the increased capacitance lowers the output frequency to a value, which in this example was 11.85 MHz, constituting the low level frequency of the modulator.

The switching of capacitor C₁ is controlled by the network comprising PIN diode D₁, transistor Q₁, inductor L₁ and resistor R₃. It will be recognized that Q₁, along with R₁, R₂₀, R₂, D₂ and D₃, constitute a switchable current source. Thus, when the data input is low (i.e., a "0"), the base of Q₁ is biased at a potential, such that a fixed current will flow from the collector through the diode D₁ so that the diode D₁ will exhibit a low rf resistance. This will bring C₁ into active connection in parallel with the capacitor C₂, thus increasing the capacitance and lowering the output frequency of the oscillator. When the data input is high (a "1"), however, Q₁ will shut off. The bias at terminal 33 through the inductor L₁ and resistor R₃ will reverse bias diode D₁ so that D₁ will now exhibit a high rf resistance (i.e., low capacitance) thereby effectively disconnecting C₁ from the oscillator circuit. This reduces the total capacitance and raises the output frequency to its higher level. The data rate in this example was 1.544 Mbitsec.

For this application, the PIN diode D₁ can be a standard switching diode with a low rf resistance (typically 1 ohm) at forward bias and a low capacitance (typically 0.7 pF) at reverse bias. A proper carrier lifetime for the output frequencies in this example would be 150 ns, and an appropriate reverse recovery time for the data rate in this example would be 50 ns.

The output of the circuit of FIG. 3, after passing through a buffer circuit (15 of FIG. 2) is then coupled to the circuit of FIG. 4. The signal is first coupled to an inverter 40 through capacitor C₁₁. Coupled between the output of the inverter and pound is a series connection of a resistor R₇ and capacitor C₈. The node between R₇ and C₈ is coupled to the noninverting (+) input of an operational amplifier (op amp), 41. A constant dc voltage at terminal, 42, in this example approximately +5V, is divided down and filtered to form a reference voltage by R₈ , R₁₀ and C₉. This reference voltage is applied to the inverting (-) input of the op amp through resistor R₉. The output of the op amp is coupled to the input of inverter 40 through resistor R₁₁. The output of inverter 40 is also coupled to a second inverter 43, which is used for buffering the output of inverter 40.

In operation, the sinusoidal signal at the input of inverter 40 will be transformed into a square wave signal of reversed polarity at the output, and then returned to the original polarity after being sent through inverter 43. Since the threshold of the inverter will vary, op amp 41 is utilized to adjust the dc level of the sinusoidal input signal so that it will always produce a fixed duty cycle for the output square wave, which, in this example is a duty cycle of 50 percent. This is achieved by coupling the output signal through a low pass filter formed by the combination of resistor R₇ and capacitor C₈, and applying it to the "+" input of the op amp. The values of the resistor R₇ and capacitor C₈ are chosen so that the resulting signal applied to the op amp input is a dc signal equal to the duty cycle of the square wave signal output of the inverter, 40, multiplied by the supply voltage. In this example, the resistance of R₇ was 10K ohms, the capacitance of C₈ was 0.1 microfarads and the supply voltage was +5 volts.

Meanwhile, resistors R₈ and R₁₀ are chosen so that the reference voltage applied to the inverting (-) input of the op amp is equal to the voltage which will give the desired duty cycle, in this example 0.5 times the supply voltage, or 2.5 volts. In this example, the resistance of the resistors R₈ and R₁₀ was 10K ohms each. The output of op amp 41 applied back to the input of inverter 40 will, therefore, adjust the dc level of the input sinusoidal signal until the noninverting (+) input of the op amp is also at 2.5 volts. At that time, the output of inverter 40 will be at the desired 50 percent duty cycle, and op amp 41 will go into a quiescent state until such time as the threshold of inverter 40 should change and alter the duty cycle.

Due to the inherent turn-on delay of laser diodes, it is known that the electrical drive duty cycle should be greater than 50 percent to achieve a 50 percent optical waveform duty cycle. In this example, the required duty cycle is approximately 60 percent. Consequently, the output of inverter 43 is coupled, through resistors R₁₂, R₁₈, and capacitor C₁₀, to inverters 44 and 45 where the operation of duty cycle adjustment is repeated by op amp 46. The purpose of R₁₂, R₁₈ and C₁₀ is to convert the square wave output of inverter 43 into a triangular wave so that duty cycle adjustment can be repeated. This time, however, resistors R₁₄ and R₁₅ are chosen to produce a reference voltage of approximately 0.4 times the supply voltage, or 1 volt, at the inverting (-) input to achieve a 40 percent duty cycle signal at the output of inverter 44 so that the output of inverter 45 produces the desired 60 percent duty cycle. The signal is then sent to the laser driver to produce the optical output signal.

It will be understood that the 60 percent duty cycle is not produced in one stage due to amplitude variations in the sine wave input to inverter 40 which occur in the oscillator circuit during the data transitions. Such amplitude variations would cause duty cycle jitter for any value other than 50 percent.

It will be appreciated from the above that any desired duty cycle can be achieved in accordance with this aspect of the invention by an appropriate choice of reference voltage.