NL9500575A - Circuit for modulation of a crystal oscillator with voltage controlled (pin) diode. - Google Patents

Description

Circuit for modulation of a crystal oscillator with voltage controlled (pin) diode

The invention relates to a circuit for modulating a crystal oscillator arranged to generate a fundamental frequency or a overtone frequency, comprising a crystal element in series with a series circuit of a self-inductance and a voltage-controlled capacitive element, wherein a modulation voltage can be connected to the junction of the capacitive element and the inductance.

Such a crystal oscillator is well known and is widely used in wireless telecommunication applications, where this oscillator must be frequency modulated (FM: frequency modulation, or FSK: frequency shift keying, frequency shift encoding).

Especially with portable and non-mains connected devices, where small physical dimensions and low energy consumption are of great importance, no simple method and no simple device is available to achieve sufficient linear FM modulation with overtone crystal oscillators. Since a quartz crystal is an electro-mechanical vibration unit with a very high quality factor, it is difficult to change its resonant frequency by more than several hundred parts per million. In the frequency range from several hundred kHz to about 30 MHz, it is possible to construct crystals that function at their fundamental frequency. These units can be quite easily "pulled" from their fundamental frequency at 670 ppm (parts per million parts per million) by means of a series impedance, which can be made either capacitive or inductive by means of an external modulating voltage (Varicap diode). However, if a crystal oscillator is to operate at higher frequencies, so-called overtone crystals must be used. In this case, the crystal element does not resonate at the fundamental frequency, but at an odd multiple thereof. This can be compared to a violin string that resonates at a higher harmonic. However, a drawback of these overtone crystal oscillators is that they can be "pulled" out of their frequency by a much smaller amount than the oscillators at the fundamental frequency. For example, for a crystal that oscillates on the third overtone, this pull-out range is nine times less than for a crystal that vibrates at its fundamental frequency. For a crystal that vibrates on the fifth overtone, the pull-out area is already twenty-five times smaller, etc.

Transmitters used in VHF telecommunication applications should exhibit a frequency deviation of approximately 4 kHz during modulation. Heretofore, this has been accomplished using an oscillator frequency below 30 MHz and the oscillator having a vibration frequency equal to the transmit frequency divided by an integer. For example, a 150 MHz transmitter uses a 150 MHz / 9 oscillator = 16.667 MHz. The frequency variation of the oscillator is multiplied by the same factor as the frequency. Although this principle works well on its own, various electronic frequency multiplication stages are required, which occupy a relatively large amount of space and dissipate electrical energy.

In modern telecommunications equipment, a frequency synthesizer is used, which can be frequency modulated with frequencies that must be higher than 50 to 100 Hz. Below these frequencies, the synthesizer's control loop is active to stabilize the oscillator. Therefore, these synthesizers can only be used in a limited number of applications. In addition, the number of electronic components and the power dissipation is greater than that of a simple crystal-controlled VHF oscillator.

Where there is limited space and the energy dissipation should be as small as possible, the simplest transmission circuit with an oscillator operating at the transmission frequency or possibly half the transmission frequency is of utmost importance to the designer. A 150 MHz transmitter using a 75 MHz crystal oscillator allows the use of a crystal with third overtone. The problem, which is solved by the present invention, is to design an oscillator with which sufficient frequency variation in the oscillator circuit can be obtained as a function of the modulation signal.

The object of the present invention is achieved with a crystal oscillator of the type described at the outset, characterized in that the capacitive element comprises a capacitance and a diode connected in parallel thereto, wherein a predetermined reference voltage is arranged in series with the diode can be switched. "The diode is preferably a pin diode. By providing such a pin diode with a modulation voltage of, for example, 5 V peak-peak or less, a frequency deviation (difference between highest and lowest frequency) of 4 kHz can easily be achieved So with a 75 MHz crystal and a transmitter frequency of about 150 MHz, the deviation is 8 kHz.

Preferably, the overtone crystal oscillator consists of a Col Pitts oscillator.

For frequencies above 30 MHz, an overtone crystal is used for mechanical reasons. A third overtone crystal can be selected for a frequency of 75 MHz. A frequency deviation of 4 kHz can then be realized in the circuit with a pin diode. Therefore, fewer multiplication steps are necessary, so that the space required is smaller and the energy dissipation is less than in the prior art.

The invention will be described in more detail below with reference to the accompanying figures, which are intended to be illustrative only and not to limit the invention.

Figure 1 describes a known oscillator, which can be FM modulated using a Varicap diode; Figure 2 shows a similar device to Figure 1, however, in which the function of the Varicap diode has been replaced by a pin diode; Figure 3 shows the modulation curve of a 150 MHz transmitter based on a 75 MHz crystal oscillator designed according to Figure 2.

Figure 1 shows a Colpitts oscillator 1, the output frequency of which can be FM modulated using a modulation voltage Vm applied across a Varicap diode D1. Such a circuit is known per se and comprises, for example, a crystal element X, which is connected in series with a first inductance L1 and the said Varicap diode D1 to earth. The other side of the crystal element X is connected to ground via a series connection of a first capacitance C1 and a second capacitance C2. A second self-inductance L2 is connected parallel to the second capacitance C2. The junction of the first capacitance C1 and the crystal element X is connected to a first gate G1 of a MOS transistor 2. A second gate G2 is provided for connection to a predetermined voltage. The MOS transistor 2 further has a drain D and a source S. The source is connected via a parallel connection of a first resistor R1 and a third capacitance C3 to the connection point of the first capacitance C1 and the second capacitance C2. During operation, an output signal can be taken from the drain D of MOS transistor 2.

The modulation voltage Vm is applied to the cathode of the Varicap diode D1 via a second resistor R2.

The operation of the circuit of figure 1 is known and does not need extensive explanation here. The crystal element X largely determines the resonant frequency of the oscillator. The inductance L1 tends to lower the resonant frequency within certain limits, a capacitance in series increases the frequency slightly. Thanks to the variable capacitance diode D1, the total impedance of L1 and Di can be made inductive or capacitive depending on the capacitance value of D1. Just diodes with a hyper-abrupt variable capacitance value, i.e. a large ratio between the maximum and minimum capacitance value C | naj (/ Cmin, a frequency deviation of about 1 kHz can be achieved. However, for this the modulation voltage must be in the order of magnitude of about 25 V. In addition, the linearity of the modulator is very low, therefore this principle cannot be used when a frequency deviation of about 4 kHz with respect to the transmission frequency is needed, in this case, as mentioned, the solution is that a lower crystal frequency is selected and various multiplication stages are connected.

Figure 2 shows a Colpitts oscillator in which the same reference signs as in Figure 1 refer to the same elements. The Colpitts oscillator of Figure 2 differs from that of Figure 1 in that the Varicap diode D1 is replaced by an (adjustable) capacitance, indicated schematically by C3. A series circuit of a pin diode D2 and a reference voltage source Uref, which serves to supply a bias voltage to the pin diode D2 during operation, is connected in parallel with the capacitance C3. However, the supply of a bias voltage during operation can also take place in other well-known ways, for example via the modulation input superimposed on the modulation voltage Vm.

The circuit of Figure 2 is based on the following insight. It has been found that with a modulation voltage of, for example, 5 v peak-peak, a frequency deviation of 4 kHz can easily be achieved. The reasons for this are twofold. For example, when the voltage Uref is set to 2.5 V and the modulation voltage Vm increases from 0 V to 2.5 V, the pin diode D2 is reverse polarized and a frequency shift is caused by an increased load on the oscillator circuit as a result of rectifying effects. The positive half period of the oscillator voltage, as present across the pin diode D2, conducts this pin diode for a longer period of time as the modulation voltage Vm increases. When the modulation voltage Vm increases from 2.5 to 5 V, the pin diode D2 becomes progressively stronger in the conductive state. It is known that a forward voltage bias pin diode functions as a variable ohmic resistor for HF currents. Normally, however, this phenomenon would stop the oscillator due to the decrease of the Q factor of the resonant circuit. However, this does not happen in the proposed circuit because of a different effect of the pin diode D2: if the pin diode D2 has a forward voltage setting, its capacitance value changes. This phenomenon is known per se, but has so far, as far as is known, not been used to effect frequency deviations in crystal oscillators. Due to the increasing forward bias voltage across diode D2, the pn barrier becomes thinner, resulting in the said increasing capacitance value of the pn barrier. Since this change in capacitance value can be in the order of magnitude of several nanofarads, a frequency deviation of 4 kHz can be easily achieved even with an overtone crystal with a frequency of 75 HHz.

At 75 MHz, the reactance of the forward capacitance of the pin diode D2 is much lower than the HF resistance of the pin diode D2, which is parallel in the electrical replacement scheme of the pin diode D2. Therefore, the ohmic resistance from that replacement scheme influences the Q factor of the resonant circuit only very slightly.

Although the described effect occurs in every diode, this effect appears to be strongest with pin diodes. When an ordinary diode is used instead of D2 in Figure 2, however, a better frequency deviation result is already obtained than with the conventional circuit according to Figure 1, so that the invention also relates to such a circuit.

The variable capacitance value of the pin diode D2 now tunes the resonant circuit in a similar manner as the Varicap diode D1 in Figure 1 does. The value of the capacitance range is only much larger and, very importantly, the capacitance value of a pin diode D2 can be varied with much lower voltages. In addition, it appears that phase noise does not cause serious interference when the pin diode D2 has a forward bias voltage. If the pin diode D2 does, the influence of C3 in the series connection of L1 and C3 is reduced. Due to the large capacitance variation, which is possible with a pin diode (larger also than with other diode types), the achievable reduction is much greater than is achievable with a Varicap instead of C3. At a low voltage, C3 is decisive: the frequency is above the resonant frequency of the crystal. With an increasing voltage, the increasing capacity of D2 becomes parallel to C3, so that the influence of C3 decreases and finally L1 becomes decisive: the frequency is then below the resonance frequency of the crystal. A favorable operating point is set by choosing Uref.

The oscillator frequency and the aodulation curve appear to be very stable over a wide temperature range. Figure 3 shows the modulation curve of a transmitter of approximately 153 KHz, based on a crystal oscillator of approximately 75 MHz, which is modulated using a modulation voltage Vm over a pin diode D2, as shown in Figure 2. Figure 3 shows that with a modulation voltage Vm of 5 V peak-peak, a frequency shift of about 15 kHz can be realized. By decreasing the value of the resistor R2, a good operation can also be obtained with modulation voltages Vm of 2 V peak-peak.

By using a pin diode D2, various multiplication stages, as used in the prior art, can be omitted, making the overall circuit smaller and reducing the power dissipation. In addition, the FM modulation can be realized with modulation voltages Vm of 5 V peak-peak or less. This is in line with current developments in electronics, where 3 V power supplies for mobile equipment are increasingly being used.

Within a frequency range of 30 to 100 MHz, a frequency deviation of at least 4 kHz can easily be realized with crystal oscillators operating on the third overtone. The deviation from ideal linearity appears to be no more than 2%.

The invention can be applied in particular for very small telemetry transmitters, data transmitters and voice transmitters, because precisely there power dissipation and dimensions are of great importance.

Although the principle of the present invention has been explained with the aid of a Colpitts oscillator (see Figures 1 and 2), it will be clear to the skilled person that the principle can also be applied to any other crystal oscillator, in which a capacitive element with voltage ning-driven variable capacity value is applied.

Claims (6)

A circuit for modulating a crystal oscillator (1) arranged to generate a fundamental or overtone frequency, comprising a crystal element (X) in series with a series circuit of a self-inductance (L1) and a voltage-controlled capacitive element (D2, C3), where a modulation voltage (Vm) can be connected to the connection point of the capacitive element (D2, C3) and the inductance (L1), characterized in that the capacitive element has a capacity (C3) and comprises a diode (D2) connected in parallel, wherein a predetermined reference voltage (Uref) can be connected in series with the diode (D2). Circuit according to claim 1, characterized in that the diode (D2) is a pin diode. Circuit according to claim 1 or 2, characterized in that the capacity (C3) is an adjustable capacity. Circuit according to claim 1, 2 or 3, characterized in that the crystal element (X) is a third overtone crystal. Circuit according to any one of the preceding claims, characterized in that the crystal oscillator (1) is a Colpitts oscillator. Circuit according to any one of the preceding claims, characterized in that the overtone frequency of the crystal element (X) is more than 30 MHz and less than 100 MHz.