Radio frequency signals are typically generated using electronic circuits called oscillators, which convert DC electrical energy into a signal of the required frequency.
The form of an oscillator is generally a gain block with some form of frequency selective feedback, such as a tuned circuit or resonator, arranged such that a self-sustaining signal is maintained by the feedback loop. There is another class of oscillator commonly used at radio frequencies, which uses a negative impedance to generate the oscillation. In both cases, to achieve good performance, a resonator with a high quality factor (Q factor) is desirable. The Q of a circuit is its centre frequency divided by the resonator 3 dB bandwidth.
A general reference on microwave and millimeter wave oscillators giving detailed explanations of this can be found in Randall W. Rhea, Oscillator Design and Computer Simulation, Noble Publishing, Atlanta, 1995 (ISBN 1-884932-30-4), pp. 1-87.
In some cases mechanical tuning of the frequency of oscillation is acceptable. However, in many applications it is desirable to be able to electronically set and/or modulate the frequency of the oscillator. This is achieved by placing an electronic component such as a varactor diode (or similar device having characteristics that can be electronically controlled) as one element of the resonant circuit. This has the disadvantage that the varactor diode limits the tuning range (compared to that of a mechanically tuned oscillator), gives a non-uniform frequency versus control signal characteristic, limits the magnitude of the oscillating signal, is difficult to integrate and is itself a source of electrical noise. In some applications, the overall tuning range is desirably increased by using a combination of electrical tuning (for fine adjustment) and mechanical tuning (for coarse adjustment).
The phase slope versus frequency characteristic of the resonator defines the phase noise and stability of the feedback oscillator. This is related to the Q factor of the resonator, and for a single pole resonator the average slope is 90 degrees over the 3 dB bandwidth, which equals filter centre frequency divided by filter Q. To obtain higher Q (hence lower noise), the bandwidth needs to be decreased. As described above, many oscillators use a varactor diode to allow the resonator frequency to be adjusted. This limits the obtainable Q and tuning range.
Optimal phase noise performance for any given resonator occurs at the frequency for which the resonator has the highest phase slope. An additional phase shift may be introduced into the feedback path, to ensure that the optimum frequency of operation is obtained, ie. ensuring that the conditions for oscillation are obtained at the frequency for which the resonator has the highest phase slope. In single pole resonators there is only one optimum point of operation (over a narrow band). This optimum setting has to be obtained by manual adjustment or by calculation.
A method for obtaining high phase slope over a wide bandwidth is advantageous. For example, for a multi-section filter, the phase slope increases with the number of sections of the filter and the phase slope is linear over an extended frequency range. Therefore, it is possible to achieve wider tuning range for a given phase slope without changing the filter. However it is then necessary to introduce a controlled phase shift in the feedback loop to adjust the overall loop phase shift to be a multiple of 360 degrees at the required operating frequency.
The open loop gain must exceed unity. This is achieved by adjusting the gain within the loop (e.g. with a variable attenuator) and/or allowing the amplifiers to compress.
In recent years monolithic microwave integrated circuits (MMICs) have been developed to reduce the cost of microwave and millimeter wave active circuits. Further information can be found in “New Developments in the Design of Microwave and Millimeter-wave Oscillators”, Institute of Electrical and Electronics Engineers—Microwave Theory and Techniques Special, Vol. 46, No. 10, October 1998, Part 2. One disadvantage of MMICs is the difficulty of connecting a negative resistance oscillator to the external circuit required to meet the desired quality factor. Alternatively, an (integrated) on-chip resonator can be used, however, these have low Q factors and limited tuning range.
One application of a microwave and millimeter wave oscillator is as the source of a signal to carry information. Any noise or unwanted perturbation of the frequency of the signal results in a reduction in the information carrying capacity of the signal.
Thus, there is a need for a low noise frequency adjustable oscillator the frequency of which oscillator is adjustable in a frequency range from 1 GHz to 200 GHz.