Yttrium iron garnet (YIG) filters are magnetically tunable bandpass filters that can be found in a variety of test and measurement systems. For example, YIG filters are commonly included in front-end sections of microwave spectrum analyzers as a preselector for applied input signals.
YIG belongs to a broader class of microwave band ferrimagnetic materials used to make microwave filters and oscillators. These materials, as applied to such applications, are referred to generally as “ferrimagnetic resonators”. Other types of garnets include YIG doped with aluminum, gallium, gadolinium, or aluminum and gadolinium, and calcium vanadium. In addition to garnets, magnetic ferrites can be used, such as magnesium, magnesium-zinc, magnesium-aluminum, nickel, nickel-aluminum, nickel-zinc, lithium, and hexagonal ferrites made with barium, for example.
FIG. 1 shows an example YIG filter 100 that can be found in a microwave spectrum analyzer or other electronic system. Although not shown in FIG. 1, YIG filter 100 is generally used in conjunction with a magnetic source such as an electromagnet. The magnetic source generates a magnetic field “H” that can be adjusted to tune YIG filter 100 to a desired frequency passband.
Referring to FIG. 1, YIG filter 100 comprises a YIG sphere 105, an input coil 110, and an output coil 115. During operation, input coil 110 receives an input signal in the microwave frequency range. The input signal produces a fluctuating magnetic field on YIG sphere 105, which causes it to resonate. The resonance of YIG sphere 105 induces an electrical current in output coil 115 to produce an output signal that is a filtered version of the input signal.
The output signal of YIG filter 100 has a frequency spectrum determined by the frequency passband of YIG sphere 105. The center frequency of the passband can be raised or lowered by increasing or decreasing the strength of magnetic field “H”, and the width of the passband can be increased or decreased by adjusting other factors such as the geometry and configuration of input and output coils 110 and 115. The passband can also be modified by varying the number of YIG spheres in the filter. For instance, many applications use three or four YIG spheres, although any number of spheres is possible. In addition, as alternatives to YIG spheres, other types of ferrite materials can be used for the filter element, such as barium hexi-ferrite, nickel zinc, or various other materials.
In some applications, YIG filter 100 is placed in a gap along a magnetic pole of an electromagnet to allow precise focusing of magnetic field “H”. In such applications, the passband and the center frequency of YIG filter 100 varies according to the magnetic flux density “B” within the magnetic gap. The magnetic flux density “B” can be modified by changing the strength of magnetic field “H” or by changing the size of the magnetic gap.
FIG. 2 shows an example front-end 200 of a microwave spectrum analyzer using a YIG filter such as that illustrated in FIG. 1. In this example, front-end 200 has a frequency range of 0-50 GHz. However, other front-end designs can be used for other frequency ranges.
Referring to FIG. 2, front-end 200 comprises an input band switch 205, low pass filter 210, preselector 220, and frequency mixers 215 and 225. Preselector 220 comprises a YIG filter that restricts the frequency spectrum of signals provided to the corresponding mixer 225.
During operation, input band switch 205 receives an input signal and transmits it to a designated one of the filters 210 or 220 according to an operating mode of the spectrum analyzer. The input signal is filtered by the designated filter and then transmitted to a corresponding one of frequency mixers 215 and 225. The respective passbands of preselectors 210 and 220 are typically designed to match to the respective mixing modes of frequency mixers 215 and 225.
YIG filters can generally provide high frequency selectivity and broad frequency tuning ranges. However, they can also suffer from frequency drift, making it difficult to accurately set and maintain a passband center frequency at a desired value. Where the passband center frequency of a YIG filter is inaccurately set or maintained in a preselector of a microwave spectrum analyzer, amplitude errors can occur in the spectrum analyzer's response.
One cause of frequency drift is heat dissipated by an electromagnet used to tune the YIG filter. The electromagnet dissipates heat through conductive coils that generate the magnetic field for tuning. The dissipated heat causes non-uniform thermal expansion of the electromagnet, which can gradually modify the passband center frequency by changing the magnetic field density “B” applied to the YIG filter. This frequency drift tends to stabilize as the thermal expansion approaches an equilibrium state. However, a typical electromagnet structure can take several minutes to reach equilibrium.
Another cause of frequency drift is thermal expansion due to changes in ambient temperature. This type of thermal expansion can be less predictable than that caused by the electromagnet, and the ambient temperature may not have a reliable equilibrium state.
Frequency drift can be especially problematic in YIG filters designed for high frequency ranges, such as 50 GHz, because these YIG filters are generally placed in a smaller magnetic gap in order to increase magnetic flux density. The small size of the magnetic gap can magnify the effects of thermal expansion in the electromagnet, which can lead to unacceptable levels of frequency drift.
What is needed, therefore, are improved techniques and technologies for stabilizing drift in YIG filters. Such improvements are especially needed for high frequency applications such as microwave spectrum analyzers.