by Marinus (Ron) Korber, Teledyne RF & Microwave
Oscillators and filters rely on resonant behavior to function, and at higher frequencies, the size of the crystal and cavity dictate the frequency of resonance. Yttrium iron garnet (YIG) has an unusual property when used as a resonator, as its resonant properties do not depend on size but on the strength of the magnetic field passing through it. This has some downsides but also some major benefits.
YIG resonators have been in use for decades in oscillators, bandpass filters, and band-reject filters, although they are rarely the first choice when small size or low power are critical metrics. However, they remain valuable in test equipment such as spectrum analyzers, clock recovery units, and military systems because they have benefits not achievable with any other technology. The advantages and disadvantages of YIG technology are shown in Table 1.
The Venerable Band-Reject (Notch) Filter
Test equipment and some military applications require extremely sensitive receivers located next to high-power transmitters, and a continuing issue is how to stop the receiver from being overwhelmed by the transmitter signal that can obscure signals of interest. A ship on which surveillance equipment sits near very high-power radar is a good example. For interference that is either predictable in frequency or dwells long enough to be measured and responded to, it is possible to notch out the unwanted signal.
A common approach is to use a switched filter bank or a tunable filter. A filter bank has switch insertion losses that mount up as the number of filters increases, while a tunable filter is more flexible. YIGs are very well suited for use in tunable filters if the receiver operates over very wide bandwidths. As an example, a single bandpass YIG filter can be made to tune from 3.5 to 50 GHz, far wider than other approaches.
A key property of a YIG sphere is the behavior of the magnetic dipoles inside it. As shown in Figure 1, with no applied field (a), the dipoles are randomly arranged. As an external magnetic field is applied, they start to align in the same direction (b), and as the applied field increases, a point is reached where all are aligned in the direction of the field, at which point it is considered to be saturated (c). Beyond this point the sphere can couple with a nearby RF field and influence it, but only if the field is at exactly the right frequency; to frequencies outside a narrow range, the sphere is invisible.
The properties of YIG spheres are such that resonant behavior is independent of the amount of material, and the resonant frequency is linearly dependent upon the magnetic field. This allows YIG spheres to be very small, typically 0.5 mm, and allows tuning over a very wide bandwidth. It also produces downsides; typical YIG devices require the size, weight, and current needed to enable electromagnets. However, their benefits can be considerable, such as rejection greater than 60 dB, steep skirts, and good shape factors.
How They Work
To understand the basic functioning of a YIG band-reject filter, consider the situation shown in Figure 2. A YIG sphere is suspended between the pole pieces of an electromagnet and tuning of the resonant frequency is achieved by varying the current. In the figure, the sphere is next to a microwave transmission line, and its impact on the signal passing between the connectors along the transmission line depends on how much coupling occurs by the coupling loop. Such loops can be made of several turns or as simple as a straight transmission line passing nearby. The amount of coupling is a critical factor in the design of the filter and is frequency dependent.
The tuning is dependent upon the current in the electromagnet, which might be 300 mA on an 8 ohm coil. It is very linear, which makes control straightforward, with 10 MHz/mA a typical example. However, tuning is not instantaneous as there is inductance in the coil, and eddy currents that cause an opposing magnetic field in the pole piece must settle before the notch can reach its final frequency.
The microwave portion of the assembly can be thought of in electrical terms (Figure 3). The inductance of the coupling loop is followed by a parallel RLC circuit which provides high impedance at resonance and low impedance out of the resonance band. The higher the value of R0 that can be achieved, the deeper the notch and the higher the rejection.
The following discussion uses a 500 MHz to 2.7 GHz filter, although the same ideas apply across any frequency band at which YIGs can operate. The graph in Figure 4 shows an overlay of measured plots of the insertion loss (S21) of a single-stage YIG band-reject filter of the type shown conceptually above as it is tuned by a changing magnetic field over its entire band.
Some features become immediately apparent. The notches are extremely narrow, with steep sides, which is a good attribute for a notch filter. The notches are also quite deep, more than 15 dB at higher frequencies, and the out-of-band insertion loss is very low. It’s a promising start, but at the low end of the range the notch is only providing 5 dB of rejection, so it is necessary to consider approaches to achieve improvements.
The first step towards increasing in-band rejection is to stack multiple filters in series. Eight stages are a common approach, although up to 16 are possible. A representation of an eight-stage filter like the one used in these measurements is shown in Figure 5. Such filters can be laid out linearly such as the one shown or like the spokes of a wheel in a circle. Placing eight stages similar to the one above in series yields the four plots shown in Figure 6. All use the same scale: a magnified 200 MHz span on the horizontal axis, 10 dB/division on the vertical, but with different center frequencies.
The Q is higher, and the notch is deeper at higher frequencies, with the depth of the notch at lower frequencies going from -5 dB to between -30 and -40 dB. This is certainly a major improvement but not enough for many applications with the shape at the bottom of the notch becoming unhelpful. The notch is also getting wider at higher frequencies, which is also undesirable.
Another technique for improving notch depth is to strengthen the influence of the YIG by increasing the degree of coupling. This is typically achieved by either increasing the size of the sphere or decreasing the size of the loop. For this example, the loops were kept constant, and the spheres increased in size.
Figure 7 shows that tighter coupling has had a noticeable impact and the low-frequency notch is much improved. However, several other factors are creeping in that are undesirable. That is, the higher frequency notches are getting much wider, and outside the main notch the insertion loss increases while tracking spurs are also becoming problematic. Tracking spurs are inherent issues with YIGs, particularly at lower frequencies, but their magnitude relative to the main notch can be influenced.
The dominant ones are usually slightly above the main notch in frequency and are most harmful to system design when they are near it, effectively expanding the width of the main notch. The purity of the YIG material and quality of the surface finish play roles in the quality of the notch and position of the spurs. Traditionally, this is about as far as an eight-stage design like this can be taken. Tighter or looser coupling can be used to vary the depth of the notch until the tracking spurs go beyond acceptable limits for the system design.
A New Way Forward
Going back to the single-stage filter example, there is another way of achieving band rejection by using a coupled loop-to-ground that forms a shunt resonator. Notches can be created in the same way as before, but there is one unique feature that can be exploited. While the performance of the series resonators degrades as the frequency gets lower, the shunt resonators exhibit the opposite behavior. The ideas are captured in Figure 8, with the original single-stage filter shown in the top section and the alternative topology in the bottom section.
The shunt resonator provides another benefit for improving performance. Removing two series stages in the eight-stage filter delivers less notch depth but also lower levels of spurs. Replacing the missing stages with shunt resonators delivers the performance shown in Figure 9.
Teledyne’s latest YIG-based notch filters combined the benefits of both approaches with deep notches that have steep skirts and suppressed tracking spurs, which results in a higher performance component. In addition, the number of shunt stages can be varied depending on the required performance trade-offs.
Most applications would ideally require narrow and consistent notches that do not vary in bandwidth as they are tuned in frequency, and while this ideal is not achievable, the new design is significantly better than previous efforts. Common metrics used to gauge this are bandwidth at 3 dB down and the same at 40 dB down. A comparison is shown in Figure 10.
Although YIG filters are not the answer for every application, for those needing the utmost notch performance over a very wide tuning range, they are hard to beat. This applies most often to applications in test equipment and the military. There are design trade-offs that can be made to achieve high performance, including the degree of coupling to the YIG sphere that improves notch depth but at the cost of intrusive spurs. Teledyne’s patented topology mitigates most of the downsides of traditional designs and provides performance closer to the ideal tunable notch than has been achieved before. The new approach is available in standard products up to 20 GHz.
For Further Reading
1. For an in-depth technical description of the shunt band-reject filter, see Teledyne patent US 9,203,129B2.
2. P.S. Carter, Jr., “Magnetically Tunable Microwave Filters Using Single-Crystal Yttrium-Iron-Garnet Resonators” IRE Transactions on Microwave Theory and Techniques, Volume: 9, Issue: 3, May 1966.
3. G. L. Mattheal, L. Young, and E. M. T. Jones, “Microwave Filters, Impedance-Matching Networks and Coupling Structures,” New York: McGraw-Hill, 1964, pp. 1027-1049.
4. Helszajn, Joseph, “YIG Resonators and Filters,” Wiley & Sons, April 1985.
5. Carter, Philip S., “Equivalent Circuit of Orthogonal-Loop-Coupled Magnetic Resonance Filters and Bandwidth Narrowing Due to Coupling Inductance “, IEEE Transactions on Microwave Theory and Techniques, Volume: 18, Issue: 2, February 1970, pp. 100-105.