Originally posted Jul 21, 2014
Like an Italian sports car, they combine impressive performance and design challenges
In a 1947 speech, Winston Churchill remarked “…it has been said that democracy is the worst form of government except all those other forms…” Today, I suspect that some microwave engineers feel the same way about YIG spheres as microwave resonator elements. They’re an incredibly useful building block for high-frequency oscillators and filters, but it takes creativity and careful design to tame their sensitive and challenging nature.
The “G” in “YIG” stands for garnet, a material better known in gemstone form. A YIG or yttrium-iron-garnet resonator element is a pinhead-sized single-crystal sphere of iron, oxygen and yttrium. These spheres resonate over a wide range of microwave frequencies, with very high Q, and the resonant frequency is tunable by a magnetic field.
That makes them perfect as tunable elements for microwave oscillators and filters, and in this post I’ll focus on their role in the YIG-tuned filters (YTFs) used as preselectors in microwave and millimeter signal analyzers.
These analyzers typically use an internal version of the external harmonic mixing techniquedescribed in the previous post. It’s an efficient way to cover a very wide range of input frequencies using different harmonics of a microwave local oscillator—itself often YIG-tuned!
However, mixers produce a number of different outputs from the same input frequencies, including high-side and low-side products plus many others, typically smaller in magnitude. These undesired mixer products will cause erroneous responses or false signals in the spectrum analyzer display, making wide-span signal analysis very confusing.
One straightforward solution to this problem is a bandpass filter in the signal analyzer that tracks the input frequency. Here’s an example:
The yellow trace is the frequency response of a YIG preselector bandpass filter as it appears at the signal analyzer IF section. The blue trace shows the raw frequency response, with the preselector bypassed.
YIG technology enables the construction of a tunable preselector filter, wider than the widest analyzer RBW, whose center frequency can be synchronously swept with the analyzer’s center frequency. This bandpass filter rejects any other signals that would cause undesirable responses in the analyzer display.
Problem solved! So why the Churchillian perspective on YIGs? It’s a matter of the costs that come with the compelling YIG benefits:
- Sensitivity is reduced: The preselector’s insertion loss has a direct impact on analyzer sensitivity.
- Stability and tuning are challenging: The preselector’s wide, magnetic tuning range comes with temperature sensitivity and a degree of hysteresis. It is a challenge to consistently tune it precisely to the desired frequency, requiring careful characterization and compensation.
- Bandwidth is limited: The preselector passband is wider than the analyzer’s widest RBW filter, but narrower than some wideband signals that would normally be measured using a digitized IF and fixed LO.
Fortunately signal analyzer designers have implemented a number of techniques to optimize preselector performance and mitigate problems, as described in Agilent Application Note 1586 Preselector Tuning for Amplitude Accuracy in Microwave Spectrum Analysis.
An alternative approach is simply to bypass the preselector for wideband measurements and whenever conditions allow. Many measured spans are not wide enough to show the undesirable mixing products, or the unwanted signal responses can be noted and ignored.
So, just as with democracy and its alternatives, YIG preselectors offer compelling benefits that far outweigh their disadvantages.
If you’d like to know more about harmonic mixing and preselection, see Chapter 7 of the new version of Application Note 150 Spectrum Analysis Basics.