YIG Oscillator Donald Benson, Ali Tatari, Amaaduddin, San Jose State University Fall 2009, EE 172 Abstract –A resonator based on YIG (Yttrium Iron Garnet) and a bipolar amplifier were implemented, targeting application as an oscillator in the < 6 GHz range. Undesired resonances from parasitic elements were noted in simulations and observed in measurement. Inadequate magnetic field strength prevented proper operation of the YIG element. A second implementation of the amplifier improved amplifier performance, and improved magnetic pole pieces concentrated the field so tuning from 1.8 GHz to 6 GHz was observed. Improvements for future work were identified I. INTRODUCTION This project began as development of a YIG based oscillator. The frequency range of up to 6 GHz was selected based on lab equipment available to students at SJSU. It was decided to make a YIG resonator and an amplifier as separate circuits with RF connectors. This allowed the team to work on them independently, characterize them, simulate how they would work together, and then test as an assembly. Fig I 1 : A commercial YIG oscillator Photo from http://www.vhfcomm.co.uk/pdf/A%20Simple%20Approach%20tyo%20YIG%20Oscil.pdf A. Background YIG (Yttrium Iron Garnet) is grown as a crystal, diced into cubes, and polished in a tumbler to form spheres. It serves as a ferro-magnetic material, and the precession of “spinning” electrons resonates at a frequency proportional to an externally applied magnetic field when excited in a different direction by an RF field. YIG provides stable, tunable frequency references in the GHz range. It is slower than Varactor-diode tuned VCOs due to the inductance of the electromagnet’s coil. As with crystal oscillators, temperature compensation (a regulated heater) can be used to eliminate frequency variation due to ambient temperature and self heating. According to the website for Microlambda Wireless, a manufacturer of YIG oscillators and filters, http://www.microlambdawireless.com/apppdfs/ytodefinitions2.pdf “Phase noise is energy generated at frequencies other than the carrier/center frequency. Phase noise is measured as power relative to the carrier/center frequency, in frequency “windows” offset from the carrier/center frequency.” Fig I 2 : Phase noise measurement Fig I 3 : Common circuit typesFig I 3 : Comparison of circuit types (the preceding pictures were all from Microlambda Wireless) Fig I 4 : alternate common—base configuration with no physical capacitors, just parasitic C & R in equivalent circuit http://www.teledynemicrowave.com/Products/Oscillators/Ultra_Low_Phase_Noise_YIG_Oscillators.aspx YIG spheres can also be used to create filters. Fig I 5 : Diagram of YIG-coupled loop filter Picture from http://www.microlambdawireless.com/apppdfs/ytfodefinitions2.pdf Note that all power will pass through this 0.012” sphere! “The coupling loops are essentially RF transmission lines that pass all RF energy. However, when these transmission lines are located close to the surface of the YIG sphere, the loop couples to the magnetic field resonating ( @ microwave frequencies ) around the YIG sphere. This coupling essentially reflects/rejects in coming frequencies that are at the same RF frequency as the RF magnetic field resonating around the YIG sphere. Rejection bandwidth is widened by increasing the number of YIG resonators and carefully “tuning” the RF coupling loops.” Limitations of YIG include power consumption (driving the coil) and slower tuning than oscillators with varactor diodes, due to the time required to change current through an inductor, and limited frequency response of magnetic cores. The former is sometimes addressed with two coils, one for coarse range setting, and the other for closed-loop control. One area of current research is combination with piezo devices to allow rapid frequency tuning by application of mechanical stress. This was proposed for use in phased array applications, where the speed of sweeping an antenna beam depends on the rate at which phase shifters can be tuned. B. Advantages of YIG YIG oscillators have good signal quality with low level of phase jitter as compared to VCOs. They also have better broad band characteristics. They have a linear tuning curve. Low Phase Noise: According to Teledynewireless.com, http://www.teledynewireless.com/Products/Oscillators/Ultra_Low_Phase_Noise_YIG_Oscillators.aspx “YIG Oscillators exploit the principle of magnetic resonance to generate a clean low phase noise microwave signal over broad tuning ranges.” YIG behaves similar to a tank circuit when placed in the air gap of an electromagnet. It also experiences magnetic resonance which helps keep the phase noise low. Wide Frequency range: YIGs come in different frequency ranges like 2 to 4 GHz, 4 -8 GHz, 8 – 12 GHZ, 12 – 18 GHZ , and 2 – 8 GHZ. They can usually go beyond their specified range. For example , a 2 -4 GHZ YIG can operate at 1.8- 1.9 GHz. Sometimes they can even go down into the megahertzrange. This all depends on the type of the YIG. http://www.vhfcomm.co.uk/pdf/A%20Simple%20Approach%20tyo%20YIG%20Oscil.pdf Wide Bandwidth Pulse Compression: YIGS can be used for wide bandwidth pulse compression. According IEEE proceedings, this was first demonstrated by Hughes in 1964 by successful compression of 90 MHz bandwidth frequency modulated pulse centered at 100 MHZ. http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=01446930 II. FIRST IMPLEMENTATION A. Amplifier An amplifier circuit configuration was selected after reviewing literature. FET implementations support higher frequencies, but BJT covers the selected frequency range and offers improved phase noise, so this is the one that was pursued. Initial simulations were performed using a non-ideal Infineon transistor available in the Microwave Office library. Fig II 1 : Diagram of Amplifier Circuit Fig II 2 : Simulated Amplifier Response With no matching networks, a flat transfer function across the desired frequency range was observed, but with S21 < 1.0 it was not clear whether gain was achieved. If most of the