The EPR signal from the sample resonator is amplified in a W-band low-noise amplifier (LNA) and downconverted to 35 GHz in the W-band downconversion mixer by mixing with the same V-band source output that was utilized in upconversion. This approach ensures frequency coherent downconversion. Path length equalization between the W-band upconversion arm and the downconversion arm is accomplished with a delay line in the V-band local oscillator (LO) circuit. The EPR signal at Q-band is amplified in a Q-band LNA, downconverted to 1 GHz in the signal mixer (by mixing with 36 GHz), and sent to the signal receiver.
The EPR signal receiver is shown in Fig. 4. The EPR signal at 1 GHz is amplified and downconverted to baseband by mixing with the output of a 1 GHz synthesizer in the synthesizer array. The phase of the 1 GHz synthesizer, which also shares the same time base with the other synthesizers, is adjusted by the user to set absorption or dispersion. Following Eq. (1), maximum Γ corresponds to dispersion, and minimum Γ to absorption. The baseband EPR signal is amplified, filtered, and applied to the analog-to-digital (A/D) converter and averager card (Acqiris Agilent model AP240-Avgr). Further processing is completed in the computer. The A/D-averager is triggered by the generator that produces the waveform utilized in frequency-sweep experiments.
Signal receiver detail. The 1 GHz LO synthesizer phase is adjusted for absorption or dispersion. The waveform generator synchronizes data collection with the frequency sweep waveform.
Frequency-sweep EPR experiments have been enabled by the development of a V-band frequency agile source. The source, detailed in Fig. 2, incorporates a YIG (yttrium-iron-garnet) tuned oscillator (YTO) and a fixed frequency Gunn diode oscillator. The YTO (Microlambda Wireless, Fremont, CA; model No. MLXS-1678RF) has exceptional frequency linearity relative to the varactor-tuned Gunn diode oscillator previously used in FM experiments [3]. The YTO output is upconverted to V-band by mixing with the 51 GHz Gunn diode oscillator output, filtered for the upper sideband, and amplified to a level sufficient to drive the W-band mixers in the W-band bridge. The isolators ensure that the oscillators, mixer, and amplifier are terminated in well-matched impedances. The low-pass filters ensure that oscillator harmonics are not injected into the mixer. The bandpass filter passes only the upper sideband from the mixer, the desired nominal frequency of 59 GHz. The YTO is utilized for nominal frequency adjustment in the bridge as well as for frequency sweeps. The 51 GHz Gunn diode oscillator is a scaled fixed-tuned version of a Q-band oscillator developed in this laboratory [8,9]. The Gunn diode (model No. MG1022-16) is from MDT, now Microsemi Corporation (Lowell, MA).
This V-band source has significantly reduced phase noise (over 20 dB) relative to the varactor-tuned oscillator previously reported [3,6]. The phase noise of the YTO is specified to be ?130 dBc/Hz at 100 kHz offset. The phase noise of the Gunn diode oscillator is estimated to be ?120 dBc/Hz at 100 kHz offset. This conservative estimate was obtained by scaling the phase noise performance of a 35 GHz Gunn diode oscillator to 51 GHz, and by accounting for skin effect, surface roughness effects of the stabilizing resonator, and increased Gunn diode noise factor at the higher operating frequency. Thus, the Gunn diode oscillator phase noise sets the phase noise floor.
In frequency-sweep EPR experiments, the output of the waveform generator (Fig. 4) is applied to driver electronics that control the microwave frequency of the YTO. This frequency control is described in the next section.
The YTO is a frequency-tunable source utilized in modern microwave instrumentation such as phase-locked loop (PLL) synthesizers. It consists of a low-noise transistor oscillator circuit that utilizes a YIG sphere as the resonator and a magnetic circuit in which the YIG resonator is immersed. The oscillator is followed by a buffer amplifier. Frequency tuning is accomplished by changing the magnetic field in which the YIG resonator is immersed.
There are two main types of YTOs: electromagnet and permanent magnet. In the electromagnet YTO, there are two coils: one for the main magnetic field of the YIG resonator (the main coil) and one for rapid frequency tuning, sweeping, and modulation (the FM coil). The main coil, which has high inductance, establishes the magnetic field of the YIG resonator. It can tune the YTO over several GHz, 10 GHz or more, but slowly. The magnetic circuit cutoff frequency (about 5 kHz) fundamentally limits the frequency tuning rate. Furthermore, the electronic circuit that energizes this coil (the driver) is usually heavily filtered and, hence, of narrow bandwidth. Otherwise, circuit noise would frequency-modulate the YTO, and the phase noise performance would degrade. The frequency-sweep tuning rate can be as high as 100 MHz/ms. The tuning characteristic is fairly linear, typically less than 0.1% deviation from linear [10], and performs well within a PLL architecture. However, the residual nonlinearity and the hysteresis of the YTO magnetic circuit fundamentally limit the linearity of the free-running YTO tuning characteristic.
The FM coil has a lower inductance, which results in a higher frequency sweep rate and a significantly lower peak frequency deviation relative to the main coil. The FM coil is very small, quite close to the YIG resonator, and essentially in an air region of the magnetic circuit. Typical peak frequency deviation and highest FM rate are on the order of 40 MHz and 2 MHz, respectively. Residual nonlinearity in the magnetic circuit is usually minimal and not specified.
The second type of YTO is the permanent magnet YTO (PMYTO). The main coil magnetic circuit is replaced by a permanent magnet that sets the nominal microwave frequency of the YTO. Some models retain a main coil to provide some tuning range, a few GHz at most. Fixed-frequency models eliminate this coil. The FM coil is retained and can be modified to increase frequency deviation, but at the expense of frequency sweep rate.
It is interesting to compare frequency sweeps across a portion of an EPR spectrum using the FM coil of a YTO with large amplitude magnetization field sweeps using the field modulation coil that is mounted on an EPR cavity (see Ref. [5]). The YIG resonant frequency is fo(MHz) ≈ 2.8Ho(G), the gyromagnetic ratio of the electron, where Ho is the magnetic field intensity in the air gap that contains the YIG resonator in the YTO magnetic circuit. The field modulation coil is physically large and often driven on the order of 1 A to establish 10 G at the sample. The FM coil on the other hand is very small and quite close to the YIG resonator. A typical YTO FM coil frequency sensitivity is on the order of 400 kHz/mA, which scales to approximately 140 G peak for 1 A peak—more than an order of magnitude greater than for the EPR field-modulation coil. In addition, a deficiency of the method of large amplitude field sweeps is that variation of field-modulation homogeneity across the sample leads to spectral blurring. In contrast, the microwavefrequency is strictly uniform across the sample. Although the RF amplitude can vary, spectral blurring does not result. Furthermore, high bandwidth electronic driver circuits and thermal management with coils in the 100 mA range are more readily achieved than in the 1 A range. Hence, frequency sweeps with a YTO have significant practical advantages over field sweeps with a modulation coil.