Replacing YIG-tuned oscillators with silicon
(Product announcement here)
ADI’s Bob Clarke and Ian Collins write;
“RF and microwave instruments, such as signal and network analyzers, require wideband, swept frequency signals to make many of their basic measurements. Yet, perversely, wideband voltage controlled oscillators (VCOs) typically have the worst phase noise due to the low Q and high KVCO (a VCO’s tuning sensitivity in MHz/V) required to maximize their tuning ranges. Yttrium iron garnet (YIG) tuned oscillators (YTOs) neatly solve this problem with good, wideband phase noise performance and an octave of frequency tuning range but can be large, expensive, and can consume hundreds of mA current due to their tuning currents. And of course they still require an external phase-locked loop (PLL) to close the loop and a voltage controlled current source to provide their tuning current.
“A YIG crystal sphere looks like a high Q LC circuit whose resonant frequency is linearly proportional to an externally applied magnetic field. The oscillator is tuned by a current through a single turn loop through an octave or more in the GHz range. YIG tuned oscillators exhibit a low level of phase jitter, and their broadband characteristics from approximately 2 GHz to 18 GHz (with a very linear tuning curve) make them a popular choice for many measurement applications.
“However, the performance gap between YIG-tuned oscillators and integrated PLL/VCO ICs is closing. For example, recent advances in integrated PLL/VCO ICs, such as the ADF4355 have much improved phase noise over their predecessors. They have also solved the wide tuning range issue by design techniques such as dividing the output frequency range into multiple, adjacent subbands, where each subband can have a dedicated band switched VCO to increase tuning range while making the band switched VCOs look like a single VCO with a moderate KVCO (shown in the figure) to the user.”
The article notes; “…even if an integrated PLL/VCO IC has a more comparable tuning range than a YIG-tuned oscillator, one problem still remains: YIG-tuned oscillators still provide 12 dB better phase noise performance than the best integrated VCOs. Even this gap can be reduced if not yet closed, by combining the outputs of multiple PLL/VCOs in parallel….” – and continues to explain how such a configuration can be implemented, and what results are to be expected.
Figure, above; The multiband VCO in the ADF4355 PLL/VCO consists of an array of oscillators, each tuning a fraction of the entire band and maintaining a uniform KVCO and thus VTUNE across the entire range. The shape of the tuning voltage plot looks like a sawtooth because each oscillator has fixed capacitors that are switched in parallel with the voltage variable capacitance one at a time to maximise each VCO’s overall tuning range.
Read the complete article on ADI’s site here.
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