A 135fsrms-Jitter 0.6-to-7.7GHz LO Generator Using a Single LC-VCO-Based Subsampling PLL and a Ring-Oscillator-Based Sub-Integer-N Frequency Multiplier

Despite the growing use of mm-wave bands, FR1 bands are still the primary spectrum for 5G communications due to their natural advantages, such as higher compatibility/interoperability with existing networks and better properties for radio transmission. To fully utilize the entire FR1 spectrum, the efficient design of an LO generator based on the meticulous plan for LO frequencies,
$f_{\text{LO}^{\mathrm{S}}}$
, is essential but very challenging. The first challenge is the ever-expanding range of FR1 (top of Fig. 4.2.1). In recent standards, FR1 is defined to cover the new-radio (NR) bands up to 7.1 GHz while still including sub-GHz existing bands. Second, LO generators should achieve very low IPN (integrated phase noise (PN)) to support high-order QAMs. To achieve IPN below -43dBc for 256-QAM, the rms jitter should be below 200fs in the upper-frequency bands. Third, LO signals should have accurate quadrature phases. To generate accurate quadrature LO signals with low IPN over the wide FR1 range, cascaded architectures are most popular (bottom left of Fig. 4.2.1). At the 1st stage, a fractional-N Pll with LC-VCOs generates low-jitter signals at relatively low frequencies. Then, frequency multipliers (FMs) increase these frequencies to higher bands. Compared to LO generators using LC-VCO-based FMs [1], architectures using ring-oscillator (RO)-based FMs [2] are much more efficient in terms of silicon area but still limited since the 1
^st
-staue PLL needs multiple LC-VCOs to make the following FM cover the entire target
$f_{\text{LO}^{\mathrm{S}}}$
. (The PLL requires at least 46% frequency-tuning range (FTR).) While a chain of frequency dividers (FDs) can be used instead of FMs [3–5], in this case, LC-VCOs and FDs operate at much higher frequencies and require more power. The problems of the conventional cascaded LO generators, i.e., large area due to multiple LC-VCOs and large power due to long FD chains, are because typical FMs and FDs that only have integer multiplication factors (Ms) and even division numbers, respectively. The area and power issues in LO generators become more serious as the number of component carriers increases for higher data-rates.