We demonstrate a 2.6 GHz chip-scale oscillator that measured phase noise better than -150 dBc/Hz at 1 MHz offset and integrated jitter of just 25 fs. The part was designed with differential out and drives a 100 Ohm differential load with a mean voltage swing of 100 to 200 mV. The phase noise at 10 kHz offset is -110 dBc/Hz. The device runs at 3.3V and Idd is just over 9mA (including buffer). Another variant is designed to have an on-chip varactor allowing tuning of 615 ppm/V over the targeted 0.5 to 1.8 V tuning range. Here, the integrated jitter degrades by 2X at 0 V Vtune and we measured 85 fs jitter at 1.5 V Vtune. Still, the integrated jitter was well below 100 fs. The all-silicon packaged part is designed to directly solder down onto a PCB. There are no bond wires used in the assembly of this device. The height of the soldered part is under 0.25 mm, and the area of the die is less than 1 mm 2. The oscillator uses a Zero Drift Resonator (ZDR) FBAR and we see about +/- 100 ppm temperature drift from -40C to +110°C. The design uses a cross-coupled architecture with the ZDR and is ac-coupled to a buffer amplifier.