We designed ultra-low-noise regenerative divide-by- 2 circuits that operate at input frequencies of 10, 20, and 40 MHz. We achieved output-referred single-sideband residual phase noise equal to -164 dBc/Hz at 10 Hz offset and estimated residual Allan deviation, σ(y)(τ) less than 3 × 10(-15)τ(-1) for a single divider, which is, to our knowledge, the lowest noise of any divider ever reported at these frequencies. To measure such a low noise, we also built a cross-spectrum measurement system that has a noise floor of -175 dBc/Hz at 10 Hz offset from the carrier frequency. The low noise of the divider and the measurement system are achieved by using custom-built mixers/phase detectors that use 2N2222A bipolar junction transistors (BJTs) in a conventional double-balanced diode ring.