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Ultra-stable room-sized environmental chambers may involve significant power variations leading to hysteresis and nonlinearities of the stabilization system. This paper describes utilization of a pulse code modulation control which significantly reduces observed nonlinearity of the chamber actuation system. Stability of environmental conditions for temperature Δ T = ± 25 mK (15–35 °C dynamic range), relative humidity Δ R h = ± 0. 1 % (3%–95% dynamic range), and pressure Δ P = ± 12 Pa (± 500 Pa dynamic range) was achieved within 27 m 3 chamber volume, which is two orders of magnitude better than the corresponding stabilization achievable using a standard linear proportional–integral–derivative (PID) controller. Utilizing this chamber we have demonstrated two orders of magnitude improvement of the operational relative frequency stability of ultrastable quartz oscillators (USO), reaching 2 × 1 0 − 13 stability at 1- 1 0 5 s averaging time. • The chamber allow the development of instruments before the environment is available. • The chambers simulate conditions to test the failure rates of the novel instruments. • Control of the environmental factors is the key to qualify the source of failure. • The chamber is tuned to qualify atomic clocks, masers, oscillators, frequency units. • A pulse code modulation control reduces nonlinearity of the chamber actuation system. [ABSTRACT FROM AUTHOR]