In modern digital communications and radar the success of communications and sensing has become extremely sensitive to phase noise as well as amplitude noise. Phase noise is the frequency domain description for jitter, which is a major component of bit-error rate (BER) and other phase error related digital communication and sensing degrading phenomenon. Where Low Noise Amplifiers (LNAs) are designed specifically for their low added noise figure, there is another type of low noise amplifier designed to maintain a low added noise figure as well as low phase noise. A Low Phase Noise Amplifier (LPNA), is a variety of LNA optimized for a lower phase noise while still ensuring a low added noise figure.
Key LPNA Electrical Specifications & Features
- Frequency range [Hz]
- Interconnect impedance [Ohms]
- Small signal gain [dB]
- Noise figure [dB]
- Phase noise [dBc/Hz @ 1kHz offset]
- Gain flatness [dB]
- Gain variance at base plate operating temperature (OTR) [dB]
- Input & output VSWR
- 1 dB compression point output power [dBm]
- Output 3rd order intercept point [dBm]
- Saturated output power [dBm]
- Spurious [dBc]
- RF power handling (maximum RF input power) [dBm]
- Reverse isolation [dB]
- Operating DC voltage [V]
- Operating DC current [A]
- Operating temperature range [degree C]
- Interconnect type [waveguide, coaxial connectorized, SMT, etc]
LPNAs have very similar electrical specifications to LNAs, with the exception that phase noise is a more prominent specification. Phase noise is the noise that is generated due to random and transient phase fluctuations in a signal. Phase jitter is the time-domain phenomenon that results in phase noise in a signal. Generally, phase noise is measured and described as the amplitude of the phase noise relative to the carrier frequency (-dBc), phase noise amplitude, at a certain distance away from the carrier signal (@ #kHz), carrier offset, and over a certain bandwidth (usually 1 Hz). It is important to specify the bandwidth, as noise power is proportional to the bandwidth.
A higher absolute value of phase noise is typically desirable. The phase noise goal of a signal chain is dependent on the overall performance goals of the system. Often, a LPNA is chosen as the overall phase noise budget for the signal chain is exceeded with a typical LNA. LPNAs may not have as good of noise figure performance as pure LNAs, so there will still need to be room in the link budget for a marginal additional added noise figure when using an LPNA.