Selecting suitable amplifiers for a frequency reference system involves many considerations. Simple selection criteria of gain and noise figure are not sufficient because the signal level is large compared to the noise level. Inherent nonlinear effects in the amplifier then mix base-band noise processes with the signal to create close-to-carrier phase modulation (PM) and amplitude modulation (AM) noise. The magnitude of the added PM and AM noise depends on the signal level and the frequency of the carrier signal and are generally not related to the noise figure. The PM and AM noise added by the amplifier(s) degrade the performance of the input reference signal causing additional frequency / phase / amplitude instability, which may be dependent on environmental parameters. In general, highly linear amplifiers exhibit lower up-conversion PM and AM noise than those with more conventional non linear characteristics.

Other issues are the input and output impedances, reverse isolation, and sensitivity of these parameters to variations in the input DC power supply and / or the environment. In many cases the signal needs to be split into several paths (channels) to be used for different functions. The relative phase of these signals is often important. The phase of the signal in one path is influenced by the reflection from the input impedance mismatch of the amplifier in the adjacent channel. Reflections from the load on one amplifier, attenuated by the reverse isolation, can also change the phase of the signal in the adjacent channel. Isolation between channels is also important.

• Selection Parameters
• Signal power
• Gain
• Multiple outputs
• Phase noise
• Phase stability with time and environment
• Amplitude noise
• Input return loss / impedance mismatch
• Output return loss / impedance mismatch
• Reverse isolation
• DC input power restriction
• Multiple power supplies required, hot switching to backup required?
• Size & weight restrictions
• Lifetime / reliability
• Commercial / military / space?
• Available lead-time
• Cost restrictions
• Environmental parameters
  • Temperature
• Humidity
• Atmospheric pressure
• Vibration
  • Voltage stability
  • Radiation environment
  • Magnetic environment
  • Load stability

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