Low noise mixer preamp

The circuit uses a low flicker noise SSM2220 pnp input stage with an OP27 output stage and an AD820 dc servo stage. It has a gain of 60dB and a <1Hz to >300kHz 3dB frequency response. It can accommodate mixer IF port dc offsets of up to 40 millivolts. It is suitable for use with PC sound cards and low frequency spectrum analysers for measuring phase noise. The offset frequency range is [20Hz, 20kHz] when using a sound card.

Originally designed for measuring mixer flicker phase noise at low offset frequencies it is also useful when measuring the phase noise of other components. With appropriate software a cross correlation phase noise measurement system can be assembled incorporating a PC sound card and a pair of these preamps.

The design derives from Enrico Rubiola's low noise mixer preamp design (http://arxiv.org/pdf/physics/0503012). The noise and power supply rejection of the input stage current source have been improved and its cost reduced. The 0.01% low drift collector load resistors (R104 and R105) and offset trimming network are replaced with cheaper 0.1% resistors to reduce cost without significantly impacting performance as the dc servo ensures that amplifier output dc level is very small.

Component selection:

• All resistors are metal film 1% except for:
  • R104 and R105 which are 0.1% metal film.
  • R110 and R111 may also be 0.1% metal film if you want high midband gain accuracy.
• All bypass capacitors are X7R ceramic, or plastic film.
• C106 (NP0/C0G) should be selected for best high frequency response. Alternatively a trimmer capacitor can be used.
• C107 and C108 are low leakage polypropylene film capacitors.
• All electrolytics are solid tantalum.