The following papers may be of some interest to anyone contemplating building a low noise low frequency (dc -100kHz) amplifier. Such an amplifier would be useful, for example, after a double balanced mixer in a conventional phase noise measurement system. http://arxiv.org/PS_cache/physics/pdf/0503/0503012.pdf http://arxiv.org/PS_cache/physics/pdf/0512/0512082.pdf
In a correlation measurement system the white noise floor can be lowered by averaging, whilst the flicker noise cannot. Thus it is important to select amplifiers with low flicker noise for such systems. Although some of the aspects of the amplifier design are questionable (e.g. lower noise current sources are possible), the very low frequency noise measurements illustrate the importance of thermal lagging and using a light tight enclosure. The seals around the kovar leads in the metal dual transistor packages transmit light into the package. All transistors are photosensitive unless the junctions are shielded. The LED in the current source is also photosensitive, however the resultant modulation of the current source output is a common mode signal. Since the LED emits light it can also generate photocurrents in all of the metal can packaged transistors. It would be interesting to look at the effect of placing a light tight shield over the LED. Another approach would be to use equivalent dual transistors (SSM2220) in a light tight package. Other useful information is the noise data for various opamps in the flicker noise region. This shows that the intrinsic low frequency noise of devices can be significantly lower than that specified in their datasheets. The datasheet noise specs are usually based on measurements by the manufacturer using a setup which isn't optimised to minimise thermal fluctuations or air currents. Since the close in phase noise of a bipolar RF amplifier is primarily due to modulation of collector base (and other) capacitances by bias voltage fluctuations, and the collector base voltages of the various transistors is temperature dependent, it is important to shield the amplifier from air currents and thermally lag the amplifier by enclosing it in a massive metal enclosure, if one wishes to achieve low close in phase noise. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
