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Message ID: 326     Entry time: Mon Aug 22 14:46:47 2011
Author: Daniel, Jan 
Type: DailyProgress 
Category: General 
Subject: Low Noise Preamplifier 

We built a Low Noise Preamplifier (D060205), and added voltage regulators for external power supply, so one can switch between battery and external supply. Some may know this one as the Ray Rai box (which it is once you put it inside a box...). The heart of the 100x amplifier is a high-gain, low-noise FET amplifier from InterFET (IF3602) that can have voltage noise as low as 0.3nV/rtHz. First a picture of the amplifier: 

RIMG0141.JPG

Terminating the preamp input with 50 Ohm, and using another 100x amplification from an SR560 (in battery mode) and AC-Coupling, we measured the following spectral noise density recorded with one of the DAQ systems that was designed by Vladimir:

 noise_FET2.png

As one can see, the (single-sided) noise spectrum is below 2 nV/sqrt(Hz), and there is almost no difference between the two cases in which the preamp has external and battery supply. The 60Hz lines are unusually strong. We already saw spectra that only had one weak 120Hz line. So our guess is that this is picked up by the cables that we rearranged recently. Anyway, it is good news that no significant noise seems to couple in from the external power supply to the preamp output.

We were wondering a little about the high noise. The specs promise something that should be weaker by a factor 5 or so. In fact, measuring the preamp noise with the network analyzer, we got noise spectra that were weaker by more than a factor 2. It is quite tricky to pin this down to some specific problem. Calibration seems to be fine since signals from the frequency generator show up with the correct amplitude in the time series. One thing that we still need to look at is some C code used to read out data from the DAQ. Some simple data averaging happens there without anti-aliasing filter, and this could cause extra noise.

We should soon have new FETs to be able to make more preamps. Also, if someone wants to have this amplifier, you can get the parts from us. Only the FETs are missing and you could nicely ask Peter King to get a FET from him. We also have a complete digikey parts list for the preamp in case you prefer to buy parts yourself. Unfortunately, we don't have a suitable box yet for the amplifiers.

 

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