| An Ultra Low Noise
(=<1ppb) Programmable Audio Oscillator
The "Uber
Oscillator" -- a design from Linear Technology's AN-67, by
Dale Eagar "Using Super Op Amps to Push Technological Frontiers,
an Ultrapure Oscillator". The author reported that the
distortion was immeasurable !!!
To see the article on Linear
Technology's website, click on "Application Notes" and scroll
down to AN-67
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| For the complete
description you should refer to Linear's Application Note.
We have produced the schematic here, using our schematic capture
program Multisim so that the design could be converted to a
printed circuit board. Speaking of circuit boards, this
design uses a quad current-feedback operational amplifier, the
LT1230, in the "Super Gain Block" (first schematic) -- these
very fast devices should be used with a printed circuit board.
The "OffPage" connectors are used to link the two schematics in
Multisim. |
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Construction: Place and
solder in the following order: resistors, integrated circuits,
capacitors, trimmer resistors, power and output connectors.
The oscillator should be placed in a shielded chassis, preferably
one with good shielding properties like a steel cookie tin.
To get the project started I used a large cookie tin and 10
D-Cells to provide +/- 7.5 volts. |
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Testing: You will need an
oscilloscope to test the oscillator. The first trick is to
get it to oscillate -- place the Output Amplitude and 2nd Harmonic
trimpots to their 50% positions. With your scope set as follows:
50mV/division vertical, 20uS/horizontal, DC coupling adjust
the 2nd Harmonic Trimpot until the output
voltage offsets some tens of millivolts above 0 volts reference.
Then slowly adjust the Gain Trimpot until
oscillation kicks in. You should get anwhere from 700 mV to
7.5 V peak to peak reading, and it won't be pretty. For a
pure sine-wave the you will have to back off the Gain
Trimpot and "twiddle" the Output Amplitude Trimpor
for a reading of 5V peak to peak. |
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Results: We ran a comparison of
the Super Oscillator and the oscillator of the Boonton 1120 THD
analyzer. Both devices were fed to a Tektronix DA4084 Distortion
Analyzer and the notched output displayed on the HP 3577a.
Spectrum Analyzer. Note that the vertical sensitivity of the
Super Oscillator graph is 1uV/division, but 2uV/division for the
Boonton. Both plots were done with a 20dB attenuator.
The average noise level of the Super Oscillator is a bit higher
than the Boonton, but the harmonics are virtually non-existent.
In fairness to the Boonton, once it settles down for a few minutes
the 2nd and 3rd harmonics become very small.
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