I recently had the opportunity to
investigate a new micropower
6-MHz LTC6255 op amp driving
a 12-bit, 250k sample/sec LTC2361
ADC. I wanted to acquire the FFT of a
pure sinusoid of about 5 kHz. The problem
is that getting the FFT of a pure
sinusoid requires, well, a pure sinusoid.
Most programmable signal generators,
however, have fairly poor noise and distortion
performance, not to mention
digital “hash” floors, compared with
dedicated op amps and good ADCs. You
can’t measure 90-dB distortion and
noise using sources that are “60 dB-ish.”
So rather than try to find and keep an
almost-ideal programmable signal generator,
I decided to build up a low-distortion
oscillator using an ultralow-distortion
LT1468-2 op amp (Figure 1).
Figure 1 This Meacham-lightbulb-stabilized, low-distortion, low-noise 5-kHz Wien-bridge sinusoidal oscillator’s RC feedback network attenuates by a factor of 3 at its midband. The bulb’s self-heating forces a gain of 3 in the op amp.
The lightbulb amplitude-stabilization
technique relies on the positive
temperature coefficient of the bulb
impedance stabilizing the gain of the
op amp to match the attenuation factor
of 3 in the Wien bridge at its center
frequency. As the output amplitude
increases, the bulb filament heats up,
increasing the impedance and reducing
the gain and, therefore, the amplitude.
I did not have immediate access to the
usually called-for 327 lamp, so I decided
to try a fairly low-power, high-voltage
bulb, like the C7 Christmas bulb shown.
At room temperature, it measured
316Ω; fresh out of the freezer (about
−15°C), it measured 270Ω. Based on
the 5W, 120V spec, it should be about
2.8k at white hot. That seemed like
plenty of impedance range to stabilize
a gain of 3, so I decided to linearize it a
bit with a series 100Ω resistor.
For a gain of 3, the bulb plus 100Ω
must be half of the 1.24k feedback
(or equal to 612Ω), so the bulb must
settle at 512Ω. Roughly calculating a
resistance temperature coefficient of (316–270Ω)/[25−(−15°C)]=1.15Ω/°C
means that the bulb filament will be
The oscillator powered up fine,
giving a nice sinusoidal 5.15-kHz output
at several volts, and independent
measurements showed the second- and
third-harmonic distortion products
to be lower than −120 dBc. I applied
the oscillator to the LTC6255 op-amp
input after blocking and adjusting the
dc level and ac amplitude, using the caps and pots as shown in Figure 2. The
ac amplitude was adjusted for −1 dBFS,
and the dc level was adjusted to center
the signal within the ADC range.
But, of course, this oscillator was purely
analog and had no “10-MHz reference
input” on the back to allow it to be
synchronized with the ADC clock. The
result is substantial spectral leakage in
the FFT, so that it looks more like a circus
tent than a single spike. Applying a
92-dB Blackman-Harris window to the
data to reduce FFT leakage produced a
fine-looking FFT (Figure 3).
Figure 2 The Wien-bridge oscillator drives the op amp and ADC pair under test. The resulting FFT is clean after windowing, but not exceptional, as Figure 3 shows.
Click image to enlarge
Figure 3 This 4096-point FFT was achieved using an unlocked oscillator with a 92-dB Blackman-Harris window. Note that the peak does not look like –1 dBFS and that there is power in the bins around the peak.
Although this FFT is accurate in
some ways, a closer inspection reveals
some problems. For example, the input
signal is −1 dBFS, but it certainly looks
graphically lower than −1 dB down.
The reason is that even an excellent
windowing function leaves some of the
fundamental power in the frequency
bins adjacent to the main spike. The
software includes these bins in its power
calculations, and rightly so, but the fact
is that the spike looks too low to make
a good photograph.