0.02 F FILM 1.62k 1.62k
+
0.02 F FILM VDC ADJUST 1 F FILM DC BLOCK
15V OUTPUT AMPLITUDE ADJUST 2k TRIMPOT –15V 1.24k
OP AMP AND ADC UNDER TEST 3.3V
CHRISTMAS LAMP C7 120V 5W
LT1468-2
−
10k
+
3.3V 324 1% 470 pF NPO 6.65k 1% 22 pF HP 3325 ADC CLOCK GENERATOR VREF CS LTC2361 SDO SCK GND 0VDD VDD
1k
LTC6255
−
AIN
100 NEW INPUT 200k
10k 1% AGILENT 33250A SINE GENERATOR
10-MHz REFERENCE
Figure 4 With the generators phase locked through the 10-MHz reference, the low-noise and -distortion Wien-bridge oscillator is gently nudged into coherence through the high-impedance, 200k resistor. Figure 5 A more accurate FFT is obtained using the same Wien-bridge oscillator but with the frequency injection locked to a coherent 5.157 kHz using an HP33250A driving the 200k resistor at the “new input.” Note that the peak is now visibly a believable –1 dBFS and that there is almost no power in the bins adjacent to the peak.
0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 0
OSCILLATOR COHERENT USING INJECTION LOCK REALLY LOOKS LIKE –1 dBFS
EDN DI5291 Fig 4.eps
DIANE
VIN=–1 dBFS, 5 kHz FS=125 kSPS VERY LITTLE POWER OUTSIDE THE PEAK HARMONICS ARE GRAPHICALLY ACCURATE SFDR=89 dB
512
1024
1536
2048
I connected the 10-MHz back-panel references and changed the 33250A frequency to 5.157 kHz, the nearest coherent bin in the FFT. The sinusoids remained in lock, and the programmable 33250A generator successfully pulled the Wien-bridge oscillator slightly away from its natural frequency and into the desired frequency. The result was a nearly ideal FFT; all of
the pertinent fundamental and distortion powers were situated in unique bins and were accurately represented (Figure 5). Programmable sinusoidal generators often have excellent phase-noise characteristics and 10-MHz locking capabilities, but they also have high-output wideband noise floors and distortion. An FFT is sensitive to all of these forms of
source corruption and also has a finite number of output bin frequencies. To test high-performance analog and mixed-signal systems, the right combination of classical Wien-bridge oscillators with programmable generators can provide a nearly perfect source with synchronous sampling, generating accurate FFTs. EDN
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december 2012 | EDN EuropE 39
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Table of Contents for the Digital Edition of EDNE December 2012