H

Peak
ID Number

Level
(dBFS16)

10,747,863
kHz IF

10,744,810
kHz IF

Delta
kHz with 3053Hz IF delta

dBc


1

1

29

252

255

3

0

Fundamental ~10.75MHz

2

3

68

496

490

6

39

Second harmonic at ~21.5MHz

3

2

70

244

235

9

41

Third harmonic at ~32.25MHz

4

7

88

7.5

19.5

12

49

Fourth harmonic at ~53.75MHz

Note that the peak at #6 in the figure does not appear to
move, nor do the 100dB ones at +475kHz and +45kHz.
The following tables capture the calculated
peaks. The columns are the harmonic, its
frequency, the number of sampling frequency intervals it represents, the
integer number of 2x Nyquist intervals, the frequency within a sampling
interval and the frequency within the final Nyquist interval. (As the signal frequency is increased it
reaches half the sampling rate and then wraps and begins “descending” in frequency. Once it hits a sampling interval or pair of
Nyquist intervals it begins ascending).
H

Fo(Hz)

Fs ratio

2NyInt

FsFrac

Fd(Hz)

1

10,744,810

10.74481

10

744810

255,190

2

21,489,620

21.48962

21

489620

489,620

3

32,234,430

32.23443

32

234430

234,430

4

42,979,240

42.97924

42

979240

20,760

H

Fo(Hz)

Fs ratio

2NyInt

FsFrac

Fd(Hz)

1

10,747,863

10.747863

10

747863

252,137

2

21,495,726

21.495726

21

495726

495,726

3

32,243,589

32.243589

32

243589

243,589

4

42,991,452

42.991452

42

991452

8,548

To evaluate the ADC and amplifiers the gain is changed to
maximum. The following figure
illustrates a g=11 with a history of g=00.
Previous measurements indicated that for Unit #2 A board the
small signal minimum gain is 18dB while the maximum is 35dB. Comparing the last
two figure’s fundamental values of 13.9dBFS16 and 27.1dBFS16 we see a change
13.2dB while we would have expected 17dB.
This is actually quite good since we will experience gain compression
given that we are almost at the full swing value of 3.3V at the amplifiers and
the full scale range on the ADC. Note
that a couple of frequencies not previously present now are (i.e the signal
just below 200kHz and the ones around 450kHz).
Lastly the noise floor is evaluated by setting the IF to ~11MHz to get it and all its harmonics on the "low" end of the ADC response. The following figure captures the spectra at an FFT of 128 (gold) and 32k (blue).
Recall that the display is in dB relative to a 16 bit
value. The rational is to keep things simple
regarding who shifts what bits where and masks off the 4 bits of zeros – to
keep things sane, it is all treated as a 16 bit value. The upshot of this is that we really have a
14 bit value (which can never achieve its maximum due to leading two bits of
zero) which is shifted up 2 bit (or multiplied by 4). The noise is calculated as taking a 14 bit
number (6.02*14+1.76 = 86dB) and shifting this value up by the multiplication
by 4 in amplitude (or adding 20*log10( 4 ) = 12 dB) to any noise floor
calculation. The FFT processing gain is
10*log(M/2) where M is the number of FFT points. M=128 =>18dB of gain, M=32k=>42dB of
gain. So the 128 point FFT should see a
noise floor of 86dB – 18dB + 12dB = 92dBFS16 while the 32k FFT should have a
noise floor of 86dB – 42dB + 12dB = 116dBFS16.
The cases in the above figure show 128point=>87 and
32k=>112. In both cases we are a
couple of dB too high. For our purposes
this is quite good and within expectations.
In general, I am very pleased with the results and the ability to account for everything observed. The AD7276 has worked out extremely well in this application.
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