Motivation

The current implementation of Digital Active Nulling (DAN) uses an integral control loop to null, i.e. cancel out the current through the amplifier. This should leave very little residual signal at the ADC.

Here, I will use G as an arbitrary gain factor.

Results

The 195 kHz DAN build seems to operate as expected. The signal is nulled out at low frequencies, but not at high frequencies (far away from the demodulator). When the gain is increased, the residual signal drops until an instability is reached, increasing the residual quickly.

Transfer Function at G = 1

Transfer Function at G = 10

Transfer Function at G = 100

Transfer Function at G = 1000

Transfer Function at G = 10000

Below here are old results from the 12 kHz DAN build

Results

I set a small carrier some frequency away from the nulling/demodulation frequency. This produces a sine wave which is to be nulled away by DAN. The following plots show the nuller amplitude, as well as the residual signal at the ADC given a signal between 3 Hz and 305 Hz (80% of Nyquist at FIR4) for various gain settings.

Transfer Function at G = 2

Transfer Function at G = 20

Digitization appears to be an issue for G = 2 and G = 20. The gain shows step-like behaviour due to the discretization of the signal in the accumulator.

Transfer Function at G = 200

Transfer Function at G = 600

At G = 200 and G = 600, DAN seems to be well behaved, nulling well at low signal frequencies and decreasing in effective gain with frequency.

Transfer Function at G = 2000

At G = 2000, DAN is unstable, which can be seen from the large residual signal at the ADC.

Conclusions/Issues

G should be chosen to be high enough to null over the required signal bandwidth, but low enough to avoid instabilities.
I don't know why the quadrature sum of the residual signal and the nuller amplitude is not constant.
I don't know why the measured signal increases at high frequency.

-- TijmenDeHaan - 27 May 2010

Attachments

Topic attachments
I	Attachment	Action	Size	Date	Who
png	dan_10.png	manage	31.3 K	2010-09-02 - 16:36	TijmenDeHaan
png	dan_1000.png	manage	23.8 K	2010-09-02 - 16:30	TijmenDeHaan
png	dan_10000.png	manage	33.1 K	2010-09-02 - 16:30	TijmenDeHaan
png	dan_100png.png	manage	25.8 K	2010-09-02 - 16:30	TijmenDeHaan
png	dan_1png.png	manage	27.7 K	2010-09-02 - 16:36	TijmenDeHaan
png	tf_gain2000png.png	manage	28.8 K	2010-05-27 - 22:00	TijmenDeHaan
png	tf_gain200png.png	manage	36.8 K	2010-05-27 - 21:59	TijmenDeHaan
png	tf_gain20png.png	manage	38.3 K	2010-05-27 - 21:59	TijmenDeHaan
png	tf_gain2png.png	manage	32.9 K	2010-05-27 - 21:59	TijmenDeHaan
png	tf_gain600png.png	manage	32.8 K	2010-05-27 - 22:12	TijmenDeHaan

This topic: ColdFeedback > WebHome > DanTransferFunction Topic revision: r3 - 2010-09-02 - TijmenDeHaan