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Here is some data depicting the measured lambda versus the absolute MAP for my Honda B20. The lambda was measured using an AEM WBO2 gauge and the RS232 output was logged by my ECU. The MAP is also logged by my ECU at about 6.5 Hz. The logging occurs whenever the MAP and RPM exceed the chosen thresholds, in this case 0.8 BAR and 3,000 rpm. There are pics of my custom ECU elsewhere on this site.

Quite a bit of data scatter isn't there? Some of it is due to transients like letting off the throttle. I've investigated electrical interference of the WBO2 gauge and have provided good separation from the ignition and capacitive decoupling of the input power. Not a big help. It may be that the wideband sensor just isn't that consistent in a noisy (pressure, vibration, electrical) environment. It may also reflect a true variation in combustion results. I will say that the engine runs flawlessly and the spark plugs read equally from cylinder to cylinder.

UPDATE:
1. The AEM WBO2 gauge's heater driver causes ringing in the cabling and this interferes with the lambda measurement. Adding a snubber circuit (diode only, no RC worked best) in the cabling near the gauge to suppress the ringing and inductive overshoot made a substantial improvement. The inductance is from the cable (about 200 nH per foot). Apparently the AEM engineers didn't recognize this and just filtered the crap out of the displayed output. The RS232 serial data output apparently is not filtered and the parsed data is noisy. AEM, you're welcome.
2. The AEM heater control loop oscillates at about 1 Hz (you can see the pulse width variation with an oscilloscope). This results in an unstable sensor temperature and may account for some of the data scatter.
3. The Bosch lambda sensor is sensitive to vibration. Reducing the amount of engine vibration coupled into the sensor reduces the noise in the lambda data.

BTW, the cable ringing is the likely source of radio noise some folks have experienced with the AEM gauge. The heater driver supplies a 5 KHz pulse-width modulated (PWM) voltage into the 3.3 ohm heater. The 1 MHz inductive ringing occurs when the driver switches off each PWM cycle. Another path for interference is the ground path. Don't connect the AEM gauge's ground in the path of the radio's ground. The AEM gauge's ground current includes the 4 amp PWM heater current.

I've been exploring variable sample rate (where the voltage step is constant) and also base 3 digital to analog conversion. Combining the two techniques gives me an interesting way of implementing a PIC microcontroller based AX25 modem using just 3 output pins rather than the normal 4. My limitation has been PIC processor speed to keep up with the rapid ramping needed as the signal crosses 0. I could do it, with some high frequency spikes from the switching, but I'd need to up the PIC's clock frequency to its full 32MHz, or I start skipping samples.

This graph is a prediction for a 4 bit binary converter using variable sample rate. The circles penned in are the actual sample points that the software is trying to follow. It is moving half a sample early to try to reduce error and therefore noise.

If the noise amplitude is taken as the maximum deviation from the target sine then I need to avoid skipping samples and try to keep close. A fixed sample rate solution could also achieve this quite well as long as the sample rate is fast enough to keep up with the fast ramping. Then the maximum error will be equal to half of the converter's resolution. The resolution of this 4 bit converter is -24dB.

The frequency content of the noise will vary here as the sample rate varies. In fact I'm frequency modulating the sample signal (d/dt of sin(t) is cos(t)), so I think I should expect the noise signal to occupy an FM type spectrum with extra harmonics from the step functions. Mix in any non-linearities in the system and that could become problematic!

Thinking of this, the better solution may be a fixed sample rate that is fast enough not to allow the noise amplitude to become too large as the waveform ramps quickly crossing zero. (That would be effectively amplitude modulating the noise). The fixed sample rate does also have the advantage of simpler code and less resource usage on the microcontroller, and noise is easy to filter in the analogue stage.

Maybe this is why we don't see variable sample rate synthesis. Searching for commercial direct synthesis solutions it looks like effort has been spent achieving very fast fixed sample rates.