Blogs

Spent a lot of time trying to low noise overclock audio preamps.  The main factor affecting you as you get an extremely high gain transducer into subterranian levels of noise is the power supply.  Every consumer gadget encounters some humming noise before it encounters thermal noise.  Even a DSLR has banding noise from a power supply ripple.

Power supply banding noise on an obsolete EOS 5D.

You would't believe how sensitive a primary microphone stage is to power supply noise.  Merely toggling a pin on a microcontroller or driving an LCD panel would cause enough ripple in the power supply to generate a hum.  2 hums of different frequency would mix & create a harmonic within human hearing.

The whole affair made us wonder if our obsolete sensors couldn't be improved with more power supply filtering.  Everything that makes money has loads of decoupling caps near each sensor, but we always flew with 1 cap for the entire autopilot & just the reference design caps for the sensors.

EEVBLOG's video on RC filters for smoothing PWM inspired us to look more at the subject.  More importantly, he showed a dual cascaded RC filter squashing the ripple more than anything else, which conflicted with our tradition of using the fewest components.  So we applied some RC filters to the king of noise: the SCP 1000.

It's an obsolete barometer, but it's all we can afford.  Tried 10ohm + 480uF, dual 10 ohm + 480uF, & 100ohm + 480uF.



The curves reflect the board cooling after soldering.  The noise decreased dramatically with the RC filter & is now in the 1/2 meter range.  The reference design obviously left a lot to be desired & a lot of noise is coming from the microcontroller itself.

The 100ohm + 480uF did slightly better than the dual 10 ohm.  So in matters of cost & weight, you're probably still better off leaving the dual cascaded RC filter at home.

Our obsolete GPS module wouldn't work at all with an RC power filter.  That has transients at too low a frequency to dampen out without an active regulator.  The barometer most certainly has its own transients, but still benefitted.

The gyros didn't benefit at all.  They're probably frequency dependant.  Left the cap on, to dampen vibration.

Maybe the transient voltages of polling a radio receiver can reduce its sensitivity?  Maybe using the interrupt pin on our 900Mhz radios would increase their sensitivity, but as soon as the 1st byte of data came in, the device would lose sensitivity for the rest of the packet, as we clocked out the data.  Maybe if we used the analog pin as a raw UART instead of the digital FIFO, it would be more sensitive.

Remarkable that 2 years later, radios haven't gone anywhere in terms of capability or parts.  There's been no major effort to go outside the 2.4Ghz band.

To our amazement, Ning now limits blog posts to 7 photos, so the rest are on blogger.  Everyone's cutting back on the freebees & firing everyone as the stock market booms.

Vika 1 flew again, after 1 year.  Her adhesive tape had settled with the propellers facing forward after all that time.  Round carbon fiber is unbreakable but impossible to build with.  The RC filter got the altitude much more stable, but heading without a magnetometer was worse than we remember.

Someone gave us an AKM8973 almost a year ago.  The datasheet seemed to have some errors in the pinout & it never did work.

Being a chronically unemployed programmer doesn't allow us to fly very often or buy the latest autopilot instead of hacking on the ghetto autopilot.  Actually pursued the LA aerospace idea for a year, but never got any interest.  The big thing these days for making money is making web applications with Ruby on Rails, but our analysis says it won't last any longer than J2EE.