Wednesday, April 8, 2015

Q measurements with shunt fixture

In the previous post I discussed both shunt and series connected unknown circuits and formulas for each, but the results I showed were only for my 1-ohm fixture and series connected unknowns.

Here I'm showing some results for shunt connected unknowns.  BTW, in both cases the unknown is a series connected L-C circuit.

Recall that I discovered that my 1-ohm fixture had enough loop reactance (from inductance) to affect the measurement significantly, but in the series configuration it could be calibrated out.  For the shunt case it wasn't so simple. So the "fixture" for shunt measurements needed to minimize the loop created by connection of the unknown.  The simplest seemed to be just a BNC Tee with a BNC receptacle connected to the open end.


The two female ports go to the PHSNA DDS and the AD8307 measurement circuit.

I made some measurements with the type 2 and type 7 toroids again and with the molded choke, and I also added a couple of air wound B&W type coils - one small and one quite a bit larger.  In all cases I selected resonating silver mica capacitors to give resonance around 7.5 to 8.5 MHz.  Just for consistency and to be somewhat "midband HF".  I also added series resistances in several instances, to see if my measured Rx would increase by the amount I added, as a method of checking accuracy.

BTW, there are no transformers or minimum loss pads here, so this is a 50 ohm environment.

Below are some results from my spreadsheet.


If the data table doesn't fit the column, click it to make it full size.

I feel like I'm getting pretty decent results here, consistent with my expectations.  

BTW, elsewhere in my spreadsheet I enter my dBm measurement with the unknown port open and again with the unknown network connected to give me the attenuation caused by the unknown.  I also enter the environment resistance (50 ohms), and it gives me Rx, the series resistance of the network, presumed to belong mainly to the inductor.  

I also enter the measured inductance of the coil and the resonant frequency and my spreadsheet gives me the Q of the circuit.

NRK 4/8/2015


Wednesday, April 1, 2015

Q measurement with PHSNA

[Edit #1:  Some of my images (graphs & schematics) shrunk to fit the column width.  Just click on images that seem to be truncated to see them full sized.]

Measuring Q is an interesting challenge.  With the PHSNA (a DDS calibrated source plus AD8307 RF power measurement instrument plus analysis software), it seems like it should be a piece of cake.  I'm already doing it with crystals, after all.

But then I remember that Qs of 100 to 200 or more, even with toroids, aren't unusual, and inductors with reactances in the 50 to 200 ohm range are common in RF / HF designs. Do the math and you see that we may be measuring fractional ohm loss resistances.  A bit of a challenge in a 50 ohm system.

One way to increase sensitivity is to put the component being tested in a lower resistance fixture, for example, 12.5 Ω is typical for crystals.  Going to extremes, a 1 Ω fixture does a lot.  A problem is that two 50 to 1 Ω minimum loss pads back to back have a combined attenuation of 45.9 dB.  So we need a strong source or sensitive detector or both. My PHSNA system has about 0 dBm RF output and the detector should go down to say, -60 dBm without much trouble.  So I probably don't need an amplifier in line.

I also got into looking at Q measurement methods in EMRFD (page 7.36).  Typically, one puts a series resonant circuit for which Q is desired in series between the source and detector.  In this example, the test circuit is put in the shunt configuration between source and detector.  I wondered how that differs in sensitivity form the series method.  So the first thing I did was do the math to solve for loss resistance in both configurations.


I derived the formula in general terms for source (and detector) resistance Rs instead of just for 1 Ω.  Rx is the value of the loss in the coil (or coil + capacitor).  For attenuation A, I get:



The ultimate purpose of this exercise though is to find Rx.  So solving the above for Rx for the shunt configuration:



The procedure is to measure power to the detector without the tested circuit installed, then measure it again with it installed in shunt (to ground, between source and detector).  'A' is the dB difference in the two measurements and here is it is a negative value.

To compare this method with the series method, I need to derive the equations for that configuration.

Series configuration:


and solving for Rx:


I was somewhat surprised when this result didn't look like the equation I've been using for crystal Rloss measurements. I finally realized it was because my math began with attenuation as a negative number but the other equation entered it as positive.  So they are equivalent.
I put the equations into an Excel spreadsheet and plotted the attenuation A against Rx for both configurations.  My thought is that the method that produces the greatest change in A per ohm change in loss resistance is the more sensitive.

Below:
1 Ω test fixture
Rx on horizontal axis
dB attenuation on vertical axis


From the above, the series method seems better for "all around" measurements, but the shunt method looks like it would be better for high Q / low loss resistance items.  In a one ohm environment.

I repeated the above plot for a 12.5 ohm fixture and again for a 50 ohm fixture and present results below:

12.5 Ω test fixture
Rx on horizontal axis
dB attenuation on vertical axis

Above, the series configuration gives a fairly constant slope of about 0.3 dB per ohm.  The shunt configuration gives much better sensitivity, up to the source resistance of 12.5 Ω which is the crossover point again.

Finally for the 50 ohm fixture case:
Below:
50 Ω test fixture
Rx on horizontal axis
dB attenuation on vertical axis


(Above) Again the change per ohm with the series method is constant but it’s down to 0.08 dB/Ω.  With the series method, much more sensitivity is achieved with about 8 dB/Ω at 1 Ω, decreasing to 0.36 dB/Ω at 15 Ω.  It's clear that with higher source / detector resistances, the shunt method is better.
It just occurred to me that if you already have a 50/50 system, by using the shunt method you don't have to go to the trouble of building a lower resistance jig.  So EMRFD rules again.
Some practical results:
I wanted to go with the 1 ohm fixture first.  Below is a schematic of one I got from Bill Carver, W7AAZ.  It uses SMT precision resistors from Mouser. Also shown is a 3 dB attenuator.  Cut off at the right is matching to the HYCAS amplifier.  Bill was showing me how to use it to measure crystal parameters.

And below, we have it in physical form:


An unexpected problem!

I did some series method measurements with the tested item installed (as shown) and with a short across the two alligator clips.  What a shock to find, in some cases, more power measured through the tested device than with the short. Negative attenuation!

This baffled me for a while but I began to suspect the inductance of the loop formed by the two alligator clips and the shorting wire between them.  I measured the attenuation of a loop about that size on my AADE L/C meter and get somewhere between 0.045 and 0.075 uH.  At 8.2 MHz, that’s 2.5 to 3 Ω of reactance.  And in a 1 Ω environment, that’s significant.  Using LTSpice, it seems to add about 7 dB attenuation over a “real” short.

But when I have my series L/C circuit under test installed, the small loop inductance gets absorbed into my test coil’s inductance and cancelled when I find the resonant peak.  That’s why the circuit under test actually has a higher power value than the so-called shorted fixture.
I saw a couple of ways around this.  One was to resonate out the stray inductance when I did my "shorted fixture" measurements.  It's best to do this reasonably close to the measurement frequency, so I needed about 5800 pF.  That worked -- I was able to see the peak and measure available power with strays cancelled out.
Another method would be to assume the calculated 45.89 dB attenuation of the fixture is accurate, measure power with source plugged right into detector, and take 45.89 dB off of that for my "shorted fixture" power.  The method of resonating out strays seemed to give slightly better accuracy.
Oh, I did use a lowpass filter following the generator.  The DDS-60 is pretty well filtered, but notes on measuring Q emphasize the need for very good harmonic suppression.
Now some measurements.  I had a couple of iron power toroids wound with turns as noted.  One was a T68-7 and the other a T50-2.  I measured inductance with my AADE meter and chose a resonating capacitor (silver mica) to resonate at something over 8 MHz.
I made a measurement of the L/C resonant circuit and a second one with some series resistance added, to see if the delta came out close to the resistor value.  I also tested a type 61 toroid and a miniature molded choke, just to do some samples with lower anticipated Q.  I plug the attenuation and source resistance values as well as inductance and frequency into an Excel spreadsheet and have it crank out loss resistance Rx and Q.


The method is  to measure the shorted fixture power as discussed above, then hook up the test specimen and do a response sweep with PHSNA.  It finds peak and minimum values.  In this case, I want the peak.  I take a peek at the plot just to make sure it's not a "false peak".  Take the difference of the two dBm readings to get dB attenuation.
So I think my accuracy is probably decent, but could be improved.  That may be my next post.
What about the shunt method?
I decided not to try that with the fixture I have now, because dealing with the stray inductance is not so simple.  It will not be absorbed into the test specimen's inductance but instead will combine with it in a more complex way.  Probably making another fixture with minimal strays is a good solution. Also, based on the graphs, the 50 ohm method might be the best overall for the shunt method.
Notes:
EMRFD is the ARRL book Experimental Methods in RF Design
PHSNA can be found at:
https://groups.yahoo.com/neo/groups/PHSNA/info
... and in an article in the spring 2014 QRP Quarterly magazine.
Nick / WA5BDU
4/1/2015


Saturday, March 23, 2013

Vanguard I transmitter

Some time ago, a group of hams (including me) celebrated an anniversary of Sputnik I by building transmitters of a similar design and power level.  Now it's time to try the first satellite borne transmitter of the USA carried within its Vanguard I spacecraft.  This time we use a single transistor instead of a single tube.  Specifically, a germanium transistor instead of silicon.  The original was on 108 MHz, I think, but that's a bit challenging and that band isn't available to hams. We're using the 20 meter (14 MHz) band.  For authenticity, we try to hold to a single transistor, germanium, with output as a result being in the low milliwatt range.

I came a little late to the party this time because I couldn't find a suitable transistor, but eventually was pointed to some on eBay that were from the old Soviet Union.  Some kind of irony going on here, I guess.  Peaceful co-existence, anyone?

Again, I'm following the trail blazed by Mike, AA1TJ who had the original schematic and made his circuit close to it.  I was able to duplicate it pretty well, but my oscillation was slow to start, so keying would be impossible.  Impatient, I switch to a circuit from Experimental Methods in RF Design.  Here's my circuit:


I measured about 30 mW out from the transmitter, which was pretty much my goal.  I haven't yet made my first QSO, but was pleased and a bit amazed to hear my signal (from Arkansas) reported on the Reverse Beacon Network in WVA, VA, NC, OH, and PA after just a few CQs, typically 8 to 10 dB above the noise.  Update:  Just had a partial QSO with WD4HHN in Florida, so things are moving.  I'm not one of those QRP operators of great faith, so I have to admit I'm amazed.  It's fun writing in the log, Power: 0.03 W.

Not wanting to have to pound on a hand key, I also made an interface circuit to my electronic keyer using a 2N4403 PNP transistor switching the positive lead from the battery.

The transmitter went together in an hour or so on a solderless prototype board, photo below:

The silver cap thing is the transistor.  The crystal is wrapped in red tape to keep its case from shorting out any adjacent wires.

Here's a page on Vanguard activities maintained by Oleg Borodin, RV3GM.

http://www.club72.su/vanguard.html

And here's Michael Rainey's blog entry on his Vanguard project from June, 2012:

http://aa1tj.blogspot.com/2012_06_01_archive.html

72-

Nick, WA5BDU

Thursday, February 28, 2013

Antenna: Mini-loop for 20 meters

I've heard so much about small transmitting loops that I needed to eventually try one even if I have no need for a compact antenna.  I first tried it a number of years back.  I thought I'd try 40 meters since that would be appropriately challenging.  Actually, it was overly challenging and I never got it to work for transmitting, as heating of the capacitors from RF current would change the resonant frequency and drive the SWR through the roof, even at 5W.  My problem was using compromise capacitors instead of butterfly, trombone, or other high quality components.

So I'm trying again, this time on 20 meters.  My loop is made from a 10 foot piece of 1/2 inch soft copper tubing, shaped more or less into a circle of about 38 inches diameter.  It's 0.14 wavelengths long on 20 meters. This kind of antenna is fairly simple technically -- the big loop forms an inductor and a capacitor selected to resonate the inductance at the operating frequency is connected across the open gap where the ends (almost) meet.  A slight bit of technical complexity occurs in coupling the radio to the loop, but it's simple in practice.

There's lots of software to help with the design.  I used RJELOOP1.EXE by Reg Edwards, G4FGQ.  You input the loop dimensions and it gives you the capacitance required, radiation efficiency, loop current and capacitor peak voltage for a given power level, etc.  Mine shows that it will be about 52% efficient, not too bad for a small antenna.  It will have 1800 peak volts across the capacitor while operating at 5 watts(!), And the loop current will be 5.6 A.  Also, I need about 44 pF capacitance for 20 meters.

The Capacitor:

It's a known fact that you don't want your variable capacitor to have contact points such as bushings, bearings or sliding contacts that the large loop current must pass through.  But I read G4ILO's loop antenna page* as part of my research and he claimed that a good quality variable of conventional design and in good condition would work fine.  So I picked a nice one from my junk box and decided to measure the resistance across the bearings before proceeding.  I hooked up a power supply and limiting resistor to shaft and frame and passed 0.5 A DC though it. I used my DMM to measure the voltage drop across the bearings, while rotating the blades to look for bad spots.  Voltage drop was in the low millivolt range and my contact resistance was about 2.5  worst case and 1  typical.  I'm good to go!

The Other Capacitor:

My capacitor is about a 10 pF to 60 pF unit.  I mounted it in the Plexiglass plate that secures the ends of the tubing and stuck on a knob.  In initial testing, I found that I couldn't zero in on a frequency with this system.  Now the resonant frequency is 14.200 MHz, I make a small adjustment and now it's 13.900 MHz.  Playing with my RJELOOP1 program some more I see that 0.165 pF change in capacitance moves the resonant frequency by 25 kHz.  Too sensitive!  One option would be to have a 10:1 reduction drive on the capacitor.  Another would be to have a large fixed capacitor and a small value variable in parallel.  The third is just to parallel a smaller capacitor with the one I have now.  I chose the third method.

I didn't have any suitable capacitors small enough, so I decided to use a 5.5 pF to 23 pF variable and put a 2 pF fixed silver mica in series with it.  A spreadsheet showed me that this arrangement would give me 0.32 pF change for the first 30 degrees of travel from fully un-meshed, but only 0.025 pF change from 150 degrees to 180 degrees (fully meshed).  So an initial setting of 50% or so gives me a good tuning range and slow tuning.


Main & fine tuning capacitors on Plexiglass plate

OK, it's ugly but remember this is an experiment!  The big one with the knob going off to the right is the fine tuning capacitor and the smaller one with its shaft going through the Plexiglass is the main unit.  If you knew where to look, you might see the 2 pF silver mica in series with the fine tuning capacitor.

The Coupling Loop:

Literature on the subject says you can use a wire loop or a ferrite toroid threaded onto the main loop as a one-turn secondary with an appropriate number of turns on the radio side.  I'd already tried the wire loop, so I threaded a FT-114-43 toroid onto the loop. RJELOOP1 says the primary should have an impedance of two to three times that of the feeder (50 ohm coax).  So I used two turns on the radio side.

This didn't work well at all.  My null was over a 2:1 SWR and looked weird -- it came back up slowly when I tuned to the low side, instead of the sharp high-low-high null I expected.

So I went back to the wire loop.  It's just that - a loop of wire connected across the coax feeder. The loop is positioned just inside the main loop, diametrically opposite the capacitor. Nothing connects to the  main loop here, so I used a piece of insulating board secured to the main loop to support the small loop and to mount a coax receptacle on.

The sources I read said a circumference of 1/5th that of the main loop would be good, so I used 24 inches of solid #12 AWG THWN.  I had a hard time getting a good match.  Rotating the loop out of the plane of the main loop is supposed to be one tuning method, but it made things worse.  Not sure what else to try, so I tried squashing the small loop down into a football shape and got some improvement -- down to 1.5:1 at resonance.

My RJELOOP1 program had given a value of 19 inches for the small loop, so I took off five inches.  That made things even worse.  I made a new loop, this time going UP to 26 inches wire length and was able at last to get 1.1:1 at resonance.  I've read that the size of this loop isn't too critical, but I guess I have to say that's not my experience.

Now everything is ready ...


Matching loop, 26 inch circumference


Hanging from a fiberglass pole
off the upper deck of my house

Trying it out:

I made my first declaration of success before I even managed a QSO.  I was able to adjust it to the desired frequency and it stayed there over the course of a couple hours as I transmitted and was idle and it hung out in the sun and cold February wind.  Eventually I did manage a QSO with another QRPer out in New Mexico.  He was weak here and I was weak there but we managed a 20 minute QSO discussing small loops and EFHWs.

I measured my 2:1 SWR bandwidth at 22 kHz, which looks adequate to me.

What next?

If I want to actually use this antenna, I should get back to one capacitor and put on a reduction drive.  Also it would be good if the capacitor were in a weatherproof box.  As-is, I need to keep this antenna out of the weather.  A way to do remote tuning would be good -- maybe use a screwdriver type drive.  Manually adjustable would be OK, but for a big project, servo controlled nulling would be really nice.

I also need to see if it will play on 18 MHz and 21 MHz.  My capacitor should have enough range, but I'm not taking it for granted.

73-

Nick, WA5BDU 
Russellville, AR
February 28, 2013

* http://www.g4ilo.com/wonder-loop.html

Sunday, December 2, 2012

Battery monitor & discharge tester


I like to use little AGM lead-acid batteries on the workbench and in the field with my QRP stuff.  The KX3 transceiver likes to have voltage up over 13V if you want full power (10W), so I bought one of those amazing little DC-DC boosters from eBay.  Using it though, I could be unaware of how low my battery's terminal voltage has gotten.  Taking it lower than about 1.8VPC (volts per cell) could be harmful to the battery, so that led to this project - a battery monitor.

The heart of the thing is an ATmel ATtiny85 8-pin MCU.  I've been moving from PICs to AVRs lately, and from assembler to C.  I originally used the ATtiny13 and did get a functioning version in its 1024 byte program memory space, but I wanted more features so I went to the '85 and the luxury of a full 8K of memory.

To keep things simple, the user interface consists of just a speaker, and LED, and a push button.  the MCU monitors battery voltage via ADC and issues a warning beep at 11.0 volts and a "tripped" beep at 10.8 volts.  A logic line output changes states when tripped and can be used to stop the battery discharge via a relay, MOSFET or transistor.  If you just want an alarm, the output line is not used.

A brief press of the switch in normal operation will cause the speaker to beep out the current voltage.  This can be in Morse, or for those who aren't Morse literate, it will give one beep for "1", two for "2" and so on, with a long beep indicating "0".  The readout is to the hundredth digit.

If you initiate a voltage report and then re-close the switch and hold until it's finished, the MCU will swap between Morse and "count beeps" reporting methods.

Holding the switch closed for > 2s while in normal operation will toggle the sense of the output line between HIGH and LOW for the non-tripped condition.  All configuration changes are saved in EEPROM and  therefore survive a shutdown and re-start.

The MCU is programmed for a trip setpoint of 10.8V and an alarm setpoint 0.2V higher.  This is also programmable:  Connect it to a variable voltage source and hold the switch while powering on the board. The voltage at the time the switch is released becomes the new trip setpoint.

The LED flashes briefly about once per second in the untripped state and changes to two brief flashes every 2.5 seconds after it trips.  I wanted to avoid the dreaded "car alarm syndrome" of continuous audible alarms driving me crazy.  The board gives two 0.5 second raspy sounds at the warning setpoint and one two second raspy sound when it trips.  Then it goes quiet.  Check the LED to see if it's tripped.

Another function is use of the board to supervise a discharge test.  After it reaches the trip setpoint and stops the discharge, it reports the discharge time in minutes. Knowing the discharge current, you can calculate the amp-hour capacity.  Additional presses of the switch while in the tripped state cause it to repeat the discharge time and the battery voltage.

So, a fair amount of function in a tiny chip & board.  One drawback is that after it trips, it's still pulling current from the battery.  But that's only about 2mA.

I tested my trusty 4 A-hr AGM and found that it had less than 1 A-hr capacity, so it went into the recycle bin.










Friday, October 7, 2011

Sputnik transmitter

For my latest, I've put aside my AVR programming and a few other projects to participate in an activity commemorating the October 4, 1957 launch of Sputnik 1, and with it, the space age. It's the brainchild of the master of archaic radio technology Mike Rainey AA1TJ, whose original post is here:

http://aa1tj.blogspot.com/2011/07/sputnik-qso-party-transmitter-prototype.html

Mike supplied me with the the two Russian "pencil tubes", a 21.060MHz crystal and a vague instruction to do something with them. I reproduced the transmitter shown in the schematic of the above link to the best of my ability.  Initially, I had trouble getting oscillation, so I reduced the size of the "lower" feedback capacitor to get it to go.  Eventually I got all the way down to 10pF, tormenting the crystal all the way up to 21.065MHz.  I had problems with low output, sometimes less than 100mW and was suspicious of my plate RF chokes so eventually went to a 1k resistor on the oscillator plate and a resonant tank on the amplifier.  That put me up to 200mW.


Here's a picture of it.  The tubes have wire leads which I soldered to a terminal strip. For keying, I did what Michael suggested and keyed the PA plate while leaving the oscillator running.  The little perf board has a TTL reed relay for keying.  By this time I'd already made my power cable for B+ and filament and realized I needed 5V for the relay.  Rather than rebuild that cable, I added the 9V battery and a 78L05.  A little foresight could have been helpful.

On the air!


I got it ready just in time for the October 4th start date.  I was fairly amazed to make four QSOs on a Tuesday afternoon with just 200mW out. They were:

N2JJ in NY
W1PID in NH
W5RZ 10 miles up the road
WB8YYY in MD

N2JJ recorded my simulated 54 year old transmitter on his Droid phone and sent me the audio file!
W5RZ wished me "DSW".
WB8YYY gave me a 238 RST, one of the strangest I've had.

Inspired by this project, I've done a lot of reading on Sputnik.  I remember very well when it was announced on the news in 1957.  We ran out and looked at the sky.

Some of the ideas I and maybe others had weren't really correct.  First, while the exact time of the launch wasn't known (the Soviets were pretty secretive), it was generally known in the US that the Soviets were very close to making their attempt. The US was also pretty close to launching, with an intended satellite launch planned during the IGY (International Geophysical Year) of 1957. It actually succeeded in January 1958.  But the possibly underestimated reaction of the public and media to Sputnik expedited the program a great deal.

Sputnik transmitted on 20 MHz, allowing ham operators and shortwave listeners worldwide to hear its beeps. Michael chose the 21 MHz ham band for our project to get reasonably close. The American satellites transmitted above 100MHz, which required somewhat more exotic receiving equipment to hear.

Friday, August 19, 2011

Barometer - Altimeter


For some reason I've thought for a while that it would be fun to build (or even buy) a barometer.  A friend had this cool military surplus altimeter he put on the seat of his car while driving through the mountains and you could see the elevation go up and down. That was pretty cool.  Barometric pressure is interesting too.  Can you predict a storm by seeing a rapid drop?

So I finally ran across this chip, or sensor on a chip, by Freescale, the MP3H6115A pressure sensor chip. It cost $9.15 from Mouser. The chip is SMT, 8-pin SSOP package.  It has 0.05” pin spacing, but the row-to-row width is wider than SOIC packages, so a different adapter or board pattern is needed.
It has an output of 15 to 115kPa or 2.2 to 16.7 psia. Supply voltage is 2.7 to 3.3 VDC, typically 3.0 VDC.  Output is in ratio to supply voltage, so it must be known and regulated.

In the photo you see I soldered the chip to a little adapter board and glued that to a scrap piece of PC board on which I also mounted a 3.3V regulator.

Only three pins are used: supply, ground, and output. So you just take that output to the ADC input of your micro and Bob's-your-uncle, instant barometer.  Altimeter.  Whatever.

You'll need some floating point routines in the program.  I used my Arduino board which has an ATmega168 MCU, I think.  You program it in C.  I'm trying to expand beyond assembler so this is a good project for me. The formula for pressure from output voltage is:

P = (Vout/Vs +0.095)/0.009 kPa  (Vs is the supply voltage.)

Since the output is in kPa or kilo-Pascals, I did a further conversion to inches of mercury (inHg), more familiar to me.  BTW, normal barometric pressure is about 101.3kPa.

I first just hooked the thing to my DMM and converted voltage readings to inHg with Excel or my calculator.  I pulled up the local airport's data on the web and compared readings.  They were off a bit more than I expected, but then I learned that airports convert their reading to sea level equivalent, making it somewhat higher.  After compensating for the difference between sea level and our airport's altitude of about 380 ft. I was closer.  In the end I did a small fudge factor to get closer yet. I also have access to atmospheric pressure at my friendly local power plant for calibration purposes.

Converting to altitude is more complicated. Rigorous formulas are complex and factor in a number of things like ambient temperature and relative humidity.  I opted for one that was simpler:

Z = (1-(P/29.9247)^0.19)/22.558E-6

Where Z is altitude in meters and P is pressure in inHg.  I converted the result to feet for display.

My display is a 4-digit 7-segment display surplus from a set-top box, BTW.  I like the look of red 7-segment numerals, but it has a few issues:  No decimal point, hard to see in outdoor light, can't make many alphabetic characters.

I make my program display pressure in inHg for about 1.5s, then text approximating "inHg" for 0.75s, then feet altitude for 1.5s and text "ASL" for 0.75s and repeat.

One curious result I noticed initially was that the display would show 380 feet for a while, then jump to 350 feet, alternating between the two.  I finally realized that this is the limitation of my 10 bit ADC.  One part in 1024 is equivalent to about 30 feet of elevation here in Russellville.  I tricked my way out of that a bit by putting in an averaging routine that uses the average of 5 readings taken over a 5 second period.  Now I get smoother changes and some artificial interpolation but my response time to a step change has been reduced to the 5 seconds it takes to fill the buffer.

Today I gave it a trial run in the Miata to the top of Mt. Nebo.  My local elevation is 380 feet.  When I got to the sign at the top of the mountain saying "Elevation 1800 feet", my altimeter was displaying 1749 ASL.  So, pretty good.  

BTW, I looked at a week's worth of barometic data and saw that the equivalent elevation change between the high and low readings was 186 feet.  So I might reasonably expect extremes to make my altitude reading off by up to +/- 93 feet if I haven't entered any compensation.




Will this project ever make it into a box with a dedicated MCU chip?  Maybe.