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PostPosted: Sun Feb 21, 2016 11:47 am 
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Please...show me a VOR ground station on 108.00 Mhz!...don't just say it.

Every VOR test set I've ever seen has it as it's test frequency....RADIATED!...in free air...



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PostPosted: Sun Feb 21, 2016 1:19 pm 
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....and somebody please explain these "square waves"..... :lol:


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PostPosted: Sun Feb 21, 2016 1:40 pm 
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CID wrote:
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Go ahead, argue this is the worst way to do it. It's about one thousand dollars cheaper than any other wheel I've seen, and I get a kick out of wheels I've designed and built myself vs. ones I bought from someone else. I'll fight for your continued right to feel "anything I bought is better than anything I made myself", though.


So....you can't build a simple oscillator and AM modulate it for under a thousand? Maybe I gave you too much credit.


When did I say that? But this uses a single $5 off the shelf part, a length of wire, and a phone. It requires no test gear to tune an oscillator or adjust the depth of modulation.

Here's your challenge then: You find another way to do it that other people who are interested can try for themselves without lifting a soldering iron. don't forget you need to be able instantly to change the frequency to 329.15MHz to test your glideslope receiver, too.

Come back when you finally have something intelligent to say; I can wait as long as it takes.


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PostPosted: Sun Feb 21, 2016 4:08 pm 
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CID wrote:
Ingenuity? Any electronics tech can figure out a multitude of ways to broadcast a VOR test signal but why reinvent the wheel or choose what is arguably the worst way to do it and with risk to damaging your computer?

Square waves are composed of a fundamental frequency with an infinite number of odd harmonics in decreasing amplitudes. Operating this gizmo is wreckless at best. Thinking it's a valid test is inadvisable.


It's not hard to solder up a simple bandpass filter if one was motivated enough.

Two caps and a resistor... that's what? $5 and 15 mins of effort? The filter would also take a little off the radiated power even if slightly.



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PostPosted: Sun Feb 21, 2016 9:01 pm 
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...it's not reckless

http://www.ic.gc.ca/eic/site/smt-gst.ns ... 10208.html



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PostPosted: Mon Feb 22, 2016 8:14 am 
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But this uses a single $5 off the shelf part, a length of wire, and a phone.


So...your phone is free? Please let me know where I can get a free phone. Also let me know where to get a free replacement when it breaks from using the way you suggest here.

As for 108.00, it looks like my information is outdated. There weren't many but the last ones (I think the last one was in Germany) changed when Europe adopted VHF FM Immunity. 108.00 was just too close to some commercial FM radio stations. There are still however some airports that use 108.10 for LOC/ILS. You can check them out here:

http://www.icao.int/apac/documents/edocs/cnsdocs/03freqlist02.pdf

photofly, airplanes are not highschool science fair experiments. I'm not going to invent or suggest a shitty way to do anything to an airplane because it's cheap. That's not to say that I think the most expensive way is always the best way but this idea is half baked. That's my opinion. Go ahead and use this to your heart's content but at least people won't just blindly do this. Maybe my opinion will have them think twice.

Now let's go on to how to test a DME with paper clip and free DME test set set.....



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PostPosted: Mon Feb 22, 2016 8:16 am 
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When did I say that?


Quote:
Go ahead, argue this is the worst way to do it. It's about one thousand dollars cheaper than any other wheel I've seen



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PostPosted: Mon Feb 22, 2016 9:22 am 
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Photofly - I, for one, am interested in a little more detail about how you synthesize the wav files. Might try this with my Pi, though with a couple modifications. Btw, do you have any way to measure the transmitted power?

SDR sure has made it easier to play around with RF these days. Bitch as some people might, there are many great things about living in our times. :)


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PostPosted: Mon Feb 22, 2016 11:46 am 
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CID wrote:
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When did I say that?


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Go ahead, argue this is the worst way to do it. It's about one thousand dollars cheaper than any other wheel I've seen

Your English comprehension sucks. That doesn't mean something can't be built for less than $1000; it means that the off the shelf ready built options are limited. Regardless of your idiocy, using a $5 single board computer as a crystal controlled synthesized RF source is a cool hack.

I'm still waiting for an intelligent contribution on this topic from you.


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PostPosted: Mon Feb 22, 2016 12:05 pm 
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Posthumane wrote:
Photofly - I, for one, am interested in a little more detail about how you synthesize the wav files. Might try this with my Pi, though with a couple modifications. Btw, do you have any way to measure the transmitted power?

SDR sure has made it easier to play around with RF these days. Bitch as some people might, there are many great things about living in our times. :)

I'll post the code tonight. I don't have any way to measure the ERP unfortunately.


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PostPosted: Mon Feb 22, 2016 12:45 pm 
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I'm still waiting for an intelligent contribution on this topic from you.


photofly, that's exactly what I was thinking about you and your posts. I'll leave you to discuss your "hack". Arguably one of the most awkward uses of a Raspberry Pi I've seen so far. But...whatever.



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PostPosted: Mon Feb 22, 2016 3:15 pm 
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For Posthumane, here's the code that churns out the baseband signal.

It's based on the code from this webpage: http://soledadpenades.com/2009/10/29/fa ... ve-module/

The wikipedia page on VOR was mostly unhelpful, because it makes something very simple sound very complicated. This reference was much more use: http://www.nist.gov/calibrations/upload/tn1069.pdf

Let me know what you try and how it goes.
Attachment:
produce_VOTwav.py 2.zip [848 Bytes]
Downloaded 11 times


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PostPosted: Mon Feb 22, 2016 4:12 pm 
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Thanks for the info, photofly. Don't let the pedants discourage you. :smt023

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PostPosted: Mon Feb 22, 2016 4:29 pm 
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There are about a million and one ways to improve this; I'm as usual vastly amused by the particular nits people choose to pick. Hoping Posthumane finds something interesting to do.

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PostPosted: Mon Feb 22, 2016 5:35 pm 
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If it's designed to give a 0 degree radial From indication...or whatever...and that's what it does, then why isn't it a valid test?

What's the difference from using 114.80 Mhz VOT? ...you're not calibrating anything. It's just a check.



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PostPosted: Mon Feb 22, 2016 5:47 pm 
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Because we value something only in proportion to the cost and difficulty of obtaining it. If a test is done with a $5 part and something you already own and carry in your pocket every day, it's impossible for it to give results as valuable and good as a device that costs $1000 from Aircraft Spruce.

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PostPosted: Mon Feb 22, 2016 5:49 pm 
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Quote:
makes something very simple sound very complicated


Actually a VOR signal is kind of complex...the most of any aircraft navigation signal that I've seen.



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PostPosted: Mon Feb 22, 2016 6:04 pm 
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You know, speaking as one who is horrible incompetent with electronics -- must have failed shop class -- I'm impressed with PF's initiatives, even though I doubt I could replicate them. I'm a bit scary with tools......

Two thoughts that came to me: First, we can all choose to live our lives as creators or critics,

And -- if one chooses to work and create (in aviation or elsewhere) solely for money as a motivation -- that will be the only reward given.

Follow your passion, and money will follow as a byproduct, along with many more rewards.

Kudos to those who create and innovate!



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PostPosted: Mon Feb 22, 2016 6:31 pm 
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According to the webpage, the range is 100m with a simple wire antenna. It's also not a perfect square wave:

http://raspi.tv/2013/rpi-gpio-0-5-2a-no ... -to-use-it

so it won't just generate nice harmonics at multiples of 108Mhz. The only way to know for sure is to check with an RF spectrum analyzer.



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PostPosted: Mon Feb 22, 2016 6:37 pm 
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NeverBlue wrote:
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makes something very simple sound very complicated


Actually a VOR signal is kind of complex...the most of any aircraft navigation signal that I've seen.

Respectfully, I disagree. The engineering required to broadcast the signal is complex, but the signal itself is very straightforward: sufficiently simple to be decoded in a 1950s era radio set without any digital logic.

CpnCrunch wrote:
According to the webpage, the range is 100m with a simple wire antenna. It's also not a perfect square wave:

http://raspi.tv/2013/rpi-gpio-0-5-2a-no ... -to-use-it

so it won't just generate nice harmonics at multiples of 108Mhz. The only way to know for sure is to check with an RF spectrum analyzer.


It will just generate harmonics at multiples of 108MHz. It may not be a square wave exactly, but it's still periodic with a very stable frequency, so it can only contain frequencies that are integer multiples of the fundamental. If it's a symmetric waveform then it can only contain odd harmonics: 1st (fundamental), 3rd, 5th etc.

As far as energy content in the unwanted harmonics goes, square waves are pretty bad. Something trapezoidal will be better.


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PostPosted: Mon Feb 22, 2016 10:04 pm 
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photofly wrote:
For Posthumane, here's the code that churns out the baseband signal.

It's based on the code from this webpage: http://soledadpenades.com/2009/10/29/fa ... ve-module/

The wikipedia page on VOR was mostly unhelpful, because it makes something very simple sound very complicated. This reference was much more use: http://www.nist.gov/calibrations/upload/tn1069.pdf

Let me know what you try and how it goes.
Attachment:
produce_VOTwav.py 2.zip


Thanks for the links. I probably won't be able to get to it for a while, but I'll give it a try sometime. SDR is a fun thing to play around with (at least if you're a nerd like me). I've been thinking recently of trying a few cheap designs for an automatic direction finder in the VHF bands. I agree with you about the wikipedia page on VOR not being very useful.

photofly wrote:
It will just generate harmonics at multiples of 108MHz. It may not be a square wave exactly, but it's still periodic with a very stable frequency, so it can only contain frequencies that are integer multiples of the fundamental. If it's a symmetric waveform then it can only contain odd harmonics: 1st (fundamental), 3rd, 5th etc.

As far as energy content in the unwanted harmonics goes, square waves are pretty bad. Something trapezoidal will be better.

CpnCrunch is not wrong here, you may in fact get some small harmonics in other frequencies, due to various non-linearities in the components that make up the Pi. A non-linear junction will often radiate a 2nd harmonic stronger than 3rd, and you can have various intermodulation products, especially when you have various active components with their own clocks on the same line as your antenna. That being said, these various harmonics will generally be orders of magnitude smaller than the base signal, and not something you have to worry much about unless you have a very sensitive receiver.

A little tidbit you might find interesting - I've worked a bit with non-linear junction detection and when you illuminate a complex electronics board with two frequencies, f1 and f2, you can get back some funny intermod products such as 2f1 +/- f2. 2f1 +/- 3f2, 3f1 +/- 2f2... The interesting bit is that different electronics will emit different intermod products which you can sometimes use to identify a particular item based on its harmonic signature.


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PostPosted: Tue Feb 23, 2016 5:02 am 
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Posthumane wrote:
A little tidbit you might find interesting - I've worked a bit with non-linear junction detection and when you illuminate a complex electronics board with two frequencies, f1 and f2, you can get back some funny intermod products such as 2f1 +/- f2. 2f1 +/- 3f2, 3f1 +/- 2f2... The interesting bit is that different electronics will emit different intermod products which you can sometimes use to identify a particular item based on its harmonic signature.


CpnCrunch's assertion - that because the output isn't a square wave, it must contain frequencies other than the integer harmonics - is plainly wrong on the mathematics, and we don't need to address it further.

Your assertion - that intermodulation distortion generates new sum-and-difference frequencies is trivially obvious: that's how we're achieving the AM modulation of the audio signal. To use your nomenclature, f1 is 108MHz, and f2 is a 10kHz bandwidth audio signal.

However if you wish to assert that IMD is throwing energy into VHF frequencies other than the integer harmonics of 108MHz, you're going to have to show there's another f2 (in the VHF band) present - not already a harmonic of 108MHz - to mix with. You can take as many sum and differences of the harmonics already present in the 108MHz "square" wave, and and they can only generate other already-present frequencies. Nothing new.

In fact there is no evidence of any other f2 present on the output. If there were it would show up as ripple or instability in the PWM output. It would actually cripple the ability of the PWM output to do the job it was designed for.


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PostPosted: Tue Feb 23, 2016 8:37 am 
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One of the issues with signals like this is that the "noise" is not limited to multiples of the fundamental frequency. As mentioned, the "square wave" won't be perfect and the modulation method is crude. The truth of the matter is there is no way to be sure what this gadget is spewing until you subject it to a test with a spectrum analyzer. There are fairly inexpensive alternatives to this that would involve a little solder and a simple oscillator so I don't know why so much effort is being spent on making this square peg fit in the round hole.....


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PostPosted: Tue Feb 23, 2016 9:15 am 
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photofly wrote:
.


However if you wish to assert that IMD is throwing energy into VHF frequencies other than the integer harmonics of 108MHz, you're going to have to show there's another f2 (in the VHF band) present - not already a harmonic of 108MHz - to mix with. You can take as many sum and differences of the harmonics already present in the 108MHz "square" wave, and and they can only generate other already-present frequencies. Nothing new.

In fact there is no evidence of any other f2 present on the output. If there were it would show up as ripple or instability in the PWM output. It would actually cripple the ability of the PWM output to do the job it was designed for.

No, I wasn't saying that there would be intermod interference in the VHF band, just that almost every active board such as a Pi would have some additional oscillations present, and those do give rise to other intermod products. There's a 1 GHz clock somewhere on the Pi zero (I haven't looked if it's internal to the processor or if it has an external xtal) and with a sensitive receiver you'd be able to see that, plus sums and differences of the harmonics of your transmission. That is all pretty academic, of course, since those would all be so small as to be trivial and they wouldn't be in a band you care about. So not at all a criticism of the design, just an interesting factoid, and only relevant to those working in fields that make use of those spurious emissions.

When I get around to trying this out, I'll see if I can throw a wideband spec-A next to it and either feed it directly or with a wideband antenna to see what's coming out of it. I suspect that things outside of the intended band will be hardly even visible above the noise floor of the DC-8GHz spec-a.


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PostPosted: Tue Feb 23, 2016 10:18 am 
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CID wrote:
There are fairly inexpensive alternatives to this that would involve a little solder and a simple oscillator so I don't know why so much effort is being spent on making this square peg fit in the round hole.....


Design and build one, then. Share your expertise. If you can do better, then... do better. (Actually I believe you'll find building a stable 108.00MHz oscillator with just a few components isn't straightforward. But you're the analogue RF design engineering expert in this thread, apparently.)

Meanwhile I'm still waiting.


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