"A radio transmitter shall be added to transmit the wind speed by changing its frequency. The frequency change shall be 1 Hz per 0.1 meter/sec. The transmitter must be custom designed and built. It cannot be a commercial product. The frequency must be in the 433 MHz ISM band or if a team member has an amateur radio license, an amateur radio free can be used. The transmitter must be able to be set to 8 different frequencies in the 433 MHz ISM band with 25 KHz separation. The team can use a commercial receiver." I cannot make heads or tails of this. Any help will be appreciated! Thanks.
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1\$\begingroup\$ I cannot make heads or tails of this Hmm, perhaps you ventured into the wrong class then ? If you do not understand any of it then I truly wonder what you're doing in that class. You must know what 433 MHz means. If you don't then you lack so much knowledge that it cannot be answered here either. \$\endgroup\$– BimpelrekkieCommented Sep 5, 2017 at 9:50
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1\$\begingroup\$ @bim or anyone who can address my curiosity - what I understand from this question is that his frequency should shift from 433000000 Hz at zero wind speed to 433000010 at a wind speed of 1 m/s. Will a receiver be able to differentiate between such close frequencies? \$\endgroup\$– WhiskeyjackCommented Sep 5, 2017 at 9:54
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\$\begingroup\$ @Whiskeyjack Maybe it can modulate 433MHz with 100 Hz for example and change that 100 Hz with 1 Hz / 0.1 m/s. It does not say that the carrier frequency must be changed, it just says "its frequency". I would consider a 100 Hz modulation also part of the transmitter frequency. Changing the carrier can be done as well but the absolute error will be an issue because it requires a very high frequency stability. Transmitter and receiver rely on their clocks to be very well synchronized and tracking. That is not a practical idea. A typical crystal oscillator is 10 ppm so 4.3 kHz at 433 MHz. \$\endgroup\$– BimpelrekkieCommented Sep 5, 2017 at 10:05
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\$\begingroup\$ "The team can use a commercial receiver" - that's a clue. If you use a commercial receiver then your transmitter must be compatible with it. So if for example your receiver expects an OOK (On Off Keyed) signal, the transmitter must produce an OOK signal. Then think about how you can send a varying frequency over this rf link. \$\endgroup\$– Bruce AbbottCommented Sep 5, 2017 at 11:36
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1\$\begingroup\$ While the specs are ridiculous (1 kHz/0.1 m/sec would be much more reasonable), they are very clear. That you cannot understand them suggests that either the course has horribly presented, or you have not been paying attention. Or the course requires prerequisites which you do not have. \$\endgroup\$– WhatRoughBeastCommented Sep 5, 2017 at 13:25
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1 Answer
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Academics.... sigh...
A radio transmitter shall be added to transmit the wind speed by changing its frequency. The frequency change shall be 1 Hz per 0.1 meter/sec. The transmitter must be custom designed and built. It cannot be a commercial product. The frequency must be in the 433 MHz ISM band
OK, so:
- WS is the wind speed in m/s
- B is the band setting (0 to 7) as they want 8 settings spaced by 25 kHz
- C is the carrier (let's say 433MHz)
You need a transmitter which outputs a continuous wave at a frequency of :
C + 25kHz*B + 10*WS
Now, the first and obvious issue here is that, if we want an accuracy of 1m/s on the wind speed, then our 433MHz local oscillator will have to have an accuracy of 10Hz. Same for the receiver. So 5Hz error for both.
That's 1.15e-8 or 0.01ppm accuracy... It can be done, think about custom made crystals lapped under a full moon with some fairy dust on top (very low yield, you can always ask Laptech for a quote), temperature-stabilized ovenized enclosure... ultra low noise circuits... like those. When the applications mention "Radar/Military" and it comes in a thick heavy metal brick with 50 screws on each side, you know it's more expensive than your car.
So your job as a budding engineer is to make a realistic cost estimate for the idiotic requirements you are provided. And then come up with a solution that will work for a budget under $10.
This happened at one of my jobs: every time the boss came back from a ski weekend and suggested a "stupendously innovative" idea, the database administrator would pop up from their screen and quip "no problem boss, that's gonna need about twelve extra servers, plus oracle licenses, sign here".
I mean, if you ask Wenzel for a quote explaining your purpose is to have the teacher learn about some hard facts of life, they'll probably have a good laugh and give you the quote.
Options include:
- Use a less dumb modulation scheme, for example encode the wind speed information into pulses or binary and transmit that by using FM or AM on your 433MHz carrier.
- It can be as simple as "every time the anemometer makes a turn, transmit a short pulse of RF."
- As said in the comments, modulate the 433MHz with a frequency like "1kHz+10*windspeed".
- Put a ESP8266 on the focal point of a parabolic dish.
- Etc.
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1\$\begingroup\$ Gotta love the people who are tasked with training the next generation of engineers but don't understand realistic design specifications. +1 for your excellent answer! \$\endgroup\$ Commented Sep 5, 2017 at 13:18
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\$\begingroup\$ "The transmitter must be able to be set to 8 different frequencies in the 433 MHz ISM band with 25 KHz separation" Other than that, accuracy is not specified. The question has plenty of stand alone educational value. \$\endgroup\$ Commented Sep 5, 2017 at 14:12
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\$\begingroup\$ @ScottSeidman with just a little adjustment it would have had so much more value and have actual applicability outside of academia. Given that the instructor is asking them to design the system from scratch this is not a 1st year course. I'd call this a very poor assignment in that context. \$\endgroup\$ Commented Sep 5, 2017 at 18:25
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\$\begingroup\$ I think its main educational value is to teach the students how to deal with unclear, badly worded specs... and help the client come up with better specs! \$\endgroup\$– bobfluxCommented Sep 5, 2017 at 18:31