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From: "Steve Andrew"
Subject: Re: Help needed with my A-level project - RF link
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
Date: Sat, 02 Nov 2002 01:08:16 GMT
NNTP-Posting-Date: Sat, 02 Nov 2002 12:08:16 EST
Organization: BigPond Internet Services (http://www.bigpond.net.au)
Steve Taylor wrote in message
> Sanjeevan wrote:
> > I'm an A-Level student who is currently contructing my end of year
> > project.
> > I intend to
> > build a simple RF transmitter, and reciever to send digital data. The
> > digital data is going
> > to have a maxmium frequeny of about 500KHz, and the carrier is going
> > to have
> > a frequency
> > around the 50MHz mark (Is this an approiate frequency? Tell me). The
> > data is
> > going to be
> > amplitude modulated onto the carrier wave,as this is within my ability.
> Don't bugger about - buy the RF modules from
> http://www.rfsolutions.co.uk - or direct from Maplin. You won't get a
> 500Khz bandwidth though.
And what exactly is he going to learn by doing that ? - Sure, it's a quick
and easy solution, and probably the correct one from an engineering point of
view, if the object of the excercise is to get data from point A to point B
via an RF link.
> > The reciever is going to consist of a tuned circuit, to only accept
> > the frequency my data is on, then it is going into a mixer. The mixer
> > will be a dual gate FET, and will have an input from a LO and the RF
> > signal, so that the output after it has been filtered will have a
> > lower intermediate freqency. This intermediate frequency will then be
> > fed into a diode to demodulate the AM signal into a digtal one. Then
> > te siganl will be fed into a schmitt trigger to clean up the signal.
> You can have a lot of fun receiving the signal, and you don't have to
> worry about emitting illegal signal levels into the spectrum
> > I want to know if the approach I going to use is approiate for this
> > project,
> > and if not i
> > want to know where I am going wrong.
> > The data I'm going to be sending is going to be in the form of 8 bits at
> > time. So for
> > example if I send BYTE A:"1 1 1 1 1 1 1 1", then send BYTE B:"0 0 0 0
> > 0 0 0
> > 0", how can I
> > prevent the reciever from picking up a mixture of BYTE A and BYTE B
> > (e.g. "1
> > 1 1 1 0 0 0
> What reference books have you read ? What do you know about receiving
> Serial data from an FSK transmitter. Hint: The usual method is not the
> bald switching on and off of the transmitter.
> > 0")? Will i need to synchronise the digital clocks on the transmitter
> > reciever.
> See above.
In addition to the above helpfull comment - You might want to investigate
FSK (frequency shift keying) as an alternative to AM. You could use a simple
1 transistor ocsillator running at around 50mHz, part of the tuned circuit
would be a varicap diode, with the logic-level data being connected to the
diode via suitable scaling resisitors to set the amount of deviation
The reciever would be a little more complicated, but there are FSK
discriminator ICs available from Motorola, assuming you are allowed to use
As far as synchronising the data is concerned - If you have a suitable gap
or time delay between each byte sent then it's quiet simple to use a couple
of resistors and capacitors to detect the gap and establish synchronisation.
Hint - See how simple radio control decoders do the job.
> > As an A-Level project, it does not have to work properly, or work at
> !!!!!!! And people wonder whether A levels have been devalued. For your
> own sense of self worth MAKING IT WORK IS THE WHOLE BLOODY POINT. No
> amount of theory or elegant maths makes up for the final product being
> crap ! If you really think it doesn't matter, you should be doing A
> level ART.
Give the guy a break ! - As he has already stated, an understanding of the
theory, and a viable design would seem to be the object of this excercise.
He may have limited test euipment - imagine trying to get his proposed setup
running with no more than a multimeter ? - I know of several 2nd and 3rd
year EE uni students who would spend most of a semester playing around with
a project like this, and still not get it to work.
> > What I want is to get the theory behind the project correct, and have
> > a basic schematic of all sub-systems which make up this project, e.g.
> > mixer, filter, amplifier.
> Read some books. One of the classic texts is by Terman.
Any good radio amateur handbook will give you most (if not all) of what you
need. One of the fun things about engineering is that there are usually at
least 10 ways to skin a cat, the art is picking the way best suited to cat
you have - and then arguing the toss with the other 9 engineers who have
each picked their own prefered skinning method. :)
Good luck to you
> Good luck.
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