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From: "Michael A. Terrell"
Subject: Re: Help needed with my A-level project - RF link
Date: Fri, 01 Nov 2002 20:18:17 -0500
Organization: Have you seen my bench? No, really! Where is it?
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References: <firstname.lastname@example.org> <4AFw9.email@example.com>
Steve Andrew wrote:
> 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
> Steve Andrew
> > Good luck.
> > Steve
Just DON'T listen to the cat!
Michael A. Terrell
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