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Post subject: Equipment and research advise Posted: Thu Sep 22, 2005 11:55 pm


Joined: Thu Sep 22, 2005 11:53 pm

Posts: 1

I've come up with an RF project that I want to build but my background is in digital electronics so I need some advice.

I want to convert a 418Mhz signal to 433Mhz and possibly change the data in the process. I've looked at several packaged receiver/transmitters but I have a lot of questions about testing and RF in general.

What are some good books or sites that will give me a better background on RF?

How do I tune a receiver/transmitter?

I assume on the receiver site I can measure the output voltage but on the transmitter side?

Do I need a oscilloscope with a bandwidth > 433Mhz or are there some other tricks/tools that are commonly used?




Post subject: World of pain!Posted: Fri Sep 23, 2005 4:39 am

You don't seem to have many replies, this seems to be happening more often on this site each time I visit! Anyway, your probably not getting replies because your question is not easy to answer.

From a pure system point of view (ignoring all the nasty RF things that always go wrong when you mess around with circuits at RF frequencies) you can achieve the signal processing you ask for as follows:-

1. Conversion from one frequency to another is just a MIXING or HETRODYNE operation. In simple terms you multiply the signal you want to change with another frequency to get the one you want. The formula is simple.............its just the difference. So if you have 400 MHz and you want 50 MHz you mix (or multiply) the 400 you have by 450 or 350. In either case the DIFFERENCE of the two frequencies is 50 MHz and this frequency will come out of what we call the I-F port of your mixer. This all sounds very easy BUT:-

(a) You need to generate 350 or 450 MHz. Easy if you have a signal generator and can allow for this in your final system. Otherwise you need to build an oscillator and maybe even a PLL (Its starting to snowball isn't it!)

(b) You need to get the levels of the signals going into the mixer correct or it wont behave.

(c) You need to impedance match everything

(d) Theres lots more ........ RF is easy isn't it!

2. "GET AT THE DATA". Well...........good luck! you need to demodulate the data which if you know how its modulated then you can probably do it. There are plenty of bits of test kit out there with demodulators on for simple AM and FM and for more funky modulation but you will need to mix the UHF signal (400 MHz ish frequency is UHF) down to a much lower frequency in order to recover the data (demodulate). Assuming you don't really need to build a receiver then and that you can get away with using a bit of lab test kit that will demodulate the signal for you, you are then faced with trying to decode the data. You can almost guarantee that the data will be coded in some way or another. For example a lot of radio data is manchester coded before it hits a modulator. Assuming you can deal with this you then need to get the bit and byte sync.

I hope that helps in some way but its probably put you off RF for life.

With regard to RF books.....your best bet is probably the ARRL handbook and the new wes haywayd book.




Post subject: Posted: Sat Sep 24, 2005 2:33 pm

Steve, thanks you've been very helpful. Pain can sometimes be fun

I was wondering what modulation was being used but I'd hoped that would be apparent when I started looking at the signal. The FCCID only revealed 417.9-418.1MHz 36KHz carrier + some information on the 32bit data stream that's being sent.

The part that was puzzling me was manipulating/viewing the signal. I knew if I was ever to troubleshoot my circuit I had to be able to see what it was doing. I'd priced used o-scopes and anything in that range was pricey plus I remembered that we only had a handful of GHz scopes in school. As soon was you said "mixing" I had a eureka moment and flashbacks of linear systems. What a nightmare class that was. I may not remember the math but the theory is still clear. If I mix another signal in I should get a sum, a difference, some harmonics, and the original signals. I should be able to pass the whole mess through a bandpass and just view the difference on a scope.

Now I need to figure out how much I want to spend on a single generator and a scope and go digging through the attic for my old school books. Sounds like I've got a lot of reading and fighting with signals a head of me

I'm sure I'll have more questions in the future but I want to know a little more of what I'm talking about first.

Thanks again for your help.


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