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Below are all of the forum threads, including all
the responses to the original posts.
Post subject: Equipment and research advise Posted: Thu Sep
22, 2005 11:55 pm
Joined: Thu Sep 22, 2005
I've come up with an RF project that I want
to build but my background is in digital electronics so I need some
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
How do I tune a receiver/transmitter?
I assume on the
receiver site I can measure the output voltage but on the transmitter
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!)
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!
"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