Post subject: Equipment and research advise Posted: Thu
Sep 22, 2005 11:55 pm
Joined: Thu Sep
22, 2005 11:53 pm
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?
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
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
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
Thanks again for your help.