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Latest UHF DATA RADIO SCHEMATIC - RF Cafe Forums
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darcyrandall2004
Post subject: Latest UHF DATA RADIO SCHEMATIC Posted:
Sat Jan 31, 2009 10:20 pm
Colonel
Joined: Tue Feb
27, 2007 6:16 am
Posts: 46
Hello All,
The good news is,
with my latest design I have achieved lock.
The bad news is that
although I may have achieved lock, the output frequency is slighlty
off the desired frequency and I am unable to view the demodulated intelligence
signal on my Marconi test set.
To get to the stage of acheiving
lock:
1. I first had to alter the components surrounding the resonator.
The VCO would not oscillate at the frequencies I had simulated. I am
guessing parasitic capacitances/inductances were responsible for this.
2. I had to remove the RF choke feeding the varactor and replace with
a 10kohm resistor.
To help diagnose why the output frequency
is slightly offset from the desired:
1. I have confirmed with a logic
analyzer that the PLL is being setup with the correct data.
2. I
modified the board, removing the 20MHz Fox oscillator and replaced with
a 10MHz oscillator from CMAC -> no change.
I witnessed
1. if I program the PLL for an output of 440MHz with a 10MHz ref, ie
R=1, FRAC=0, INT=44, MOD=400 then with my Marconi test I have been able
to tune the reference osc so that I can achieve roughly 440MHz with
an error of 100kHz(my channel spacings are supposed to be 25kHz).
2.If I program the PLL or an output of 450.325MHz with a 10MHz ref,
ie R=1, FRAC =13, INT=45, MOD=400 I end up with a signal at roughly
448MHz.
I also think it is worth pointing out that the digital
lock detect output is typically a value of 2.5V that occasionaly dips
to 0.
Does anyone have any ideas as to why I am not achieving
the expected output frequency?
Does anyone have any ideas as
to different things a can try to diagnose the problem of both the wrong
output frequency and the lack of the moduating intelligence signal?
I am thinking that the reference oscillator is not oscillating at
the correct frequency, yet I cant understand why as it was purchased
off the shelf.
There is no shielding on my board. Typically, should
I be able to tune a UHF transmitter regardless of wether shielding is
in place or not?
I would like to say cheers to everyone on this
forum for helping me get this far.
My schematics are here:
http://members.iinet.net.au/~darcy/working/paging_tx_13.pdf
I
will post a photo of the board I am working with in time. Perhaps the
state of it will provide a hint as to why it is not working properly.
Thanks again
_________________
Regards, Darcy Randall,
Perth, Western Australia
Top
darcyrandall2004
Post subject: Re: Latest UHF DATA RADIO SCHEMATICPosted: Mon Feb 02,
2009 2:06 am
Colonel
Joined: Tue Feb 27, 2007 6:16
am
Posts: 46
IT WORKS!!!!
Yeehaw. After changing the loop
filter components and the amount of attenuation prior to the PLL, I
have been able to successfully view a demodulated square wave on my
Marconi test set at the desired 450.325MHz frequency. Granted there
are other oscillations present on my waveform and the design wouldnt
be emc compliant but regardless the result is still satisfying.
This being my first RF design has taken me about two years on a
part time basis.
Thankyou to everyone particularly IR for their
help.
cheers
_________________
Regards, Darcy Randall,
Perth, Western Australia
Top
IR
Post subject:
Re: Latest UHF DATA RADIO SCHEMATICPosted: Mon Feb 02, 2009 2:10 pm
Site Admin
Joined: Mon Jun 27, 2005 2:02 pm
Posts:
373
Location: Germany
Congratulations Darcy!
Was my pleasure.
My first RF design was about 8 years ago and I also had to learn
through ''the legs''. Back then there was no one to help me... I am
keen now to help others.
All the best, and always consult us
here!!
_________________
Best regards,
- IR
Posted 11/12/2012
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