Latest UHF DATA RADIO SCHEMATIC - RF Cafe Forums
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Post subject: Latest UHF DATA RADIO SCHEMATIC Posted: Sat Jan 31, 2009 10:20 pm
Joined: Tue Feb 27, 2007 6:16 am
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
1. I have confirmed with a logic analyzer that the PLL is being setup with the correct data.
modified the board, removing the 20MHz Fox oscillator and replaced with a 10MHz oscillator from CMAC -> no change.
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.
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
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
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:
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.
Regards, Darcy Randall, Perth,
Post subject: Re: Latest UHF DATA RADIO
SCHEMATICPosted: Mon Feb 02, 2009 2:06 am
Joined: Tue Feb 27, 2007 6:16 am
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.
Regards, Darcy Randall, Perth, Western Australia
Re: Latest UHF DATA RADIO SCHEMATICPosted: Mon Feb 02, 2009 2:10 pm
Joined: Mon Jun
27, 2005 2:02 pm
Was my pleasure.
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!!