Latest UHF DATA RADIO SCHEMATIC - RF Cafe Forums
Post subject: Latest UHF DATA RADIO SCHEMATIC
Posted: Sat Jan 31, 2009 10:20 pm
Tue Feb 27, 2007 6:16 am
good news is, with my latest design I have achieved lock.
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.
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.
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.
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
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
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:
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
Regards, Darcy Randall, Perth, Western Australia
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.
to everyone particularly IR for their help.
Regards, Darcy Randall, Perth, Western
Post subject: Re: Latest
UHF DATA RADIO SCHEMATICPosted: Mon Feb 02, 2009 2:10 pm
Joined: Mon Jun 27, 2005 2:02 pm
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!!