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Latest UHF DATA RADIO SCHEMATIC - RF Cafe Forums

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Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


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


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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


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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








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