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UHF DATA RADIO SCHEMATIC - RF Cafe Forums

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darcyrandall2004
Post subject: UHF DATA RADIO SCHEMATIC Posted: Mon Apr 02, 2007 9:41 am

Colonel


Joined: Tue Feb 27, 2007 6:16 am
Posts: 46
Hello. For the sake of my own interest and learning I am attempting to create a UHF FM data radio that will operate at 450.325MHz.

To test the unit, I have a processor that I attach, that outputs a TTL POCSAG signal.



Of the prototypes I have created, I have had some success with the VCO, generating a signal around 450.325MHz. (For some reason this prototype also includes a signal at 122MHz).

Could I please get some feedback as to whether it looks like I am headed in the correct direction.

My biggest problems have been associated with creating the modulated signal. Either I cant create the modulated signal or I cant adjust the level.

Is this means of generating FM correct?

The FOX914E TCXO operaties at 20MHz. I have chosen R= 800, B=562 and A=29.

Your help is very much appreciated.

Regards Darcy Randall from Perth, Western Australia.


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nubbage
Post subject: Posted: Tue Apr 03, 2007 3:51 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi
Whilst I am not a PLL guru, it seems to me there is much going wrong around the VCO circuitry of Q1.
For one thing the values of C35 and C27 seem way too high.
Then the way the varactor D1 is connected is far too brutal, that is to say it has far too high an influence over the 450MHz center frequency. It should be isolated somewhat from the (Colpits??) VCO tank circuit. Otherwise any stray fluctuations in the control voltage Vtune emerging from the loop filter, will shift the output frequency around. Thus you need to reduce the frequency sensitivity/dependency of the VCO on the capacitance of the varactor. If you remove the POCSAG drive modulation, monitor the frequency on a scanning receiver, or examine the spectrum on a Spec An and see what is happening. Quite an important element around the story you give is R2 in IC2A. It needs to be very stable and low noise (cermet or similar). Check what the waveform out of IC2A looks like as the pot R2 is varied. The level should change very smoothly with no transients. Does this range of voltage variation match closely the specification for the dynamic range of the FOX914E input?
The spurious at 122MHz is probably due to resonances in the RFCs, particularly I7. Try changing the value of the inductor, or place a VHF ferrite bead very near to I7, and see if the spurious changes frequency.


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darcyrandall2004
Post subject: Posted: Tue Apr 03, 2007 7:15 am

Colonel


Joined: Tue Feb 27, 2007 6:16 am
Posts: 46
Hello Nubbage.

I intended C35 to act as a block to DC.
C27 to act as a short to AC signals and a DC block.

I will replace R2 to with an appropriate trimpot and I shall experiment with replacing the RF chokes to determine the source of the 122MHz signal.

With regards to the connection of the varactor diode. How would you do this?

Would you like to see a plot of tuning voltage versus VCO frequency?

Can you recommend a better practice design for a VCO?

Cheers

Regards Darcy Randall, Perth, Western Australia


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nubbage
Post subject: Posted: Tue Apr 03, 2007 9:54 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Darcy
The high values of C35 and C27 (1 µF) will unfortunately carry a penalty of adverse parasitic reactance due to lead inductance, a may also suffer high dielectric loss at UHF. You could reduce these values somewhat. Experiment until the VCO behaviour becomes erratic, say with values like 1 nF.
The oscillator schematic is fundamentally (I think from memory) a Colpitts Oscillator, which is very widely used in VCOs, so no problem with that aspect.
A voltage vs capacitance curve would be useful. The delta C you need to present to the VCO tuned circuit I6/C31/Vc should be about 10% of C31. In the arrangement you have, Vc will be biased around an average value of 100% of C31, which is so high that the deviation will be excessive, and even load the circuit Q down so much that oscillation ceases. I have a few VCO circuits in my various notebooks, so I will look up the general concensus arrangement. Sadly I will be away for the next 10 days however.
I hope that helps.
Nubbage


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IR
Post subject: Posted: Tue Apr 03, 2007 10:07 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Darcy,

While I am not an PLL expert myself and always use commercial components to design PLL loops, I found the following link that can be useful for your studying purposes:

http://www.zen118213.zen.co.uk/RFMicrow ... lpitts.pdf

And always remember: A good engineer should know to open the right book in the right page.

Good luck!


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nubbage
Post subject: Posted: Wed Apr 04, 2007 5:48 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
And "the most useful development tool is a hack-saw" (Chief Engineer, Links Division Plessey Co. now deceased).
Hi Darcy:
Another problem with your original circuit is that if the output voltage from Q1 emitter is high enough (>1 volt) it can swing the varactor into positive conduction, killing the Q of the oscillator tuned circuit I6/C31. This is avoided by using 2 varactors back-to-back (commoned at their cathodes in the case of positive Vtune). Most circuits I have looked at ground the coil I6, connect the emitter of Q1 directly to the junction of C31 (A) and another C31(B), each C31 value at double the original 5pF (ie 10pF each). One end of C31(B) goes to ground, the end of C31(A) goes to the hot end of I6. The two varactors then are commoned at their cathodes, and one anode goes to ground, the other to the hot end of I6. I11 (the RFC) is then connected to the commoned cathodes of the varactor tuning diodes. The base of Q1 is isolated to dc from the tuned circuit by means of a 100pF capacitor. The circuitry around R26 and R28 might then need slight revision.
If you still have problems after this and the web page quoted by IR, I can up-load a copy of a 900MHz VCO which you could modify easily to 450MHz.







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