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Below are all of the forum threads, including all
the responses to the original posts.
Post subject: UHF DATA RADIO SCHEMATIC Posted:
Mon Apr 02, 2007 9:41 am
Joined: Tue Feb
27, 2007 6:16 am
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
Could I please get some feedback as to whether it
looks like I am headed in the correct direction.
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.
help is very much appreciated.
Regards Darcy Randall from Perth,
Posted: Tue Apr 03, 2007 3:51 am
Fri Feb 17, 2006 12:07 pm
Location: London UK
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
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
Posted: Tue Apr 03, 2007 7:15 am
Tue Feb 27, 2007 6:16 am
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?
Randall, Perth, Western Australia
Post subject: Posted: Tue Apr 03, 2007 9:54 am
Joined: Fri Feb 17, 2006 12:07 pm
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.
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.
Posted: Tue Apr 03, 2007 10:07 am
Mon Jun 27, 2005 2:02 pm
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:
And always remember: A good engineer should know
to open the right book in the right page.
Post subject: Posted: Wed Apr 04, 2007 5:48
Joined: Fri Feb 17, 2006 12:07 pm
Location: London UK
And "the most useful development tool
is a hack-saw" (Chief Engineer, Links Division Plessey Co. now deceased).
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.