Right then...

I've scanned the current schematic for the Teletext Data Separator from=20
my lab notebook. Here it is in all its scribbly, hand-drawn,=20
chickenscratched glory:
   http://www.philpem.me.uk/temp/ttxpll.pdf

And the ETSI Teletext Specification is here:

http://www.etsi.org/deliver/etsi_i_ets/300700_300799/300706/01_60/ets_30070=
6e01p.pdf

   (FYI: only pages 14 through about 20 are of any interest to this=20
discussion)

For reference, here's what I want to do:

   - I want to take a Teletext-format NRZ data stream and extract the=20
clock, so I can then extract the data stream
   - This is to archive Teletext pages before the digital switch-over.=20
"Archival for posterity". That and it seemed like a fun project (until I=20
started designing the PLL!)
   - Data rate is 6.9375MHz, plus or minus 25ppm.
   - Data packets are 45 bytes long:
     - 3 byte preamble (01010101 01010101 11100100) - even parity
     - 2-byte address (Hamming coded, odd parity)
     - 40 bytes of data (odd parity)
   - The PLL needs to lock by the end of the second preamble byte, ready=20
to receive the framing byte (11100100).

   - I've scrapped the 74HCT4046 PLL I was using, in favour of a Hogge=20
phase detector and VCXO.
   - My VCXO is based on a 74HCT04 and uses a varactor for tuning.
   - The VCXO's frequency ranges from 6.93738MHz (0V across varactor) to=20
6.93758MHz (5V across varactor). Somewhat asymmetrical, but close to=20
what's required (6.9375MHz at 2.5V is the ideal). The range on this may=20
be a little narrow -- 25ppm is 6.937327MHz to 6.937673MHz.

The problem at the moment is that while the output data and clock are=20
valid (or *look* valid on the scope), the PLL doesn't seem to be=20
locking. The voltage at "VCO Vin" seems to rise and fall in a sawtooth=20
(and a VERY slow one at that) but there doesn't seem to be a point where=20
VCO_Vin stays at one particular value.

My theories at the moment are:
   - The circuit is built on breadboard. Stray track-to-track=20
capacitance is screwing up the loop filter, VCO or both.
   - The VCO range just plain isn't wide enough. Thus, the loop keeps=20
searching, but never hits the right phase/frequency.
   - There isn't enough current going into the varactor to make the=20
frequency change significantly. This seems plausible -- I tested the=20
VCXO by wiring it up to a lab power supply and varying the output voltage.
   - The loop filter just plain isn't right.

Can anyone offer any insight or assistance?

Thanks,
--=20
Phil.
piclist@philpem.me.uk
http://www.philpem.me.uk/
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