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G8MNY > TECH 30.08.03 20:23l 113 Lines 5928 Bytes #999 (0) @ WW
BID : 13086_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: Off air lock for ref osc.updated
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Sent: 030830/1805Z @:GB7CIP.#32.GBR.EU #:13086 [Caterham] $:13086_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY
Here is my method of locking the master osc in a PLL signal generator to BBC
625 line TV syncs. (BBC Ceasium ref better than 1/10^9), obtained from any TV
or video. This can be better than other lock sources there ie less fading etc.
NB. It is of no use for anything other than analogue UHF TV main station or off
air fed repeaters. Digital sources are NOT accurate as they only use local
timebases. In the UK only BBC has accurate syncs, CH4 is quite accurate, ITV
are poor, CH5 are no use as it is usualy from a digital sat feed!
This design will work with any 1MHz clock that has (or can have) a varicap to
trim its frequency. So it can easily be used on a frequency counter if a 1MHz
clock can be derived or extracted & will greatly improve the accuracy &
temperature problems of simple Xtal timebase.
FEATURES
Automatic switch to passive free run mode with no Video.
Reasonably immune to TV sync noise, eg ghosts etc.
Lock up speed up circuit.
THE CIRCUIT
220u + +12v SYNC PRESENT SWITCHED 12V
ÚÄ´ÃÄÄÁÂÄÄÄÂÄÄÄe ÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄ +12V
Ï ³ 22K \ / ³ PRESET _³_ D3_³_ ³
T1 ³ ³ T2ÄÂÄ+ ³ 4K7 /_\ \_/ PHASE Speed 100K
OFF AIR u1 ³/e ÃÄÄÄÄÁÄ´ÃÄ´ ³ ³D1 ³ COMP up ³
VIDEO Ä´ÃÄÂÄ´ 100K 2u ³8 1K ³ 10K Úij>³ÄÄ´ To Master
INPUT ³ ³ ³\____³Ä´ÃÄ¿ ÚÄÄÄÄÄ¿3 ³ ³ ÃÄÄÂÄ47KÄ´ D4ÃÄÄÂÄ> Osc
³ ³ ³ ³ 15p ³ ³ 555 ³ÄÄ)Ä220KÄ´ ³/ ³ Ãij<³ÄÄ´ ³ Varicap
75R ³ ³ ³ 220K ³ 7³ÄÄ´ ÃÄÄÂÄ´T3 === ³ D5³ ³+
³ ³ ³ === ÃÄÄijCV ³ 12K _³_ ³ ³\e ³22n ÃÄÄ1MÄÄ´ === Tant
³ 1M 10K ³1n5 ³ 5³ 2,6³ÄÄ´ D2\_/ ³ ³ ³ +³ 100K ³100uF
³ ³ ³ ³ === ÀÄÄÄÄÄÙ === ³ 56K ³ ³ === ³ ³
³ ³ ³ ³ ³1n ³1 3n3³ 3K* ³ ³ ³ Tant³2u ³ ³
ÚÄÁÄÄÄÁÄÄÄÁÄÄÁÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄ)ÄÄÄÁÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÁÄ
Ï SYNC AIR LOCKED ³ 20:1 ATTEN
STRIPPER 15,625Hz 15,625 ³
Hz ³
eg 64uS ³ 5ÚÄÄÄ+5vÄÄÄ¿5
T1 any small PNP HFE 100, ³ ÚÄÄÄÄÄÄ¿1 ÚÄÄÄÄÄÄ¿1 TTL
T3 NPN HFE 100. Àij74LS93³ÄÄij74LS93³ÄÄÄ<1MHz
D1 is a germanium/Schottky. 11ÀÄÄÄÄÄÄÙ 11ÀÄÄÄÄÄÄÙ
D2-5 are common 1N4148. ³2,3,10 ³2,3,10
ÚÁÄÄÄÄÄÄÄÄÄÄÁÄ
Ï DIVIDE BY 64
HOW IT WORKS
The input can be terminated or not onto 75R. T1 strips the syncs of the video,
which are filtered almost to a saw tooth with the 1n5. The average DC level of
T1 collector changed from high to low with the presents of video & this turns
on T2 to power up & enable the rest of the circuit.
The 555 15,625Hz (64uS) astable osc is fed with only a few mV of the recovered
syncs on its CV pin 5, this makes the astable lock up only over a narrow range
so it free wheels over noisy & missing syncs during the frame sync pulse.
The 555 output is compared in TR3 with the local 1MHz clock after that has been
divided by 64 (8x8 to give 64uS) in 2x 74LS93. Diode D2 & 'select on test' 3K*
are to temperature compensate T3 so there is no change in phase sensitivity.
D1 & D3 isolate the control output when there is no video.
T3 O/P is lightly filtered on its collector so that the phase errors can be
scoped for setting up. The final varicap control line is attenuated & heavily
filtered to remove all traces of TV variables & clock pulse etc. This would
cause long lock up times of several minutes, so an out of lock speed up
circuit is included.
The master oscillator varicap feed has 2 100K Rs across the 12V, so that when
there is no video the osc free runs at the normal frequency. Some accurate osc
modules have internal thermistors for thermal tracking the Xtal with a varicap,
this circuit is able to drive that load without the 2 100Ks.
On applying video the two 15,625Hz phases will be well out of phase & the
control voltage will be > ñ 0.5V off the normal varicap voltage, so diodes D4/5
will conduct & bypass the 20:1 attenuator/filter speeding up lock. When in lock
the attenuator insures only a small amount of heavily filtered control is
applied.
TESTING
Lock the scope to incoming video. Scope the filtered output syncs on T1
collector. Check that T2 is turned on with video being present, & check that
the it is off & the varicap voltage returns to the normal setting with no
video.
Initially set up the 555 astable preset so that pin 3 output square wave is in
line lock with the video.
Check the divider outputs are OK & produce a 15,625Hz square wave. Using a high
Z probe (1/10) on T3 collector, check that there is a level somewhere between
the power rails (it will have some line & frame sync content on it), this
DC level indicates lock up. Now adjust the scope timebase to 1/25 Sec to show
2 video frame syncs, and accurately set the 555 preset for minimum disturbance
during both frame syncs.
Use your osc/counter for some accurate work, eg listen 70cms SSB carrier or
measure 70cms frequency. Remove video & adjust your master osc trimmer for the
same frequency. This should leave the non-locked accuracy as near to the off
air as possible, but still with drift etc.
LOCK INDICATION
Not shown here, but I use a couple of transistors to sample the two 15,625Hz
phases again, but with slight CR windowing so that they do not turn on when the
unit is properly locked. A long time constant to stop indicating lock, until
everthing has settled down. This feeds a tri-state LED transistor switch
circuit to indicate NORMAL (OFF) TV SYNCS (RED) & TV LOCK (GREEN).
/QSL
73 de G8MNY @ GB7CIP
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