Keith Gooley VK5OQ quartz crystal oscillator temperature stabilis

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Keith Gooley VK5OQ quartz crystal oscillator temperature stabilis

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Reference Oscillator
Keith Gooley VK5OQ
quartz crystal oscillator temperature stabilised enclosure used reference counter narrow-band mode receiver transmitter UHF/SHF
need this reference arose shack when using counter measure frequencies region finding errors order kHz. This that much, percentage even significant part bandwidth narrow-band signal. frequency increased, requirements stability accuracy reference become more demanding. original reference counter simple microprocessor crystal CMOS type oscillator. calibrated some years obviously drifted off. idea this more stable reference born. ultimate lock frequency horizontal sync pulses signal along lines
Photo
networks accurate standards generate their sync pulses. derives reference from Global Positioning System signals.
Design
There main causes drift frequency quartz crystal oscillators, namely temperature changes aging crystal itself. make high
stability oscillator therefore requires that both these causes drift compensated for. Putting crystal often, associated oscillator components temperature-stabilised oven long been means reducing removing effects temperature. There much average amateur about aging crystal other than obtaining best quality crystal
Figure Oven Controller
Amateur Radio, August 2001
that afforded given requirements application. More recent reference oscillator designs temperature sensor such thermistor adjust frequency oscillator based sensor output. This done directly coupling thermistor varicap more accuracy required microprocessor used. temperature reading brought into micro converter lookup table used determine output voltage required varicap correct frequency. digital representation output voltage then applied converter then varicap. This latter method made very accurate table values micro tailored particular crystal. These methods compensating temperature changes generally result very power consumption oscillator compensation therefore well suited modern battery operated equipment. However, they easy amateur reproduce, building reference. Therefore have chosen take "tried true" method putting whole oscillator buffer temperature stabilised enclosure with good insulation minimise power consumption heater once internal temperature stabilised. tackled design building oven controller first reasoned that couldn't that right, there point continuing. turned out, controller works very well point that temperature variations thermistor difficult determine, certainly less than degree Temperature variations thermistor thing secret make thermal coupling between heater, thermistor crystal tight possible. heater power transistor large TO-218 package screwed copper heat spreader. thermistor soldered under transistor fixing screw, making good thermal contact. crystal clamped heat spreader alongside heater transistor. layout illustrated photographs. heat spreader length copper bar.
Photo
Circuit Description Oven Controller
Refer Fig.1, oven controller circuit. This circuit feedback control system which temperature sensed thermistor error between reference amplified used control heater. precision voltage reference volts, providing precise supply resistance bridge thermistor, TH1. latter resistance about deg. imbalance bridge applied which amplifies passes correction voltage heater driver transistor, temperature low, higher than plus input opamp, goes positive causing output rise. This results drawing more current, which flows base TR3. TR3's emitter current rises until voltage across causes turn This shunts current away from base, limiting emitter current about This configuration current limiting transistor (TR2) base circuit another protect latter from excess current very useful one, which quite widely applied. fact commonly used output circuits IC's, both digital analogue. dissipates about watts when hard heats oscillator components thermistor. thermistor voltage falls lowering
correction voltage into opamp which reduces current therefore TR3. diode ensures that when opamp output lower limit which about volt, remains since diode voltage drops (1.3 volts) required turn components frequency compensation control loop. Without them loop unstable with noise picked opamp input causing large amplitude oscillations opamp output. addition, loop tends "hunt" with heater going from hard hard off. resulting temperature fluctuations cause oscillator frequency fluctuate well, after all, idea oven controller keep temperature constant within close limits possible. Close thermal coupling between thermistor heater transistor makes stabilising control loop easier. found that placing thermistor heat spreader away from transistor fixing screw resulted thermal delay which difficult compensate for. drill hole where thermistor placed initially seen photo compensation close ideal with only little overshoot temperature during warm-up.
Oscillator
oscillator circuit, shows details Colpitts crystal oscillator buffer amplifier. This version Colpitts oscillator using darlington
Amateur Radio, August 2001
Figure 10MHzReference Oscillator
Amateur Radio, August 2001
Figure
Figure
connection transistors, TR2. have used past found more reliable than single transistor version. crystal parallel resonance frequency frequency adjusted changing capacitance seen crystal. Both electrical mechanical adjustments provided varicap good quality dielectric trimmer. With
trimpot about centre range, varying frequency tuning voltage from zero volts causes oscillator frequency increase about high gain darlington connected transistors enables larger values feedback capacitors used. This provides improved isolation crystal from variations
transistor parameters. Transistor parameter variation temperature minimised clamping oscillator transistors heat spreader alongside crystal. Likewise varicap placed thermal contact with heat spreader. thermal grease mating surfaces help keep temperature more uniform. oscillator output lightly
Figure Clamp dimensions
Amateur Radio, August 2001
Photo
coupled dual gate mosfet TR3, through capacitor very feedback capacitance order 0.02 therefore there very little effect oscillator from influences buffer output. drain circuit contains autotransformer, with centre coupling signal base output emitter follower, TR4. resistor, included series with emitter output transistor make output impedance close ohms. output power prototype about 1.6-volt into ohms.
Construction
sections reference constructed circuit board separated copper heat spreader. Photo layout diagram Fig. show arrangement components. heat spreader held onto circuit board with screws spacers separate from circuit board, though retrospect, this probably necessary even desirable. There clamp over TR3, crystal, been removed photograph. Springy copper leaves seen soldered underside clamp press crystal transistors onto heat spreader. thermistor soldered solder under heater transistor mounting screw. thermistor originally comes with wire leads solder directly solder lug, scrape paint
side thermistor, unsolder wire solder that side thermistor previously tinned solder lug. Place thermistor close possible mounting hole lug, leaving enough space screw head. insulating sleeve should placed over remaining wire, which hole circuit board. oscillator transistors plastic TO-92 packages mounted flat side down heat spreader. varicap placed heatsink compound adjacent transistors. small ferrite toroid used core autotransformer, with bifilar winding turns (about SWG) enamelled copper wire. Toroid types FT23-43 FT-37-43 would fine. circuit board piece single sided copper laminate with components mounted copper side which acts ground plane whole circuit. Most component leads through holes board non-grounded ones have holes slightly countersunk prevent shorting ground. Inter-connections made underneath board using mostly just component leads. This method that have developed after less than satisfactory results making medium density PCBs home. board using computer package (Protel) normal were making instead etching board, overlay diagram template drill holes.
First thoroughly clean copper side board with steel wool water then tape layout diagram over copper side. punch make marks through paper where holes Next remove paper drill holes with smallest size drill. find good most components. larger holes bored correct size later. have countersink those holes where leads pass through board without connecting ground plane. find much easier make solder joint ground hole countersunk, mark with felt those holes where ground connection made. Most inks these pens soldered through don't worry about preventing good solder connection. mark earth holes countersink them don't worry miss still possible solder lead ground plane around countersunk hole. just takes little more solder. unmarked holes countersunk. this very carefully with small drill bit. about right. Only slight countersink required, just enough remove burr around hole. countersink diameter plenty. board ready loading components except thing. good idea this stage spray copper side with lacquer keep shiny looking. using sockets useful away plastic housing socket above pins which grounded. Doing makes much easier solder directly ground plane once socket inserted board. This most applicable corner earth logic opamp IC's done with earthed pins. Terminals power input signal output made pins soldered isolated pads ground plane. These pads with counterbore available from Farnell Electronics (Cat. No.146-413) They aren't cheap nice cutting round pads ground plane. isn't necessary these isolated pads, though. hole drilled size appropriate that have will retained countersunk hole solder joint under board. Components placed
Amateur Radio, August 2001
board time underside connections made. Solder connections rather than putting many components then turning board over. This will prevent components falling when turn board over make solder joints. Inevitably some connections will required which cannot made with existing component leads. make shorter connections, have retained large number offcuts component leads flat that when short connection required those. longer links single strand kynar insulated wire, type used wire wrapping ordinary hookup wire will This sound messy time consuming compared with simply dropping components into soldering them place, have found preferable generating layout, transferring coated board using toner transfer process then etching board XYL's laundry often with entirely satisfactory results. etching pattern provided bottom side board those wish etch their own. Double sided laminate should used side protected from etching covering with adhesive tape adhesive "Contact" film. board original mounted fabricated from zinc annealed steel. corners were welded local workshop $10. coats self priming spray paint gave nice finish box. secured with screws opposite corners going into threaded spacers screwed bottom box. Insulation from sheet thick polystyrene board with pieces sides box. diecast would serve very well don't want roll your even plastic "zippy box" would although metal preferred screening
Testing
built tested oven controller section first. When first power having checked your wiring, monitor supply current. should start between falling level dependant well board insulated. Initially, probably isn't insulated all,
current will vary depending draughts blowing across board. Check that reference voltage correct, volts with LM329 device shown circuit. Look voltage output opamp. should free from large variations oscillation. have used similar components original similar type construction, compensation components oven controller should need altered. thermal coupling between heater transistor thermistor changed substantially reference voltage changed, will most likely have suppress oscillation control loop. find these circumstances that make comply with following equation, loop will stabilise. 1/[2*pi*(C1*R7)] where frequency oscillation loop. chose value then worked value based above equation. This certainly rigorous design based control system theory "good enough" method. With some rudimentary insulation over board, folded towel other piece cloth, supply current should stabilise about minutes after cold switch settle somewhere between depending insulation ambient temperature. Testing oscillator section starts with setting about midrange. Ensure that regulator output voltage close volts. Measure output level with diode probe oscilloscope. about volts peak peak into ohms output. Mount circuit board box, insulation ready frequency. Couple little this signal into receiver tuned WWVH with receiver USB, Adjust note comfortable audio frequency. Adjust coupling oscillator that hear audio beat note itself beating frequency difference between oscillator WWVH. should then able adjust oscillator zero beat with Time Frequency Standard signal.
Components
Some comments component selection will assist those wishing duplicate this reference oscillator. Many components this design were selected they were junk which been swelled considerably recent years wealth electronic equipment many types being available prices being given away thrown out. Therefore where have used specialised component because hand will endeavour give commonly available substitute.
Oven Controller:
reference diode LM329 available from Farnell 411-530). zener diode would bridge output differential very sensitive supply voltage bridge. volt zeners have lowest temperature coefficient. Thermistor could substitute device (Dick Smith R1797) change have spare thermistors stuck. Opamp non-critical. single dual opamp will bipolar input. Suitable alternatives LF351, TL071, TL072, LM358 should plastic film capacitor, non-polarised. TO-220 TO-218 transistor don't with fully insulated tab. copper heat spreader which crystal other components mounted length copper bar. Aluminium could substituted aluminium half thermal conductivity copper.
Oscillator components
Crystal. This oscillator stability thought that could good enough stability used crystal unknown source from junk box. want best stability though, crystal should ordered especially oven operation temperature oven. case this deg. oscillation mode parallel, capacitance. Varicap. Almost reverse biased diode will give enough frequency variation varicap proper will have
Amateur Radio, August 2001
higher 1N914's even power diodes such 1N4001 series have been used varicaps. There lots varicaps available tuners, radios like there shouldn't need resort using power diode. Transistors. 2N5770 2N706 with tighter specs latter could substituted. BF199F (Altronics Z1106) would work these frequencies BC547 would probably work well, would 2N2222 PN2222. plastic case transistor preferred metal case devices usually have collector connected case. Dual gate mosfets, which could substituted, MFE131, BFR84 BF981. could substituted output emitter follower.
Performance
Output Frequency 10,000.000 (adjusted) Warm time Frequency within after minutes Frequency drift order X10-8) Output signal level8 Power consumption volts warm minutes ambient
stability control circuit. best frequency stability oscillator sought this instance intended that oscillator locked horizontal sync pulses television signal. method construction described which does involve etching PCB.
References.
Pogson, TV-Derived Time Frequency Standard" Electronics Australia, July October 1989. Kimberley, Kenneth VK2PY, Amateur Radio Engineering Project" Amateur Radio, September October 1986
Conclusion
oven controlled reference oscillator suitable home construction described. oven temperature controller proportional type some hints have been given achieve
Remembrance Contest
Pacific ended 15th August 1945. Many Radio Amateurs served Services, both active duty home. Some never returned from service. Remembrance Contest held weekend closest 15th August each year mark respect those died. When look back sacrifice servicemen women civilians, stayed behind enemy lines died, acknowledge debt them continued democratic life Australia. However such times also good look forward hope have learnt something from these sacrifices. this year have chosen focus present future focus attention signallers today. pictures cover with this article show have both women field. They still awfully young. Some their equipment still backpack whip aerial, more computer keyboard monitor screen. frequencies used higher well satellites important links total system. forget past, make sure have learnt from lessons dearly paid apply them future.
Photo top: Donna Feltus LCPL Williams.
Photo right: Julie Meredith Cristian Birzer.
LEST FORGET
Photographs from Captain Sandra Turner Brigade, Army Reserve Unit Tasmania. personnel members Signal Squadron. Kathryn Thomas took photos. wish thank them their contribution annual Remembrance.
Amateur Radio, August 2001

Keith Gooley VK5OQ quartz crystal oscillator temperature stabilis

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