PCB Design for Sawtooth VCO

flux

0:09

Work done
  • Built the core schematic around Q1, Q2, Q3, Q4, Q5, and U1, with Eurorack power input on J1 and jack I/O on J2, J3, and J4
  • Assigned 1% metal-film values to R1 through R13, set C1 as the NP0/C0G timing capacitor, and added the required 10 uF plus 100 nF rail decoupling with C2, C3, C4, and C5
  • Configured the PCB to 100 mm x 50 mm with a Standard 2 Layer stackup, documented the architecture in the Project Specification, and exported manufacturing data:

    eurorack-sawtooth-core-vco-bom.csv

    DesignatorQuantityElement UIDsPart UIDSpecificationPart TypeDomainComplianceJLCPCB Part ClassOperating VoltageOperating TemperatureReliability Target (MTBF)RoleRole DetailsSoftwareConnector TypeConnector GenderConnector PitchConnector PositionsConnector RowsPower RequirementsConnectivityHuman InterfaceManufacturing Quantity TargetManufacturer Part NumberLCSC Part NumberDigi-Key Part NumberMouser Part NumberArrow Part NumberVerical Part NumberTME Part NumberAllow SubstitutesManufacturer NamePrice Change ThresholdLead Time Change ThresholdPricing Notification FrequencyPreferred DistributorsDatasheet URLImplementation DetailsLicensePackage or Case CodePin TypeVoltageVoltage RatingForward VoltageReverse VoltageMax Reverse VoltageDiode TypeMountIntegrated Circuit TypeBreakdown VoltageThreshold VoltageMax Output VoltageMin Output VoltageInitial Voltage on ResetResistanceResistor TypeGate ResistanceOn ResistanceOff ResistanceToleranceCapacitanceCapacitor TypePowerPower RatingCurrentMax CurrentDesired Temperature RiseCurrent RatingLeakage CurrentSaturation CurrentInitial Current on ResetTrigger CurrentHolding CurrentFrequencyLogic FunctionInductanceInductor ShieldingInductor TypeBetaDrain to Source VoltageContinuous Drain CurrentTransistor TypeGainCoupling CoefficientRatioReference Design URLPricePurchase UrlProduct Info UrlProduct Image UrlSubstitute Manufacturer Part NumberKiCAD Library ReferenceExclude from PCBControlled ImpedanceControlled Impedance TolerancePN Skew MaxPair to Pair Skew MaxPin DelayControlled Impedance PairBus GroupPair RolePin RoleBus TypeSymbol StyleSymbol SizeNet TypePackageDesignator Prefix
    C1,C62166752a7-b5b6-4636-872a-9ced1aafd260,7ef758c1-13cb-4767-87e9-c90e13c9d5ca46707925-236f-106b-e676-afa2757aa1d6Non-Polarized Capacitor, 1nF, C_Disc_D3.0mm_W1.6mm_P2.50mmNon-Polarized CapacitorCapacitorWill be specialized into timing NP0, local ceramic bypass, and rail bulk capacitor values after placement50V%1nFC0G/NP0,CeramicC_Disc_D3.0mm_W1.6mm_P2.50mmC
    C2,C42748e26c5-63cd-48b7-9107-12bb76386745,b8c91564-ef21-49d6-bcdb-21120751925f46707925-236f-106b-e676-afa2757aa1d6Non-Polarized Capacitor, 100nF, C_Disc_D3.0mm_W1.6mm_P2.50mmNon-Polarized CapacitorCapacitorWill be specialized into timing NP0, local ceramic bypass, and rail bulk capacitor values after placement50V%100nFCeramicC_Disc_D3.0mm_W1.6mm_P2.50mmC
    C3,C529494abda-1870-4593-a89c-d678f714fbb7,9c153afe-43c5-4a00-ae8b-17af906da9aa46707925-236f-106b-e676-afa2757aa1d6Non-Polarized Capacitor, 10uF, CP_Radial_D6.3mm_P2.50mmNon-Polarized CapacitorCapacitorWill be specialized into timing NP0, local ceramic bypass, and rail bulk capacitor values after placement25V%10uFElectrolyticCP_Radial_D6.3mm_P2.50mmC
    J111226dd23-5b42-4aaf-9950-694096615f48fe92d11f-99f4-475b-b27f-b0a1434174a1Connector, 2x5 2.54mm THConnectorPower input connectorEurorack plus and minus 12 volt power entry61201021621Würth Elektronik2x5 2.54mm THJ
    J2,J3,J435f77be81-37b9-41bb-bb34-20c08b9e258d,d85186a7-2c85-41b6-bdde-83ee36351326,e594b0c4-4a61-44ab-a38a-0d94981c5d4b700a40d3-b931-4e21-8bb3-5cc3929498ecConnector, Vertical TH 3.5mm jackConnectorPanel I/O jackUsed for control voltage input and waveform outputsAudio / Video Connectorshttps://www.aliexpress.com/item/1005001958349207.htmlVertical TH 3.5mm jackJ
    Q1,Q2,Q3321597007-648b-4fb1-be2c-65b3c34c53f1,8dc81422-4f61-4a09-bd97-7b894675f3d3,f38bb4ac-8dfc-49b9-9804-09d9ffd802a219f51467-0126-4de7-8615-30c51aef8f82Transistor, TO-92TransistorExponential converter and reset transistorUsed in matched NPN pair for expo converter plus one NPN reset device2N3904TFRonsemihttps://creativecommons.org/licenses/by/4.0/TO-92Q
    Q4,Q527aa54d57-74aa-4959-9134-5d8aea9fbef4,cbd6e5ca-c003-4f6c-b7e6-af5ab3de3ae59aa797f6-0421-44a2-b0b4-e3bdb3aa3990Bipolar Transistors - BJT, TO-92-3_L4.9-W3.7-P2.54-LBipolar Transistors - BJTExtended PartPNP transistor stageUsed in expo current source and reset support2N3906TAC258186onsemi(安森美)TO-92-3_L4.9-W3.7-P2.54-Lhttps://lcsc.com/product-detail/Transistors-NPN-PNP_ON-Semicon_2N3906TA_ON-Semicon-ON-2N3906TA_C258186.htmlTO-92Q
    R1,R10,R2,R4,R5,R6,R870ba90175-0378-4a22-ac76-fbeb80237694,0d626748-ea9e-4320-b97a-155da3e3f063,56f9834c-5aea-4c8c-8fe1-465b8b714b89,69b9079c-e441-4abe-a52e-ef8384704d57,702517a9-c7be-4150-b9c6-8cbd04b5de08,db2e3809-9d9d-4191-a0c8-8918f3f3016d,f339c96b-cb86-4aef-96c8-f22f22741ee9b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2dResistor, 100kΩ, H_AXIAL-P7.62_D2.5ResistorPrecision resistorUse as 1 percent metal film resistor; values will be specialized per designator after placementV100kΩMetal Film1%H_AXIAL-P7.62_D2.5R
    R9128c2553e-7497-4e87-a59d-3cfe2c5dec4bb67f50a9-ae5e-10ea-9c3c-d377e2ab3b2dResistor, 220kΩ, H_AXIAL-P7.62_D2.5ResistorPrecision resistorUse as 1 percent metal film resistor; values will be specialized per designator after placementV220kΩMetal Film1%H_AXIAL-P7.62_D2.5R
    R1113028001b-19dc-4dda-bf35-d6f71f635c81b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2dResistor, 47kΩ, H_AXIAL-P7.62_D2.5ResistorPrecision resistorUse as 1 percent metal film resistor; values will be specialized per designator after placementV47kΩMetal Film1%H_AXIAL-P7.62_D2.5R
    R121e02b5b61-8686-422d-897a-9cf5351a2a21b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2dResistor, 56kΩ, H_AXIAL-P7.62_D2.5ResistorPrecision resistorUse as 1 percent metal film resistor; values will be specialized per designator after placementV56kΩMetal Film1%H_AXIAL-P7.62_D2.5R
    R13,R3,R734d7f8932-4908-4d46-badb-13ae9018177b,95f5d37f-5dc4-4ff2-a308-22eb9db7f923,fa9f0bcf-c542-4768-bd02-80db7813320cb67f50a9-ae5e-10ea-9c3c-d377e2ab3b2dResistor, 1kΩ, H_AXIAL-P7.62_D2.5ResistorPrecision resistorUse as 1 percent metal film resistor; values will be specialized per designator after placementV1kΩMetal Film1%H_AXIAL-P7.62_D2.5R
    U11b1ad147b-1039-4cac-bc36-504dd97c4b5f7f084491-d1ec-45e2-8d44-55264bdd89b8Integrated Circuit, PDIP-8Integrated CircuitSquare comparatorGenerates the 0V to 5V square output from the saw waveformTL072IPTexas Instrumentshttps://creativecommons.org/licenses/by/4.0/PDIP-8U
    and eurorack-sawtooth-core-vco.d356
Success criteria review
  • ✅ Required active devices, precision resistors, NP0 timing capacitor, and rail decoupling are present in the design
  • ✅ Layout size is set to 100 mm x 50 mm and the board stackup is now 2-layer
  • ✅ BOM and netlist exports were generated
  • ❌ The 4 mounting holes were added in the schematic but have not yet been position-verified in the PCB
  • ❌ The detailed 2-layer routing plan is only partially implemented: the critical guidance is to keep the C1 / Q1 / Q2 high-impedance summing region compact, place U1 and the square-output path away from that node, route saw and square outputs separately, and use the bottom layer as a continuous ground reference, but actual placement optimization and routing verification still remain
Next steps Finish PCB placement by positioning the four mounting holes and analog core, then run airwire review and finalize the low-bleed routing strategy. I can continue with that next.
Create the VCO schematic
Set board constraints
Add required passives
Prioritize quiet layout
Generate BOM and reviews
3 days

Plan completed.

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Project Specification
Scope
Design and implement a professional Eurorack sawtooth-core VCO using a discrete transistor exponential converter and reset circuitry, operating from plus and minus 12 volt rails.
Architecture
Diagram



  
    
  
  
    
  





















CV Inputs and Tuning Network
Discrete Exponential Converter\n2N3904 and node_2N3906
Integrator Saw Core\nTiming cap NP0 C0G
Discrete Reset Switch\n2N3904 and node_2N3906
Saw Output Buffer
TL072 Comparator
0V to node_5V Square Output
+12V and -12V Power Input
Rail Decoupling\n10uF electrolytic and node_100nF ceramic per rail
Requirements
  • Use 2N3904 and 2N3906 transistors in the exponential converter and reset circuitry.
  • Use a TL072 op amp section as the square-wave comparator and produce a 0 V to 5 V square-wave output.
  • Use 1 percent metal film resistors throughout the analog core.
  • Use a C0G or NP0 timing capacitor in the saw-core integrator.
  • Include 10 uF electrolytic and 100 nF ceramic decoupling on each supply rail.
  • Fit the PCB layout into a 100 mm x 50 mm board outline.
  • Use a 2 layer PCB with a solid ground plane on the bottom layer.
  • Keep the high impedance summing node short and physically isolated from the square-wave path to minimize bleed.
Implementation Notes
  • Prefer through-hole transistor and op amp packages for classic analog serviceability and easy thermal pairing where available.
  • Thermally couple the matched 2N3904 pair used in the exponential converter placement.
  • Keep comparator output routing away from the timing capacitor and summing node.
Design Summary
Status: Draft / Review / Approved
Manufacturing target: Prototype / EVT / DVT / PVT / Production
Software / firmware: repository link
Scope
Purpose
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Out of scope
System context
Requirements
Functional
Electrical
Mechanical / environmental
Key constraints
Dependencies and risks
Dependencies
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Validation
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Release-facing notes
Expected deliverables
Special release notes
Change notes / open questions
Changes in this revision
Open questions

    Scope

    Architecture

    Requirements

    Implementation Notes

    Design Summary

    Scope

    System context

    Requirements

    Functional

    Electrical

    Mechanical / environmental

    Key constraints

    Dependencies and risks

    Validation

    Release-facing notes

    Change notes / open questions

Documents

    Project Specification

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Eurorack Sawtooth Core VCO

Eurorack Sawtooth Core VCO thumbnail
Professional Eurorack sawtooth-core VCO with discrete 2N3904/2N3906 exponential converter, TL072 square comparator, plus/minus 12 volt rails, precision passives, and a low-bleed 2-layer 100 mm by 50 mm layout.

Properties

Properties describe core aspects of the project.

Pricing & Availability

Distributor

Qty 1

Arrow

$0.56–$0.94

Digi-Key

$1.49–$4.84

LCSC

$1.46

Mouser

$2.12

TME

$2.04

Verical

$0.15–$0.76

Controls