Softrock Lite II Softrock Lite II: Operational Amplifiers
Band: 20m

Introduction

General Info About the Stage

Theory of Operation

The low-level In-Phase (I) and Quadrature (Q) IF signals from the Mixer are sampled over capacitors C5 and C6 (in the Mixer stage).

The opamps in this stage amplify the I and Q baseband signals by a factor of approximately 499, to compensate for the attenuation as a result of subharmonic quadrature sampling .


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Stage Schematic

04_opamp stage schematic

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Board Layouts

Board Top

04_opamp stage topside
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Board Bottom

04_opamp stage underside
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Operational Amplifiers Bill of Materials
( 20m band option)

(details for installation of each component are provided in the step instructions, further down the page)

CheckTypeCategory ComponentCountMarkingImage
☐ Capacitor Ceramic 390 pF 5% 2 391 390 pF 5%
☐ Capacitor Ceramic 4.7 uF 10% 16V X7R RAD 1 475 4.7 uF 10% 16V X7R RAD
☐ IC SOIC-8 LT6231 dual op-amp 1 LT6231 ESD!!! LT6231 dual op-amp
☐ Resistor 1/4W 10 ohm 1/4W 1% 2 br-blk-blk-gld-br 10 ohm 1/4W 1%
☐ Resistor 1/6W 100 1/6W 5% 2 br-blk-br-gld 100 1/6W 5%
☐ Resistor 1/4W 1 k 1/4W 1% 2 br-blk-blk-br-br 1 k 1/4W 1%
☐ Resistor 1/4W 4.99 k 1/4W 1% 2 y-w-w-br-br 4.99 k 1/4W 1%

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Detailed Build Steps

Install OpAmp

Install OpAmp

Install the LT6231 operational amplifier to the bottomside of the board.

See hints on installing SMT ICs.

  • Orient U4 on its pads so that the pin 1 corner of the IC matches the small “1" (it also looks like a “0”) mark in the copper on the bottom side of the board. In general, pin 1 of an SOIC packaged IC is in the lower left corner of the package when the printing on the package top reads upright, from left to right.
  • Tack-solder one corner pin of U4 and reheat the tacked pin as necessary to line up U4 on its pads properly.
  • Double-check the orientation of U4 and the line up of the IC on its pads with magnification and good lighting. You do NOT want to install U4 oriented incorrectly. If all is well, carefully solder the rest of the leads to their pads.
CheckDesignation Component (top/bottom) OrientationMarking Image Band Notes
☐ U4 LT6231 dual op-amp ((bottom)) LT6231 ESD!!! LT6231 dual op-amp any

Install band-specific components

Install the band-specific capacitors and resistors.

See hints on orienting and installing resistors.

See hints on identifying and installing ceramic caps.

CheckDesignation Component (top/bottom) OrientationMarking Image Band Notes
☐ R05 10 ohm 1/4W 1% (top) E-W br-blk-blk-gld-br 10 ohm 1/4W 1% 20m
☐ R06 10 ohm 1/4W 1% (top) E-W br-blk-blk-gld-br 10 ohm 1/4W 1% 20m

Install remaining components

See hints on orienting and installing resistors.

See hints on identifying and installing ceramic caps.

CheckDesignation Component (top/bottom) OrientationMarking Image Band Notes
☐ C08 390 pF 5% (top) 391 390 pF 5% any
☐ C09 390 pF 5% (top) 391 390 pF 5% any
☐ C07 4.7 uF 10% 16V X7R RAD (top) 475 4.7 uF 10% 16V X7R RAD any
☐ R07 4.99 k 1/4W 1% (top) S-N y-w-w-br-br 4.99 k 1/4W 1% any
☐ R03 1 k 1/4W 1% (top) W-E br-blk-blk-br-br 1 k 1/4W 1% any
☐ R08 4.99 k 1/4W 1% (top) E-W y-w-w-br-br 4.99 k 1/4W 1% any
☐ R04 1 k 1/4W 1% (top) W-E br-blk-blk-br-br 1 k 1/4W 1% any
☐ R09 100 1/6W 5% (top) E-W br-blk-br-gld 100 1/6W 5% any
☐ R10 100 1/6W 5% (top) E-W br-blk-br-gld 100 1/6W 5% any

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Completed Photos

Note: the completed pictures are of the 40m option, which the author built. Other band options (which the author did not build) will appear slightly different (especially the inductors, whose windings and cores will vary by band) for the band-specific components.

View of Completed Topside

04_opamp stage completed topside

View of Completed Underside

04_opamp stage completed underside
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Testing

Overview

Warning

Take appropriate ESD precautions in these tests, since you will be working around the sensitive OpAmp IC

Visual Check

Test Setup

Using very good lighting and magnification, carefully inspect the solder joints to identify bridges, cold joints, or poor contacts.

Pay especial attention to the joints on the OpAmp IC pins. If necessary, touch up the joints with your iron and/or some flux. Wick up any excess.

Current Draw

Test Setup

  • In each test, the ammeter must be placed in series between the positive lead of the power source and the board's positive power-in "+" terminal.
  • In one test there is also a 100 ohm resistor in the series "chain" as well.
  • in the second test, the setup is the same except that the current-liminting resistor is removed

Apply 12 Vdc to the board for this test

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
With 100 ohm current-limiting resistormA< 3026.6_______
Without current limiting resistormA< 3026.9_______

Voltage Tests

Test Setup

Measure the voltages with respect to ground for each of the pins of U4. Tage care to measure at the actual IC pin rather than the pad, so as to ensure you are measuring the PIN voltage

expected voltages are indicated in the table below:

  • 5 V (range of 4.5 - 5.4)
  • 2.5 V (approx 50% of the 5V rail value)
Voltage Tests

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Pin 1Vdc2.52.46_______
Pin 2Vdc2.52.46_______
Pin 3Vdc2.52.46_______
Pin 4Vdc00_______
Pin 5Vdc2.52.46_______
Pin 6Vdc2.52.46_______
Pin 7Vdc2.52.46_______
Pin 8Vdc54.94_______

Functional Test

Test Setup

You will test the DC gain of each of the op-amps by connecting resistors RB from each op-amp inverting input to circuit ground. Introducing the "bridging" resistor RB will result in a test current equal to 2.5 / Rt which will be balanced by the current fed back from each op-amp's output through each feedback resistor, RF (i.e., R7 or R8). Each op-amp output will increase in voltage by 2.5 * RF/ RB from the nominal DC level of 2.5 volts.

The value of the "bridging" resistor (RB) is 10 kΩ:

  • Test the First OpAmp

    Power up the circuit and measure the voltage at pin 1 of the op-amp (hairpin of R8). It should be ~2.5 Vdc
  • Power off and use clip leads to connect RB between the hairpin of R6 and circuit ground. (This provides an input resistance(Ri) of 10 kΩ
  • Power up and measure the output voltage at the hairpin of the feedback resistor R8. You should get:, with RB=10 kΩ and R8=4.99 kΩ: ~3.75 Vdc.
  • Remove RB and the output voltage at R8 should go back to ~2.5 Vdc.

  • Test the Second OpAmp

    Power up the circuit and measure the voltage at pin 7 of the op-amp (hairpin of R7). It should be ~2.5 Vdc
  • Power off and use clip leads to connect RB between the hairpin of R5 and circuit ground. (This provides an input resistance(Ri) of 10 kΩ
  • Power up and measure the output voltage at the hairpin of the feedback resistor R7. You should get, with RB=10 kΩ and R7=4.99 kΩ: ~3.75 Vdc.
  • Remove RB and the output voltage at R7 should go back to ~2.5 Vdc.

The diagram below shows the test points. The yellow dots show the output voltage measurement points. The white points show the bridging resistor connection points (connect the bridging resistor RB between the ground and a white point). To measure the voltage at yellow point "A", use white point "A" for the bridge; same with points "B".

An Excel spreadsheet with a calculator for this test is available for you to plug in your bridging resistor ohms (Rt) and your pin 1 or pin 7 normal voltages (Ebias) and predict the expected voltage when bridged (Eout).

<a name=Functional Test"

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
R7 (yellow "A")- unbridgedVdc2.52.48_______
R8 (yellow "B") - unbridgedVdc2.52.46_______
R7 (yellow "A") bridging white "A" to groundVdc3.753.72_______
R8 (yellow "B") bridging white "B" to groundVdc3.753.69_______

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