Local Oscillator Introduction


Band-Specific Components

Some of the components in this phase are band-specific. The Bills of Material below specify the common components and the 40m-specific components. If you are building another band's kit, you should refer to the following chart for their component values:

Theory of Operation

The Local Oscillator stage implements a basic Colpitts Crystal Oscillator with a buffer stage to increase the signal level. In the lower band kits, the oscillator produces a signal that is at the crystal's specified fundamental frequency.

The output of this stage is divided by 4 in the follow-on "Divider" stage, producing:

In reality, for each frequency the crystal circuit will oscillate at a slightly lower frequency (~ - 1 kHz), due to the capacitive divider (C10/C11) pulling the crystal down somewhat. The effect is more pronounced for the higher bands.

(go directly to build notes)

Local Oscillator Schematic

(Resistor testpoints (hairpin, top, or left-hand lead), as physically installed on the board, are marked in the schematic with red dots)

(Click for Full Schematic)
Local Oscillatorschematic

(go directly to build notes)

Local Oscillator Bill of Materials

Stage Bill of Materials

(resistor images and color codes courtesy of WIlfried, DL5SWB's R-Color Code program)

C010.1 uF(smt) black stripe (smt) black stripeSMT 1206Local Oscillator
C020.1 uF(smt) black stripe (smt) black stripeSMT 1206Local Oscillator
JP1shunt wire (cut-off lead)Cutoffhorizontalband-specific - see chart. Use a cut-off resistor lead to shunt between holes 1 & 2 of JP1Local Oscillator
Q12N3904 NPN Transistor2N3904 2N3904TO-92Local Oscillator
Q22N3906 PNP transistor2N3906 2N3906TO-92Local Oscillator
R0110 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WN-SLocal Oscillator
R0210 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4WW-ELocal Oscillator
R0310 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4WN-SLocal Oscillator
R04499 1/4W 1%y-w-w-blk-br y-w-w-blk-br1/4WW-ELocal Oscillator
R051 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4WE-WLocal Oscillator
R07499 1/4W 1%y-w-w-blk-br y-w-w-blk-br1/4WS-NLocal Oscillator
C03100 pF 5%101 101Ceramicband-specific - see chartLocal Oscillator
C0522 pF 5%22J 22JCeramicLocal Oscillator
C04100 pF 5%101 101Ceramic band-specific - see chart. Used only in 160, 30, and 20m kitsLocal Oscillator
X128.224 MHz Crystal28.224 or 28.2C69Xtalband-specific - see chartLocal Oscillator
R0622.1k 1/4W 1%r-r-brn-r-br r-r-brn-r-br1/4Wonly used in 30m KitLocal Oscillator

Local Oscillator Summary Build Notes

Local Oscillator Detailed Build Notes

Bottom of the Board

Local Oscillator Bottom View

Install SMT Caps

Install the two SMT Caps (C1 and C2) on the bottom side of the board

C010.1 uF(smt) black stripe (smt) black stripeSMT 1206
C020.1 uF(smt) black stripe (smt) black stripeSMT 1206

Top of the Board

Local Oscillator Top View

Install JP1 Bridge loop

Mount a small wire loop to bridge the x4 holes, holes 1 and 2, of JP1. (This will result in each crystal frequency being divided by four in the clocking of the QSD and QSE circuits. The center frequency resulting from the x4 jumper will be approximately the crystal frequency divided by four, or 7.056 MHz)

JP1shunt wire (cut-off lead)Cutoffhorizontal
band-specific - see chart. Use a cut-off resistor lead to shunt between holes 1 & 2 of JP1

Install Transistors

Install the transistors, Q1 and Q2. Use the body shape silk screen marking to help in proper placement of each transistor.

Caution: be sure to carefully observe the markings on the transistors and distinguish the 3904 (Q1) from the 3906 (Q2) Getting these mixed up could damage the transistors and cause excessive load on the voltage regulator!

  • Q1 2N3904 (next to X1)
  • Q2 2N3906 (left of top mounting hole)

Q12N3904 NPN Transistor2N3904 2N3904TO-92
Q22N3906 PNP transistor2N3906 2N3906TO-92

Install Resistors

Install resistors R1 through R5 and R7. Resistors are typically mounted on the board in a hairpin fashion with the body of each resistor located over its silkscreen circle. This is important because the test points (red dots on the schematics) are at the tops of the hairpins, are based on the assumption that this installation technique was used.

Normally, you will only need R6 for the 30m rig. R6 in the SoftRock Lite kit is only needed if the clock signal from Q2 to U2 does not result in proper clocking of U2. A value of R6 between 12k and 22.1k may be used if needed.

R0110 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WN-S
R0210 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4WW-E
R0310 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4WN-S
R04499 1/4W 1%y-w-w-blk-br y-w-w-blk-br1/4WW-E
R051 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4WE-W
R07499 1/4W 1%y-w-w-blk-br y-w-w-blk-br1/4WS-N
R0622.1k 1/4W 1%r-r-brn-r-br r-r-brn-r-br1/4W
only used in 30m Kit

Install Ceramic Capacitors

Note: C4 is only used in the 160, 30, and 20m kits

C03100 pF 5%101 101Ceramic
band-specific - see chart
C0522 pF 5%22J 22JCeramic
C04100 pF 5%101 101Ceramic 
band-specific - see chart. Used only in 160, 30, and 20m kits

Install Crystal

Mount the HC49 crystal mounting in the upper right corner of the board, mounting it vertically to the board. A small plated-through hole in the lower left corner of the crystal mounting position (blue hole in illustration at right) provides a place for a grounding wire to be soldered to the metal crystal case. The grounding wire also provides additional mechanical support for the crystal. Make sure the crystal is mounted slightly above the board since a trace runs on top of the board from one of the crystal holes to the base of Q1. You can use a piece of cardboard or wire insulation between the bottom of the crystal and the board to get the desired standoff distance while mounting X1.

X128.224 MHz Crystal28.224 or 28.2C69Xtal
band-specific - see chart

Local Oscillator Completed Stage

Top of the Board

View of Completed Top

Bottom of the Board

View of Completed Bottom

Local Oscillator Testing

Visual Inspection

Test Setup

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

Current draw (using 9V battery)

Test Setup

  • connect a 1k ohm resistor in series with the positive power lead
  • apply 9 Vdc and measure the current draw with the limiting resistor in place
  • remove the current limiting resistor
  • apply 9 Vdc and measure the current draw without the limiting resistor

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Current draw with 9 Vdc and NO limiting resistormA10 - 1210.9_______
Current draw with 9 Vdc and 10k limiting resistormA< 96_______

Voltage Tests

Test Setup

  • Power up the board
  • measure the voltages with respect to ground
Voltage Tests

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
R1 hairpin leadVdc4.5 - 5.44.92_______
Collector of Q1/Emitter of Q2 (R2 hairpin)Vdc4.4 - 5.34.87_______
Base of Q1 (see bottomside view, above)Vdc2.1 - 2.62.34_______
Emitter of Q1 (R4 hairpin - s/b 0.7 V lesss than Q1 baseVdc1.4 - 1.91.65_______
Base of Q2 (R5 hairpin)Vdc4.4 - 5.34.87_______
Jumper lead of JP1Vdc2.9 - 3.53.21_______

LO Output

Test Setup

  • The frequency of the LOs output should be 4 times the desired center frequency 7.056 MHz, or 28.224 MHz.
  • Check this if you have a good frequency counter and/or scope, or a receiver you can tune into 28.224 MHz.
  • Scope measurements may be taken IF you have a high quality, calibrated scope with correctly compensated probes
  • Note: if your kit is the 20m kit, this is a little different. The Upgraded 20m SoftRock Lite kit uses 1/3 sub-harmonic sampling to give 20m receive function. The center frequency is approximately 3 * 18.73 / 4 = 14.047 MHz. The loss in sensitivity associated with the 1/3 sub-harmonic sampling, about 3 or 4 dB, is made up by 5x gain, (compared to the 40m SoftRock Lite), in the I / Q audio stage where a low-noise LT6231 op-amp is used in lieu of the TVL2462CD opamps

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Radio tuned to 28.224 MHz, short wire antennareceivesYes or No (want YES)Yes_______
Scope - probe on Jp1 jumper leadMHz (approx)28.22427.78_______
p_p voltage (scope) - square waveVp-p3.3n/a_______