Softrock Lite II 02_Local Oscillator
Local Oscillator Introduction
Theory of Operation
The Local Oscillator stage implements a basic Colpitts Crystal Oscillator with a buffer stage to increase the signal level. The oscillator produces a signal that is at the crystal's specified fundamental frequency.
See the table in the lower right-hand corner of the schematic below for the frequencies produced by this stage, for the appropriate band/kit.
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.
Local Oscillator Schematic
(Resistor testpoints (hairpin, top, or left-hand lead) are marked with red dots)
(Click for Full Schematic)
(above schematic has clickable areas that can be used for navigation)
Local Oscillator Bill of Materials
Detailed Bill of Materials
| Check | Designation | Component | Marking | Category | Orientation | Notes | Circuit |
|---|---|---|---|---|---|---|---|
| [____ ] | C10 | band-specific | Local Oscillator | ||||
| [____ ] | C11 | band-specific | Not used at all for 40m & 15m versions | Local Oscillator | |||
| [____ ] | C12 | 22 pF 5% | 22J | Ceramic | Local Oscillator | ||
| [____ ] | C20 | 0.01 uF |
 | SMT 1206 | Local Oscillator | ||
| [____ ] | Q1 | 2N3904 NPN Transistor | 2N3904
 | TO-92 | Local Oscillator | ||
| [____ ] | Q2 | 2N3906 PNP transistor | 2N3906
| TO-92 | Local Oscillator | ||
| [____ ] | R11 | 10 ohm 1/4W 1% | br-blk-blk-gl-br
 | 1/4W | W-E | Local Oscillator | |
| [____ ] | R12 | 10 k 1/4W 1% | br-blk-blk-r-br
 | 1/4W | E-W | Local Oscillator | |
| [____ ] | R13 | 10 k 1/4W 1% | br-blk-blk-r-br
| 1/4W | E-W | Local Oscillator | |
| [____ ] | R14 | 475 1/4W 1% | y-v-grn-bl-br
 | 1/4W | E-W | Local Oscillator | |
| [____ ] | R15 | 1 k 1/4W 1% | br-blk-blk-br-br
 | 1/4W | N-S | Local Oscillator | |
| [____ ] | R16 | 22.1k 1/4W 1% | r-r-blk-r-br
 | 1/4W | N-S | Local Oscillator | |
| [____ ] | R17 | 475 1/4W 1% | y-v-grn-bl-br
| 1/4W | E-W | Local Oscillator | |
| [____ ] | X1 | band-specific | Local Oscillator |
Band Specific Items for 160m Band
| Check | Designation | Component | Marking | Category | Orientation | Notes | Circuit |
|---|---|---|---|---|---|---|---|
| [____ ] | C10 | 330 pF 5% | 331 | Ceramic | Local Oscillator | ||
| [____ ] | C11 | 180 pF 5% | 181 | Ceramic | Not used at all for 40m & 15m versions | Local Oscillator | |
| [____ ] | X1 | 7.3728 MHz | Xtal | Local Oscillator |
Band Specific Items for 80m Band
| Check | Designation | Component | Marking | Category | Orientation | Notes | Circuit |
|---|---|---|---|---|---|---|---|
| [____ ] | C10 | 180 pF 5% | 181 | Ceramic | Local Oscillator | ||
| [____ ] | C11 | 100 pF 5% | 101 | Ceramic | Not used at all for 40m & 15m versions | Local Oscillator | |
| [____ ] | X1 | 14.06 MHz | Xtal | Local Oscillator |
Band Specific Items for 40m Band
| Check | Designation | Component | Marking | Category | Orientation | Notes | Circuit |
|---|---|---|---|---|---|---|---|
| [____ ] | C10 | 100 pF 5% | 101 | Ceramic | Local Oscillator | ||
| [____ ] | C11 | not used | unused | Not used at all for 40m & 15m versions | Local Oscillator | ||
| [____ ] | X1 | 28.224 MHz Crystal | 28.224 or 28.2C69 | Xtal | Local Oscillator |
Band Specific Items for 30m Band
| Check | Designation | Component | Marking | Category | Orientation | Notes | Circuit |
|---|---|---|---|---|---|---|---|
| [____ ] | C10 | 180 pF 5% | 181 | Ceramic | Local Oscillator | ||
| [____ ] | C11 | 100 pF 5% | 101 | Ceramic | Not used at all for 40m & 15m versions | Local Oscillator | |
| [____ ] | X1 | 13.5 MHz | Xtal | Local Oscillator |
Band Specific Items for 20m Band
| Check | Designation | Component | Marking | Category | Orientation | Notes | Circuit |
|---|---|---|---|---|---|---|---|
| [____ ] | C10 | 180 pF 5% | 181 | Ceramic | Local Oscillator | ||
| [____ ] | C11 | 100 pF 5% | 101 | Ceramic | Not used at all for 40m & 15m versions | Local Oscillator | |
| [____ ] | X1 | 18.73 MHz | Xtal | Local Oscillator |
Band Specific Items for 15m Band
| Check | Designation | Component | Marking | Category | Orientation | Notes | Circuit |
|---|---|---|---|---|---|---|---|
| [____ ] | C10 | 100 pF 5% | 101 | Ceramic | Local Oscillator | ||
| [____ ] | C11 | not used | unused | Not used at all for 40m & 15m versions | Local Oscillator | ||
| [____ ] | X1 | 28.06 MHz | Xtal | Local Oscillator |
Local Oscillator Summary Build Notes
- Install SMT cap
- Install Crystal
- Install Ceramic Capacitors
- Install transistors
- Install Resistors
- Test the Stage
Local Oscillator Detailed Build Notes
Bottom of the Board
Top of the Board
Install Crystal
See Band-specific Components chart for value.
Mount the HC49 crystal mounting in the upper left 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 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. 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.
| Check | Designation | Component | Marking | Category | Orientation | Notes |
|---|---|---|---|---|---|---|
| [____ ] | X1 | band-specific |
Install Ceramic Capacitors
See Band-specific Capacitors chart for value.
| Check | Designation | Component | Marking | Category | Orientation | Notes |
|---|---|---|---|---|---|---|
| [____ ] | C12 | 22 pF 5% | 22J | Ceramic | ||
| [____ ] | C10 | band-specific | ||||
| [____ ] | C11 | band-specific | Not used at all for 40m & 15m versions |
Install transistors
Mount the two transistors being careful to orient them according to the pattern in the silkscreen.
Take care not to get 2N3904 and 2N3906 mixed up. Carefully check the last digit.
| Check | Designation | Component | Marking | Category | Orientation | Notes |
|---|---|---|---|---|---|---|
| [____ ] | Q1 | 2N3904 NPN Transistor | 2N3904
| TO-92 | ||
| [____ ] | Q2 | 2N3906 PNP transistor | 2N3906
| TO-92 |
Install Resistors
| Check | Designation | Component | Marking | Category | Orientation | Notes |
|---|---|---|---|---|---|---|
| [____ ] | R17 | 475 1/4W 1% | y-v-grn-bl-br
| 1/4W | E-W | |
| [____ ] | R16 | 22.1k 1/4W 1% | r-r-blk-r-br
| 1/4W | N-S | |
| [____ ] | R15 | 1 k 1/4W 1% | br-blk-blk-br-br
| 1/4W | N-S | |
| [____ ] | R14 | 475 1/4W 1% | y-v-grn-bl-br
| 1/4W | E-W | |
| [____ ] | R13 | 10 k 1/4W 1% | br-blk-blk-r-br
| 1/4W | E-W | |
| [____ ] | R12 | 10 k 1/4W 1% | br-blk-blk-r-br
| 1/4W | E-W | |
| [____ ] | R11 | 10 ohm 1/4W 1% | br-blk-blk-gl-br
| 1/4W | W-E |
Local Oscillator Completed Stage
Top of the Board
Bottom of the Board
Local Oscillator Testing
Visual Check
Test Setup
Using very good lighting and magnification, carefully inspect the solder joints to identify bridges, cold joints, or poor contacts.Current Draw
Test Setup
- connect a 1k ohm resistor in series with the positive power lead
- apply 12 Vdc and measure the current draw with the limiting resistor in place
- remove the current limiting resistor
- apply 12 Vdc and measure the current draw without the limiting resistor
Test Measurements
| Testpoint | Units | Nominal Value | Author's | Yours |
|---|---|---|---|---|
| With the limiting resistor | mA | < 12 | 1.8 | _______ |
| Without the limiting resistor | mA | <4 | 1.8 | _______ |
Voltage Tests
Test Setup
- Power the board
- Measure the testpoint voltages with respect to ground
Note that some of the voltages measured may have ac components, which, depending upon your DMM, may average in with the dc voltages to produce higher apparent dc voltages than theory would suggest.
Author measured the dc voltage at R17 using a scope and got ~2.6 Vdc
Test Measurements
| Testpoint | Units | Nominal Value | Author's | Yours |
|---|---|---|---|---|
| R11 hairpin | Vdc | 4.5 - 5 | 4.9 | _______ |
| R15 hairpin | Vdc | < R11 hairpin | 4.7 | _______ |
| R12 hairpin | Vdc | < 2.5 | 2.3 | _______ |
| R17 hairpin | Vdc | > 2.5 | 4.2 | _______ |
LO Output Test
Test Setup
- On the 3 lower bands, the frequency of the LO’s output should be 4 times the desired center frequency e.g., 28.224 MHz for a desired center frequency of 7.056 MHz).
- If your kit is the 30m, 20m, or 15m kit, this is a little different. The higher band SoftRock Lite kits use 1/3 sub-harmonic sampling to give receive function. The center frequency is approximately 3 * XtalFrequency / 4 in 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 lower band SoftRock Lite kits), in the I / Q audio stage where a low-noise LT6231 op-amp is used in lieu of the TVL2462CD opamps
- The crystal frequency is band-specific, as follows:
| Designation | Band | Frequency |
|---|---|---|
| X1 | 160m | 7.3728 MHz |
| X1 | 80m | 14.06 MHz |
| X1 | 40m | 28.224 MHz |
| X1 | 30m | 13.5 MHz |
| X1 | 20m | 18.73 MHz |
| X1 | 15m | 28.06 MHz |
- You can use a ham receiver tuned to the appropriate crystal frequency. You should hear the LO's frequency.
- Scope measurements may be taken IF you have a high quality, calibrated scope with correctly compensated probes
- Note: 1/3 sub-harmonic sampling does reverse the spectrum. Changing the audio cable connections to the SoftRock Lite circuit board from tip to ring and ring to tip will correct the reversed spectrum so that the SDR software works the same for the higher band receivers as with the lower band receivers. (See Cecil K5NWA'a explanation of the sub-harmonic sampling in his message on the Yahoo Softrock group.
