Local Oscillator Introduction


Note: The crystals included with the Lite II Combined kit are:

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.

Sub-harmonic Sampling

Alan, G4ZFQ points out that on the 30m, 20m, 15m receivers, the Local Oscillator produces a signal that is 4/3 times the desired center frequency as opposed to the 4x the center frequency output for the lower band models.

"Subharmonic" works like this:- The LO outputs a 13.5MHz signl that goes to the dividers /4, resulting in 1 3.375MHz square wave ( rich in odd harmonics) being fed to the mixer. At the mixer, a strong 3rd harmonic is present on the clock inputs, along with the fundamental of 3.375 MHz. The 3.375 fundamental multiplied by 3 yields the third harmonic of 10.125MHZ. The Bandpass filter (BPF) performs the essential function of severely attenuating any signals centered around the 3.375MHz fundamental frequency and first harmonmic, but allows 30m signals centering around the third harmonic of the 3.375MHz LO output. The result is that the mixer is dealing with signals in the passband, centering on 10.125MHz, as though the dividers were passing a fundamental frequency of 10.125 to the mixer. BPFs are all that stop Softrocks from working on unwanted frequencies!

(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

(above schematic has clickable areas that can be used for navigation)

(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)

11 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W
210 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4W
110 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4W
122.1 k 1/4W 1%r-r-brn-r-br r-r-brn-r-br1/4W
2475 1/4W 1%y-v-grn-bl-br y-v-grn-bl-br1/4W
122 pF 5%22J 22JCeramic
10.01 uF(smt) (smt)SMT 1206
12N3904 NPN Transistor2N3904 2N3904TO-92
12N3906 PNP transistor2N3906 2N3906TO-92

Band Specific Items for 160m Band

C10330 pF 5%331 331Ceramic  Local Oscillator
C11180 pF 5%181 181Ceramic Not used at all for 40m & 15m versionsLocal Oscillator
X17.3728 MHzECS-73-18-4XENXtal  Local Oscillator

Band Specific Items for 80m Band

C10180 pF 5%181 181Ceramic  Local Oscillator
C11100 pF 5%101 101Ceramic Not used at all for 40m & 15m versionsLocal Oscillator
X114.089 MHzABLS-14.08919MHZ-10-J4YXtal  6_10_2010Local Oscillator

Band Specific Items for 40m Band

C10100 pF 5%101 101Ceramic  Local Oscillator
C11unused capacitorUnused Not used at all for 40m & 15m versionsLocal Oscillator
X128.224 MHzHC49US-28.224MABJ-UBXtal  Local Oscillator

Band Specific Items for 30m Band

C10180 pF 5%181 181Ceramic  Local Oscillator
C11100 pF 5%101 101Ceramic Not used at all for 40m & 15m versionsLocal Oscillator
X113.5 MHzHC49US-13.500MABJ-UBXtal  Local Oscillator

Band Specific Items for 20m Band

C10180 pF 5%181 181Ceramic  Local Oscillator
C11100 pF 5%101 101Ceramic Not used at all for 40m & 15m versionsLocal Oscillator
X118.73 MHz18.730 1108Xtal  Local Oscillator

Local Oscillator Summary Build Notes

Local Oscillator Detailed Build Notes

Bottom of the Board

Local Oscillator Bottom View

Install SMT cap

See hints on installing SMT Caps.

C200.01 uF(smt) (smt)SMT 1206 

Top of the Board

Local Oscillator Top View

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. This can help prevent the two plated holes beneath the crystal from shorting out on the crystal case.

Install Crystal photo
160m7.3728 MHz (Xtal)ECS-73-18-4XEN
80m14.089 MHz (Xtal)ABLS-14.08919MHZ-10-J4Y
40m28.224 MHz (Xtal)HC49US-28.224MABJ-UB
30m13.5 MHz (Xtal)HC49US-13.500MABJ-UB
20m18.73 MHz (Xtal)18.730 1108

Install Ceramic Capacitors

See Band-specific Capacitors chart for value.

See hints on identifying and installing Ceramic Capacitors.

Install Ceramic Capacitors photo
C1222 pF 5%22J 22JCeramic 
160m330 pF 5% (Ceramic)331
80m180 pF 5% (Ceramic)181
40m100 pF 5% (Ceramic)101
30m180 pF 5% (Ceramic)181
20m180 pF 5% (Ceramic)181
160m180 pF 5% (Ceramic)181
80m100 pF 5% (Ceramic)101
40munused capacitor (Unused)
30m100 pF 5% (Ceramic)101
20m100 pF 5% (Ceramic)101
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.

Install transistors photo
Q12N3904 NPN Transistor2N3904 2N3904TO-92 
Q22N3906 PNP transistor2N3906 2N3906TO-92 

Install Resistors

See hints on installing and orienting resistors

Install Resistors photo
R1110 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WW-E
R14475 1/4W 1%y-v-grn-bl-br y-v-grn-bl-br1/4WE-W
R17475 1/4W 1%y-v-grn-bl-br y-v-grn-bl-br1/4WE-W
R151 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4WN-S
R1210 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4WE-W
R1310 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4WE-W
R1622.1 k 1/4W 1%r-r-brn-r-br r-r-brn-r-br1/4WN-S

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
(measurements courtesy of Leonard KC0WOX)

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
With the 1k limiting resistormA< 97.3_______
Without current limiting resistormA< 2014.1_______

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. Per Alan, G4ZFQ, This voltage (at R17) is not critical and can vary a lot, partly depending on the crystal. The important thing is that the LO's RF output is a good healthy signal and is detectable on an external RX (or counter or scope).

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
R11 hairpinVdc4.5 - 54.9_______
R15 hairpinVdc< R11 hairpin4.7_______
R12 hairpinVdc< 2.52.3_______
R17 hairpinVdc> 2.04.2_______

LO Output Test

Test Setup

  • On the 3 lower bands, the frequency of the LOs 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:
X1160m7.3728 MHz
X1 80m1489 MHz
X1 40m28.224 MHz
X1 30m13.5 MHz
X1 20m18.73 MHz
X1 15m28.06 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.
<a name=LO Output Test"
Link to Power Supply Stage Link to Dividers Stage