Mixer Introduction


From the builder's standpoint, the Mixer stage consists solely of the installation of the input resistors (R1 and R2) and the integrating capacitors, C5 and C6. The installation of the IC, U3, was performed in the Dividers Stage.

Theory of Operation

The RF input signal, filtered by the BPF is applied in antiphase to the inputs 1B and 2B of the Mixer U3.

The two LO signals from the Divider Stage operate the switches which connect R1 to C5 and connects R2 to C6 during the first clock cycle.

When the LO Clock changes 90 degrees later, the connections reverse: R1 now connects to C6 (Q) and R2 connects to C5 (I).

This switching sequence then repeats itself.

The resulting RF input signal is sampled over capacitors C5 and C6 as the Intermediate Frequency (IF)

On the lower bands (160m, 80m, and 40m) the dividers are clocked at the desired center frequency, which is in the pass band for the incoming RF.

On the higher bands, the situation in the Softrocl RX is a little different. The clocking frequency is at the one-third sub harmonic of the desired center frequency and is NOT within the passband of the incoming RF.

For example, consider the 20m RX:

  • For 20m, the dividers are clocked at about 18.73 MHz and their output QSD clock is 18.73 MHz / 4 = 4.682 MHz
  • The third harmonic of that clock frequency is 3 * 4.682 = 14.047 MHz.
  • The 20m signals in the BPF's passband will be sampled at that 3rd harmonic; however, the sampling will not yield as strong an I/Q pair as does the sampling technique used in the lower bands. Hence, the higher gain OpAmps for the higher band kits.
  • It is like looking at a rotating wheel with a strobe flashing once for every three revolutions of the wheel. The rotation speed of the wheel is down converted but the image is not as bright as it would be if you flashed the strobe at the rotation speed of the wheel.

If you are interested, you might want to review the "Tayloe Mixer" operation. While the Softrock mixer is not a pure Tayloe mixer, the theoretical discussion on Taylo mixers helps with understanding how this process works.

(go directly to build notes)

Mixer 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)

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

(go directly to build notes)

Mixer Bill of Materials

Stage Bill of Materials

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

210 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4W
20.047 uF 5%473 473Ceramic

Mixer Summary Build Notes

Mixer Detailed Build Notes

Bottom of the Board

Mixer Bottom View

Top of the Board

Mixer Top View

Install Topside Components

Install R1 and R2

See hints on orienting and installing resistorss.

Install integrating capacitors, C5 and C6

See hints on identifying and installing Ceramic Capacitors.

Install Topside Components photo
R0110 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WW-E
R0210 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WW-E
C050.047 uF 5%473 473Ceramicvert
C060.047 uF 5%473 473Ceramicvert

Mixer Testing


Test Setup

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

Visual Inspection

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 Mixer 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 100 ohm current-limiting resistor is removed
  • The mixer stage should not appreciably change the current draw from preceding stages.

Apply 12 Vdc to the board for this test

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
With the 100 ohm current limiting resistormA< 3026.1_______
Without the current limiting resistormA< 3026.4_______

Voltage Tests

Test Setup

Power up the board and measure the pin voltages with respect to ground (on the pins, not the pads) of U3, per the table below

Voltage Tests

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Pin 14 (QSD clk_0 on topside)Vdc2.52.47_______
Pin 2 (QSD clk_1 on topsode)Vdc2.52.47_______
Pins 3 & 12Vdc2.3 - 2.52.44_______
Pins 6 & 11Vdc2.3 - 2.52.44_______
Pin 7 (1A)Vdc2.3 - 2.52.44_______
Pin 9 (2A)Vdc2.3 - 2.52.44_______
Pin 16Vdc54.94_______
Pin 8Vdc00_______
Pins 1 & 15Vdc00_______
Go to BPF Stage Go to Dividers Stage Go to Power Supply Stage Go to OpAmp Stage View FST 3253 Datasheet