Mixer Introduction

Theory of Operation

This is the "Quadrature Sampling Detector" (QSD) stage of the radio. The filtered RF from the BPF stage is clocked through the mixer against the two quadrature signals from the divider stage. The output of the mixer is two audio frequency signals representing the difference between the incoming RF and the clocking signals.

In the most of the kits, the clocking signals are at the desired center frequency. In the 20m and 15m kits, the clocking signals are at one-third of the desired center frequency. With the incoming RF having been filtered to reject out-of-band signals, the RF in the pass-band reacts to and is sampled against the third harmonic of the clocking signals, resulting in audio frequency outputs that represent the difference between the third harmonic (which is the desired center frequency) and the filtered, incoming RF. This technique is called "1/3 sub harmonic sampling". It causes us to need a higher gain opamp stage in the kits using this feature. In addition, because of the odd harmonic sampling, the I and Q signals are reversed, necessitating a reversal of the I and Q connections in the final stage.

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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)
Mixerschematic

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Mixer Bill of Materials

Stage Bill of Materials

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

CheckDesignationComponentMarkingCategoryOrientationNotesCircuit
U4FST3253 mux/demux switchFST3253 FST3253SOIC-16Mixer
C100.1 uF(smt) black stripe (smt) black stripeSMT 1206Mixer
R0810 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WW-EMixer
R0910 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WS-NMixer
R1610 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4WE-WMixer
R171 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4WE-WMixer

Mixer Summary Build Notes

Mixer Detailed Build Notes

Bottom of the Board

Mixer Bottom View

Install U4

CheckDesignationComponentMarkingCategoryOrientationNotes
U4FST3253 mux/demux switchFST3253 FST3253SOIC-16
Take ESD precautions

Install SMT Cap

CheckDesignationComponentMarkingCategoryOrientationNotes
C100.1 uF(smt) black stripe (smt) black stripeSMT 1206

Top of the Board

Mixer Top View

Install Resistors

CheckDesignationComponentMarkingCategoryOrientationNotes
R0810 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WW-E
R0910 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WS-N
R1610 k 1/4W 1%br-blk-blk-r-br br-blk-blk-r-br1/4WE-W
R171 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4WE-W

Mixer Completed Stage

Top of the Board

View of Completed Top

Bottom of the Board

View of Completed Bottom

Mixer Testing

ESD

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.

Resistance Measurements

Test Setup

With power OFF, measure the resistance WRT (with respect to) ground at each of the secondary leads of T1 (see graphic below).

Resistance Measurements

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Point 1 to groundohm700 - 800790_______
Point 2 to groundohm700 - 800790_______
Point 3 to groundohm700 - 800790_______
Point 4 to groundohm700 - 800790_______

Current Draw

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 1k ohm current limiting resistormA< 96.2_______
Current draw without the limiting resistormA25 - 3530.8_______

U4 pin voltages

Test Setup

Power up the board and measure the pin voltages (on the pins, not the pads) of U4, per the table below

Make sure you have enabled the shunt wire between the "/ENRX" terminal and ground, as directed in the Power Supply Stage. If that has not been done, you will get incorrect voltages in the Pin1/Pin15 test and the mixer will not work.

U4 pin voltages

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Pin 14 (S0)Vdc2.3 - 2.42.29_______
Pin 2 (S1)Vdc2.3 - 2.42.31_______
Pin3 & 12Vdc2.3 - 2.42.34_______
Pin 6 & 11Vdc2.3 - 2.42.34_______
Pin 7 (1A)Vdc2.3 - 2.42.34_______
Pin 9 (2A)Vdc2.3 - 2.42.34_______
Pin 16 (Vcc)Vdc54.94_______
Pin 8 (gnd)Vdc00_______
Pins 1 and 15mVdc400-500450_______

Mixer Output - Scope Test

Test Setup

The 40m SoftRock Lite voltage gain from RF volts in to I or Q volts out should be about 11. Thus a 50 mV RMS RF input should give a p-p audio volts output on the I and Q outputs of about 1.6 volts.

  • Inject an RF input frequency of 7.042 MHz at 50 mv amplitude. This level makes it easier to look at with a scope and it's not overloading our board. With a center frequency of 7.056 MHz, this should cause a 14 kHz audio frequency at the tip/ring outputs of the board.
  • Connect a scopeís probes to the ring and to the tip pads on the board and setup the scope for X-Y view, triggering on Channel 1.
  • Measure the frequency and p-p voltage. With a 50mV p-p input RF signal you should see a 1.6 V p-p output at an audio frequency that is equal to the difference between the RF frequency and the LO Center frequency.
  • The signals at the ring and tip outputs should be 90 degrees out of phase with one another.
  • The image below shows the waveform that results from feeding a 50 mV 7/042 MHz signal into the receiverís antenna terminals.
  • The I / Q outputs should be clean sine waves if the RF input is not too high. The frequency of the sine waves should be the difference between the center frequency of the SoftRock and the RF input frequency. Phase relationship between the two sine waves, which one is leading, depends on if the RF input is above or below the SoftRock center frequency.
Mixer Output - Scope Test