Theory of Operation

The mixer stage acts like two traditional direct conversion mixers operating in tandem. Each takes in half of the filtered RF from the bandpass filter stage and one of the quadrature center frequency signals, then "mixes" them, with an output being the traditional mixer products, in this case, two audio frequency signals that represent the difference between the two inputs (RF and Local Oscillator). These two signals are referred to as the I (in-phase) and Q (Quadrature) signals and are fed into the high gain Op-Amps stage for amplification and delivery to the audio outputs (and, thence, to the PC's sound card). Resistors R11 and R12 form a voltage divider to produce approximately 50 mV dc at pins 1 and 15 to enable the mixer's operation when the /RXEN is grounded.


This is a subset of the overall schematic. Note: red dot indicates resistor testpoints (hairpin, top, or left-hand lead)
06 Mixer

Bill of Materials

C220.01uFSMT 1206
C230.01uFSMT 1206
Lead wireconnectorInstall a short, stout wire from a cut-off lead between the hole marked /RXEN and the ground hole to its left

Summary Build Notes

  • Install SMT IC U7 (bottom)
  • Install 2 SMT capacitors (bottom)
  • Install 4 resistors
  • Install ground strap for /RXEN
  • Test the Stage

Detailed Build Notes

Bottom of the Board

Install U7


Install SMT Capacitors

Dotted lines are for R11 leads
Watch out for solder splash when soldering C22 - the pads are very close to the holes for R11's leads! You might want to insert R11's leads into their holes prior to installing C22.
C220.01uFSMT 1206
C230.01uFSMT 1206

Top of the Board

Install Resistors


Install ground strap for RX Enable

Lead wireconnectorInstall a short, stout wire from a cut-off lead between the hole marked /RXEN and the ground hole to its left

Completed Stage


mixer topside


mixer bottomside


Note: Some tests in this stage require you to have built and plugged in at least one bandpass filter.

If you have not yet done so, you can still conduct the current and voltage tests provided you short pins 7, 8, and 9 of J1 together to provide the DC equivalent of the T100 secondaries.

Current Draw(DMM)

  • Current numbers here are for the CMOS version of the Si570.  You will need to adjust these up by about 14 mA for the LVDS version.
  • Power the board up (author has been using an 11.6 Vdc battery pack
  • Measure the current draw and 5 V rail voltage with a 1K Ω limiting resistor
  • Measure the current draw without the limiting resistor.
TestpointNominal ValueAuthor'sYours
Current Limited mA6-10 mA7.5 mA______
Current limited 5V rail1-2 Vdc971 mV______
Non limited draw mA90-100 mA97.4 mA______

Pin Voltages (DMM)

Measure U7 Pin Voltages

  • Using a DMM, measure the dc voltage (with respect to ground) of the pins of U7. 
  • It is best to test for these voltages at the actual pins (not the pads), thereby ensuring correct soldering of the pins to the pads.
TestpointNominal ValueAuthor'sYours
U7, Pin 165 Vdc 4.97 Vdc______
U7, Pin 80 Vdc  0 Vdc______
U7, Pins 1and 1550 mVdc  49 mVdc______
U7, Pin 22.5 Vdc  2.48 Vdc______
U7, Pin 142.5 Vdc  2.48 Vdc______
U7, Pin 72.5 Vdc  2.48 Vdc______
U7, Pin 92.5 Vdc  2.48 Vdc______

If the voltage at pins 1 and 15 is not in the area of 50 mV, then the mixer will not be enabled and there will be no outputs at pins 7 and 9.

If you see a high (~5 Vdc) voltage at pins 1 and 15, check your /ENRX to be sure it is grounded

Dividers STage OpAmps Voltage Divider OpAmps Stage BPF Stage