Quadrature Sampling I/O Introduction


This stage completes the circuitry for input of the RXTX V6.3's Quadrature Sampling Detector (QSD) results and the Quadrature Sampline Exciter (QSE) results into the PTT selection stage.

It is always a good idea to double check the bill of materials in this set of documentation with the official BOM on the Mobo site. Discrepancies may be introduced inadvertently, as the changes are sometimes not communicated or discovered, making timely update of this site a problem.

Theory of Operation

The RXTX V6.3 still is responsible for the basic SDR hardware functions of quadrature sampling detector (QSD) and quadrature sampling exciter (QSE).

The Softrock's QSD hardware receives its RF signals, switched from the antenna at P17 via the select logic of U8, through transformer T3 and thence to plug K4 (in the place of the output from the RXTX's original bandpass filter board). These signals then appear at the Softrock;s Mixer stage to be converted down to the baseband I and Q signals that are fed to the PC's soundcard.

The Softrock's QSE stage upconverts the transmit I and Q signals from the PC's soundcard into RF. These QSE outputs from the Softrock are introduced to the MOBO via P19 and transformer T4 into the PTT select logic (U8) and on to the driver transistor's base (Q6).

The design provides for the insertion of a pre-amp/attenuator board into the line from P17 (antenna). However, that functionality is not covered in these notes.

(go directly to build notes)

Quadrature Sampling I/O Schematic

(Click for Full Schematic)
Quadrature Sampling I/Oschematic


(go directly to build notes)

Quadrature Sampling I/O Bill of Materials

Stage Bill of Materials

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

1BN-43-2402 (no markings!)none noneBinocular core
1BN-43-2402 (no markings!)none noneBinocular core
1Magnetic Wire, enameled #30Magnetic
10.1 uFSMT 0805

Quadrature Sampling I/O Summary Build Notes

Quadrature Sampling I/O Detailed Build Notes

Bottom of the Board

Quadrature Sampling I/O Bottom View

Install Bottomside Capacitor

C270.1 uFSMT 0805

Top of the Board

Quadrature Sampling I/O Top View

Wind and Install Transformers

Both transformers, T3 and T4, are wound with #30 wire onto BN43-2402 binocular cores. You can calculate the wire-length required for a given number of turns using this handy calculator (tx to Dohn N8EWY)


For a binocular core, each pass where the wire ends up at the same side where it started the pass counts as a turn

Exercise care in winding the binocular cores to direct the wire into each core hole so that the hard material of the core does not scrape off portions of the wire’s enamel insulation. (It may be advisable to lightly spin with one’s fingers a small drill bit in each of the binocular core hole openings to put a smooth chamfer at both ends and to remove sharp edges that may cut through the enamel insulation of a wire.)

The builder may want to review the materials provided in the build notes for the PA/Filter stage of the RXTX V6.3 (concerning winding and installing binocular transformers).

Transformer T3 - QSD RF Input

T3 (50 to 50 ohms) = P6 S3x2: There are two windings, primary (P) and secondary (S).

  • Primary (P) is 6 turns single #30 wire (~7").
  • Secondaries (S) are 3 turns #30 (~7"), bifilar .
  • There are various ways of winding, but it may be easier to do the following:
      Do the bifilar winding first. After doing the 3 turns bifilar secondaries, cut the leads short, to about 1/2". Use an ohmmeter to identify which leads are paired and use a piece of colored insulation or other means to identify and distinguish the sets of secondary windings.
    • Then do the Primary, leaving the leads long, say 1". This way, you can tell which is Primary and which is Secondary.
  • Another way is to use red wire for Primary and green wire for Secondaries, or vice versa.
Transformer T4 - QSE RF Output

T4 (50 to 100 ohms) = P4 S3x2: There are two windings, primary (P) and Secondary (S).

  • Primary is 4 turns single #30 wire (~5").
  • Secondaries are 3 turns #30 (7"), bifilar.
Installing the Transformers

The builder may wish to review the transformer installation instructions in the Softrock HF BPF kit to see graphically how binocular transformers are wound and installed.

When you have completed the windings you will have - for each transformer - one set of primary leads and two sets of secondary leads. There will, thus, be three wires coming out of each side of the binocular core: one primary lead (P) and two secondary leads (S1 and S2). There should be continuity between the S1-S1 leads and also continuity between the S2-S2 leads.

Carefully remove the insulation from all leads up to within an eighth of an inch from the core (to avoid shorts) and lightly tin the leads

Mount the transformer and its leads according to the graphic below. The leads should allow for the transformer to "stand off" from the board just a tiny amount.

Solder the leads from the bottom of the board

Wind and Install Transformers photo

Quadrature Sampling I/O Completed Stage

Top of the Board

View of Completed Top

Bottom of the Board

View of Completed Bottom

Quadrature Sampling I/O Testing

T3 Continuity

Test Setup

Board does not need power for this test. This tests the soldering of the leads of T3 by testing for continuity at points other than the immediately soldered leads.

T3 Continuity

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
A to BOhm0TBD_______
C to DOhm0TBD_______

T4 Continuity Test

Test Setup

Similarly, test the continuity for T4

T4 Continuity Test

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
A to BOhm0TBD_______
C to DOhm0TBD_______

Test Program Requirements

Test Setup