Output Transformer for QRP Labs Transceivers: QDX, QDX-M and QMX

Contents

• 1. Introduction & Background
• 2. RWTST for 12V Transceivers
• 3. WTST for 9V Transceivers
• 4. Conventional Wound Transformer Assembly
• 5. Document Revision History

1. Introduction & Background

The QDX, QDX-M, and QMX transceivers utilize a push-pull pair of BS170 transistors that drive an output transformer wound with 0.60mm enameled wire on a ferrite binocular core. The transformer is wound with a 3:2 turns ratio for 12V supply operation and a 3:3 ratio for 9V supply operation.

The introduction of high-band QDX units (20m, 17m, 15m, 12m, 11m, and 10m) in December 2022 revealed a problem in approximately 30% of 9V units and a smaller number of 12V units, exhibiting low output power and poor efficiency on the 10m band. Investigations indicated that the low power issue was related to the binocular core. It is proposed that in some cases, due to component tolerances (ferrite material), a low-Q parasitic resonance affects the 10m output power performance.

After extensive experimentation, Ross EX0AA designed the WTST (Weird Twisted Sisters Transformer) for the 9V case, which effectively eliminated the parasitic resonance and restored proper operation across all bands from 20m to 10m, with improved efficiency. The WTST method also provides improved performance on low-band QDX units (80m, 60m, 40m, 30m, and 20m) due to better coupling and symmetry.

Ross EX0AA later developed the RWTST (Really Weird Twisted Sisters Transformer) for 12V supply operation, featuring a 3:2 ratio. This design also eliminates parasitic resonance and improves performance on both low-band and high-band versions. These transformer designs comprehensively solved the issue.

This document provides instructions for building one of three transformer types:

1. WTST for 9V operation (3:3 ratio)
2. RWTST for 12V operation (3:2 ratio)
3. Conventional transformer for 9V (3:3 ratio) or 12V (3:2 ratio)

Recommendation: It is strongly recommended to use WTST (9V) or RWTST (12V) for high-band transceivers. These improved transformers are also recommended for low-band transceivers to enhance PA efficiency. The conventional transformer is easier to build and suitable for frequencies below 10m or 12m, with the only drawback being the potential parasitic resonance affecting 10m performance.

Summary of Recommendations:

• These transformers are compatible with QDX, QDX-M, and QMX.
• For 12V operation, the RWTST is recommended.
• For 9V operation, the WTST is recommended.
• The conventional transformer can be used if preferred.

Important Points Regarding Transformer Installation

Regardless of the transformer winding method, the following points are critical:

• Use only the specified amount of wire, as the 0.60mm wire is also used for other inductors in the kit.
• When the transformer is wound, bend the wires to align with the PCB holes.
• Cut the wires so approximately 1cm protrudes towards the PCB.
• Scrape the enamel from this 1cm section. Heating to remove enamel is difficult; scraping or sanding is preferred.
• Do not tin the wires before installation, as this makes it difficult to insert the thick 0.60mm wire through the PCB holes.
• Using a drill to twist the wires is necessary for an even twist; hand twisting will not yield the same result.
• The tightness of the twists affects coupling and symmetry. Tighter twists generally lead to better performance, provided they remain neat.
• When de-burring the binocular core, be gentle as the ferrite is brittle. Sharp edges can damage the wire enamel.
• All transformer methods use 0.60mm enameled copper wire.

2. RWTST for 12V Transceivers (“Really Weird Twisted Sisters Transformer”)

Only for 12V operation!

Summary in pictorial form:

[Diagram showing the winding process for RWTST]

Step 1: Cut approximately 25cm of 0.6mm copper enameled wire, fold it in two, and twist the top 2cm. Place each leg of the wire hairpin into each hole in the binocular core.

Step 2: Fold each of the two wires protruding from the other end of the binocular across the opposite hole.

Step 3: Cut another 20cm (approx) of wire and lay it equidistant across the holes as shown.

Step 4: Clockwise twist each side of the wire laying across to each side of the two wires coming out of the binocular core, laying them across the other hole.

Step 5: Loosely twist each side. Place one end in an electric drill chuck and firmly hold the other ends of the two wires.

Step 6: Carefully squeeze the drill trigger to create a neat twisted pair. This can be done by hand without a vise or other fixing. Best performance is achieved with tight and symmetrical twists.

Step 7: Push one side through so that the wire laying across and twisted goes through the hole it was laying across the center, and NOT the hole it came through. Note that some force may be required, and enamel may be scraped off. This is not a critical issue if bare copper touches the ferrite, but it is important to avoid bare copper touching other bare copper wires. Proceed slowly.

Step 8: Pull the twisted pair tight all the way through the binocular.

Step 9: Push the second twisted pair through the hole it was laying at over.

Step 10: Carefully pull tight.

Step 11: Slide one twisted pair back through the opposite hole, away from the twisted hairpin loop.

Step 12: Slide the other twisted pair back through, away from the twisted hairpin loop, and pull both tight.

Step 13: Untwist the two pairs at the opposite end of the binocular.

The bottom twisted hairpin loop is the primary, connecting to the BS170 transistor drains. Two of the top four wires are the secondary winding; the other two, with continuity to the twisted loop, are twisted together to form the center tap.

Step 14: Cut the twisted hairpin loop and untwist the few turns outside the transformer core. Scrape the enamel from the ends of all wires for testing with a DVM in continuity mode. Identify the two wires on the right-hand side with continuity between them; these are the SECONDARY. The remaining two wires in the middle are the primary center tap and should be twisted together.

Testing: Use a DVM. Points labeled 'P' (primary) should show continuity. Points labeled 'S' (secondary) should show continuity. There should be NO continuity between 'P' and 'S'. If there is, a mistake has been made, or there are shorts.

The Really Weird Twisted Sisters Transformer is now complete and ready for installation in your QDX / QDX-M / QMX PCB.

Remember: RWTST transformers are for the 12V Supply option build of QDX, QDX-M, and QMX transceivers.

3. WTST for 9V Transceivers (“Weird Twisted Sisters Transformer”)

Step 1: Cut two lengths of wire: one 20cm, one 24cm approx.

[Image showing wire lengths measured with a ruler]

Step 2: Make about a 2cm hairpin twist in the middle of the longer wire.

[Image showing the hairpin twist on the longer wire]

Step 3: Use a small electric drill on slow mode. Insert one end of the wire into the chuck and the other end, gripped by small pliers. Hold the trigger for short periods until a tight, even twist is obtained (approx. 3 twists per cm).

[Image showing the drill and pliers setup for twisting wire]

Step 4: Bend the twisted pair into a hairpin with a small 2cm twist at the top and two legs.

[Image showing the bent twisted pair]

Step 5: Pass each leg through the binocular core. Wind a neat, tight transformer half-turn at a time (crucial for symmetry) on each side, until each wire pair passes back through the binocular twice, resulting in 1 1/2 turns per pair. Ensure the half-turns inside the holes are as symmetrical as possible for best performance. Each pass of a twisted pair through a binocular hole counts as half a turn.

• Initial hairpin pass: ½ turn
• Passing each pair back through opposite holes: 1 turn total
• Passing twisted pairs back again through holes: 1.5 turns

When joined, this creates a 3:3 transformer with excellent symmetry and coupling.

[Image showing the result at 1 turn]

[Image showing the result at 1.5 turns]

Step 6: Untwist all wires outside the binocular core.

Step 7: Cut the hairpin loop.

[Image showing wires after cutting the hairpin loop]

Step 8: Scrape the wire enamel from all wire ends for testing with a DVM in continuity mode. Identify the two wires on the right-hand side with continuity between them; these are the SECONDARY. The remaining two wires in the middle are the primary center tap and should be twisted together.

Testing: Use a DVM. Points labeled 'P' (primary) should show continuity. Points labeled 'S' (secondary) should show continuity. There should be NO continuity between 'P' and 'S'. If there is, a mistake has been made, or there are shorts.

The Weird Twisted Sisters Transformer is now complete and ready for installation in your QDX / QDX-M / QMX PCB.

Remember: WTST transformers are for the 9V Supply option build of QDX, QDX-M, and QMX transceivers. Do not use with 12V supply, it will fry the output transistors!

4. Conventional Wound Transformer Assembly

Remember that the wire may also be used for other inductors in the transceiver; use only the necessary amount.

The transformer has two windings. The primary is always 3 turns, center-tapped. The secondary is 3 turns for 9V supply operation, or 2 turns if building your transceiver for 12V operation.

In the nomenclature of binocular cored transformers, “1 turn” means the wire passes through both sets of holes and ends up back where it started.

Assembly is best done in the following steps. Please read all steps before commencing.

Step 1: Carefully unwrap the 0.6mm (AWG #22) wire and straighten it, ensuring no kinks.

Step 2: Pass the wire through both holes, starting at the top left. This is the first turn of the 3-turn primary winding.

[Image showing Step 2: wire passing through holes]

Step 3: Pass the wire through the top hole of the binocular core, from back to front (left to right). This is the next half-turn of the primary winding, bringing the total turns to 1.5. Now, create the center tap.

[Image showing Step 3: wire passing through top hole for center tap]

Step 4: Pass the wire back through the bottom hole of the binocular core, from right to left. Do not pull it tight. Leave a small loop as shown in the photograph; this will be soldered to the center-tap pad on the PCB.

[Image showing Step 4: wire passing through bottom hole with loop]

Step 5: Wind the wire through both the top and bottom holes of the binocular core, pulling it slightly tight. This forms the final turn of the 3-turn primary winding. The wire should exit the bottom left hole, with about 1cm protrusion.

[Image showing Step 5: final turn of primary winding]

Step 6: Squeeze the center-tap wires together to avoid confusion.

Step 7: Start the 3-turn secondary winding by pushing the wire from right to left through the bottom hole, then from left to right through the top hole. This is the first turn.

[Image showing Step 7: first turn of secondary winding]

Step 8: Push the wires through both holes two more times to create the second and third turns. For the secondary, wind only 2 turns if building for 12V operation (3:2 ratio). Cut the wire, leaving about 1cm spare.

[Image showing Step 8: completing secondary turns]

Step 9: Bend all wires to point downward, aligning them for insertion into the PCB holes.

[Image showing Step 9: wires bent for PCB insertion]

Step 10: Cut the center-tap loop. The wires formed by cutting the loop should be longer than the ends of the secondary winding to prevent mix-ups. Twist these center wires together tightly for clarity.

[Image showing Step 10: center tap loop cut and twisted]

The transformer is now ready for installation in the PCB!

Ensure the number of secondary turns matches your supply voltage:

• 2 turns (3:2 ratio) for 12V supply operation
• 3 turns (3:3 ratio) for 9V supply operation

Warning: Do not use a 9V transformer (3:3 turns ratio) with a 12V supply, as this will likely damage the PA transistors.

5. Document Revision History