This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.
Charging forward with my project to build VK3YE’s ‘Beach 40’ 40m DSB transceiver, today I tackled the diode ring mixer. The mixer serves as the product detector, converting RF signals down to audio. It also serves as the balanced modulator, turning audio from the microphone into a double-sideband signal at the appropriate frequency.
This is a particularly simple diode ring arrangement, with only one output transformer to wind. No trifilar windings here:
The local oscillator signal is provided by the VFO I built, which has an available power of at least +7 dBm, as required for good linearity in a diode ring. In receive mode, HF signals from the antenna get converted down to baseband (audio) and pass out the audio port. In transmit mode, the signal flow is reverse, and signal from the microphone travels in through the audio port and modulates the local-oscillator, creating a double-sideband suppressed-carrier HF signal, which is then routed to an RF amplifier stage before ending up back at the antenna.
Two balance controls are provided for nulling out the carrier at the RF port – a 200-Ohm trimpot (which also serves as the LO injection point) and a ~40pF trimcap, which balances the nominally 22pF cap on the either side of the ring. No attempt was made to match the 4 diodes, they were just the first four 1n4148s I pulled from the bag.
The following picture pretty well illustrates my process of building one of these little modules. First, I re-draw the schematic by hand, just to make sure I’ve got a handle on it. Then, I’ll identify the different networks of attachment within the circuit, as each one of these will want to be its own isolated pad. I use that to make a rough sketch of the pad layout (bottom-left in this picture). Much erasing and re-doing happens at this stage! Once that’s mostly sorted, I’ll do a more precise sketch of the pad layout using the actual component sizes (bottom right). Then I’ll duplicate that design in pencil on the copper clad, and have at it with the Dremel, followed by the soldering iron.
This balance pot, by the way, came out of the mysterious silver radio I got at the DeKalb Hamfest back in May. Huzzah for re-using old parts!
Upon testing, I’m encountering a bit of difficulty totally balancing this mixer. From my understanding of how the diode ring works, with no signals present at the audio port, there should (ideally) be nothing appearing at the RF output port port. I was expecting to see a little feedthrough of the LO, but the lowest I can seem to acheive by tweaking both the balance pot and the trimmer capacitor still leaves about 50 mV P-P of LO signal at the RF port, which seems like a bit much. Then again, the LO signal is at around 4V P-P going into the mixer, so proportionally, that’s not too shabby. (not quite 40 dB). Some of that could also just be radiating straight from the LO, or from the 12″ clip-lead connecting the VFO and the mixer at this point.
For the time being, I’m going to note the balance issue and move on. From my reading, I understand the proper termination of the mixer is essential for good balance. Since the surrounding components are not in place yet, I can’t imagine I have proper termination on this thing. I’ll return to this down the road as the transceiver takes shape.
Hear you on the air!