Discrete Transceiver Mute Circuit Analysis and Troubleshooting

Fixing a persistent problem

As Project Yamhill nears actual physical implementation, it’s time to start building some more real-time, two-way communication. We’re starting to get some beta testing and discussions going! If you’d like to participate in Project Yamhill chat, please click on the link below. Thank you!

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Party Line 80 Rev A PCB with plenty of bodges

I’ve mentioned my new Party Line 80 direct conversion CW transceiver a few times now in this publication. The main impetus behind this rig is to leverage some existing design work that I’ve previously done, along with some other well-known circuits to create an inexpensive and fun to build all-discrete component transceiver. Although I do plan to sell it at some point, I’d like to also use it in some promotions for this Substack publication (hopefully I’ll have some more details on this in the coming month or two).

I created the Rev A PCB back when I was submitting all of my Project Yamhill boards to the manufacturer, knowing that the design was pretty close to being complete, but still had a few minor flaws that needed to be corrected. I’ve become a big adherent of the fail fast, iterate fast school of design, so I reckoned it would be best to get some kind of initial design down, knowing it would need some corrections later.

The Problem

For a first attempt, this design wasn’t the worst effort I’ve turned out. It was ‘usable’, but the flaws meant that it wouldn’t be pleasant to use. So I definitely needed to correct all of the remaining problems in order that this rig would be something that ops would actually enjoy using, and not just a toy to build and then let sit in a box, never to be touched again. Besides, I would like to leverage the basic design to put the rig on other bands later, and there’s no way this would be a viable commercial product if it couldn’t meet the basic operating characteristics that a modern ham would expect.

Party Line 80 initial mute circuit

The most glaring remaining problem with the transceiver was a popping impulse noise heard at the beginning of every keydown period. This was an issue that I had fought from the beginning (I even detailed the problem in another post on this publication), and I had improved the problem about 80% from where it initially started. However, there was still a bit of this noise on every transmission, and it was prominent enough to bother me. I knew it would be fatiguing to use for real QSOs and wouldn’t be acceptable to leave in this state.

I started this radio with a mute circuit that I’ve used before, which you can see in the schematic above. It consists of a series switch element and a shunt switch element (both common 2N7000 MOSFETs). The series switch is held on during receive with a 100 kΩ pullup resistor to +12 VDC. On transmit, the keyed +12 VDC rail turns on the shunt switch, and the series switch is turned off by being pulled to ground by the key line. There is a 10 μF capacitor on the series pullup resistor that gives the series switch a bit of a delay to turn back on the audio when the key is released.

Characterizing the Issues