The uSDX is not a great receiver or transmitter but it is usable and amazingly small. I bought mine through AliExpress. "uSDR uSDX 10/15/17/20/30/40/60/80m 8 Band SDR All Mode HF SSB QRP Transceiver Compatible with uSDX QCX-SSB".
Here's a recording of how a fairly week signal on 40m sounds:
The first unit I bought arrived with a broken screen. The AliExpress vendor provided free shipping for the return and refunded my payment.
I previously built the same circuit but this fully built version is more compact.
Manuel, DL2MAN who is the original designer of the uSDX sandwich contacted me with some comments about this (and similar) clone of his design. He has a video commentary and makes the following points:
- His design is used but there is no credit to him at all
- The license on the design was for non-commercial use only
- He was never asked about a commercial license
- The selling price is way above the actual cost (he thinks it would cost about US$40-$50) particularly as they are not supporting the developer
- No effort was made with documentation, they've added no value for their margin
- There are some changes to the schematic:
- Output transistors have been changed but other parts of the circuit have not been adjusted
- Harmonic suppression will be worse than the original design (and way below legal requirements)
- Firmware upgrading is not easy as it was in the original design
- The battery charging circuit is noisy
- A physical design problem can short circuit the battery in the version with a battery