The SignaLink USB is one of the most common external sound card interfaces for digital modes in ham radio. It has a lot of good features and overall is a very useful piece of equipment. It also has a high noise floor as shipped from the factory which can inhibit the sound card’s ability to decode week signal stations as effectively as it should.
Due to the SignaLink’s use of unregulated USB power to drive its amplifier circuits, not only is the internal noise floor higher than it could be, it also varies from computer to computer. Fortunately, there is a very simple mod that resolves this limitation which was printed in the August 2014 issue of QST magazine.
The mod involves removing one surface mount resistor from the circuit board and installing a new resister between two other connection points on the board.
The results of comparing the noise floor before and after performing this mod are pretty striking, with between a 10 and 30 dB lower noise floor depending on how noisy the 5V USB power supply is. As you can see in the comparisons shown below, the low frequency noise is greatly reduced as is a significant birdy at around 1000 Hz. This also demonstrates how different two 5V power sources can be with the PC much cleaner than the USB hub.
Test 1 – Shack PC Power
Test 2 – USB Hub Power
This is about as easy of a mod as can be done on a piece of electronics and I highly recommend doing this if you use a SignaLink USB for low signal work.