ELEKTOR FORUMS

Hai TomR,
Welcome to Elektor forum.
I feel that the USB is designed around 5V DC. I am confused when you said that you have supplied 9V external power supply? Please confine to 5V and if possible a linear power supply instead of modern SMPS like the one we use for Cellphone etc

While some of the PC's USB power supplies suffer from insufficient current capacity, they tend to be Noisy , many a time
Hope you might have gone thro' the "All About USB" where Mr Burkhard Kainka Has dealt with the Power filtering related issues with due emphasis.

While external powering of USB could be tried, due care is needed that the power supply under consideration is clean from Noise and spikes.
Also please check whether the RF inductors mentioned are of proper power rating to handle the needed milli amps of load. it might also be help if you can provide the voltage measurements at various points of power section like, before and after R1 and R4. Vcc_HF needs special care.

Hope to hear your results.
Sarma

PS: when external power is tried, the usb power needs to be isolated and a termination. You may use a nominal resistor and an LED . and then supply the external power at just the pint where the isolation is effected just after its pin 1.