SDR: External power supply doesn't work

[tomr]

I have added the external 9v power supply. The SDR still works fine when the power supply is switched off. However nothing happens when I turn the power supply on - it has no effect on the noise at the center (IF) frequency.

I've triple-checked all the components and connections, but can find nothing wrong.

Has anyone else added the power supply and have it work properly?

Or can someone tell me how to troubleshoot the problem?

-Tom-

[sarma]

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.

[tomr]

Hello Sarma.

Thank you very much for your reply. I have not responded until now because I was expecting the forum software to send me a notification that never arrived.

I should have provided more detail in my original post. The circuit that I used is one I found on B. Kainka's website at 'http://www.b-kainka.de/sdrusb.html under the heading "External Power Supply". (I do not speak German, so I'm depending on a web translation service.) It specifies a 9-volt input. I am using a 9-volt battery as the DC source and was planning to add an external power supply at some point in the future.

I have had some success with the external supply but it seems to be limited to the lower HF frequencies , e.g. 75 meters. In most cases I see no change when switching to the external supply.

I was unable to obtain the Schottky diode (1N5818) and used the alternative 1N4004. Would I see significantly better results with the Schottky?


Anyway, thanks again for your reply.

-Tom-'