Previous installments of this series (1, 2 and 3) have documented a frequency wobble I'd noticed with an SDR-1000 radio when trying to receive QRSS with it. When I last worked on this I'd determined that the case fan seemed to be the cause of the frequency wobble as it went away when the case fan was not running.
I Got back to working on it this weekend and it looks like the case fan I've got in the radio is putting some noise on the power lines, as adding 3200 uF of capacitance to the main 12V feed to the radio, and a 1000 uF capacitor to the power input to the fan has smoothed out the wobble quite a bit as can be seen in the screen capture below.
There's still a bit of wobble but it is considerably better than before adding the capacitors as can been seen from a picture taken from before I installed the capacitors.
Both pictures are of WWV at 15 MHz and a 0.17 Hz resolution in spectran.
Before I tried adding the capacitors I did try mounting the fan on some rubber washers to give it some isolation from mechanical vibration -- that did not seem to help at all.
I'd really like to get rid of the remaining wobble, so will add some more capacitors at strategic locations on the SDR boards themselves next time I have the radio apart. Hoping a few caps on the SDR boards themselves will totally damp out the wobble. Also need to do some experiments driving it with an external reference oscillator and see how it does. Would be nice to have it driven with a temperature compensated oscillator as the 200 MHz oscillator in the radio currently seems pretty sensitive to temperature.
Comments to: Bill Tracey (kd5tfd@ewjt.com)
Last Updated: 3 October 2004
Copyright © Bill Tracey 2004