Juan
The VCO control line will be extreemly sensitive to noise; testing VCOs with a power supply often gives more data about the power supply noise than the VCO performance. If you want to open the loop and control the VCO directly, I would suggest controlling the VCO using a battery with a potentiometer (10 turn if possible, or 'padded down' with resistors to give a reduced VCO control range) and maybe a large value parallel capacitor or two.
regards
Grant G8UBN
Juan Rivera wrote:
John,
I’ll tackle that tomorrow. For this prototype we can actually do exactly what you suggest and just move the PCB back one set of standoffs and leave the Can-Do module where it is. Of course the SMA connectors will be inaccessible with the cover on but I don’t run it that way when testing anyway, so it won’t matter.
Getting the Can-do module desoldered won’t be much fun but it should be possible. That’s a job for next week. I think I’ll find tomorrow that U17 is also a contributor to the noise. Perhaps I should cut into the VCO control loop and control the VCO with an external supply if it starts to look like a culprit. Right now I’m out of variable supplies since all three are tied up. I don’t have an oscilloscope that I trust but I may be able to use the SDR-IQ with a scope probe to look at the VCO control loop since it goes down to 500 Hz. That will be an interesting test. This little radio is very useful.
73,
Juan