[eagle] Re: New 70 cm Receiver Test Results

[Grant Hodgson]

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