Tom,

 

John had also asked me to adjust the power supply voltage slowly over its entire input range while watching for spurs.  I couldn't see anything with my spectrum analyzer but the SDR-IQ's noise floor is much lower and it's very fast.  I'll try that test this weekend.  Those spurs might be switching noise from the supply.

 

One of the few pieces of test equipment I actually purchaed new was a low-cost tektronix oscilloscope.  It's about 15 years old now and not functioning all that well.  I'll give the SDR-IQ a try with a scope probe on the PLL loop and report back this weekend.

 

73,

 

Juan

 

On 6/7/07, Tom Clark, K3IO <K3IO@verizon.net> wrote:

Juan -- a couple of thoughts that came to my mind based on your blow -by-blow report:

1. You might command a move off 435.000. As I recall, the synthesizer reference is at 10.000 MHz. I could conceive an internal spur generated by the reference oscillator.
2. The 6.8 kHz spur interval is certainly weird, but it sounds like it might be due to the synthesizer chip's RC low-pass filter with a time constant ~ 1/6.8 kHz = ~140�s. Is it possible that something inside the loop is oscillating? Try a scope probe on the VCO's tuning voltage.
3. On topic #2, is it possible that you can see microphonic on the tuning voltage?
4. I'm interested to see that you also find the IQ as a very useful piece of test equipment. Here is another test you might want to try: Remember that the IQ does down to (nearly) DC. You could try using it to look at the spectrum of the synthesizer tuning voltage over the ~0 -20 kHz range. If the chip is locked, there should be very little noise; but with the spectrum you are showing, I wouldn't be surprised to see noise that mimics the 10.7 MHz power spectra you show here

73, Tom