[eagle] Re: New 70 cm Receiver Test Results

John B. Stephensen kd6ozh at comcast.net
Sat Jun 9 06:42:57 PDT 2007

Since the noise is always symmetrical around the received signal, the switcher noise must be modulating the VCO control voltages. There are two switchers - one on the receiver PCB and one on the CAN-DO module. We must determine which is causing the problem or whether both are. Can you cut the trace on the PCB and vary the DC voltage to the receiver independently of the voltage to the CAN-Do module? 

I hope that you can tap each component and find the microphionic one. Good candidates for microphonic components are the capacitors in the PLL feedback loop. The capacitors in the integrator are film types so they shouldn't be microphonic but perhaps SMT parts aren't as good as leaded parts. The ceramic capactors in the feedback path might also cause problems and could be changed to film capacitors. Another possibility may be the 4.7 and 10 uF bypass capacitors on the outputs of the linear regulators. These are ceramic because of concerns about tantalums in space but they might have good piezoelectric properties and could be switched to 1206 size tantalum capacitors. There could also be a cold solder joint somewhere. 


  ----- Original Message ----- 
  From: Juan Rivera 
  To: John B. Stephensen ; eagle at amsat.org 
  Cc: Bill Ress ; Dave Black (Home) ; Dave Black (Work) ; Dave hartzell ; David Smith ; Don Ferguson ; Juan. Rivera (Home) ; Juan.Rivera (Work) ; Samsonoff at Mac. Com 
  Sent: Friday, June 08, 2007 03:45 UTC
  Subject: New 70 cm Receiver Test Results

  Re: 70 cm Prototype Receiver Testing


  John, and group,


  I set up my shop this evening to take a look at the microphonic issue that I had noticed before Dayton.  I decided to use my new SDR-IQ software defined radio instead of my HP 8566B spectrum analyzer.  The SDR-IQ has a lower noise floor, is quiet, and doesn't heat up my shack.  Of course it tops out at 30 MHz and the 8566B goes up to 22 GHz.  But for this application all I needed was something to look at the IF output at 10.7 MHz.


  Before I got to the microphonics I noticed spurs and phase noise that I had not previously seen with the 8566B - a classic case of going to drain the swamp and then ending up fighting alligators...


  I've posted screen shots and test details on EaglePedia's 70 cm receiver page.  Click here for a link to that top level page.


  Then go down to the Project Status area and open up Troubleshooting Resumes.  I wouldn't get discouraged.  This is why we built the prototype in the first place - to find all the little flaws that need attention.


  More to follow in a few days...


  P.S. I add new material to the end of these logs so if you've already read the earlier posting just scroll down to the new material at the end.  I always try to note the date and time I last modified the log at the top so you can see if there is anything new since your last visit.







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