[eagle] Re: Source of 70 cm Microphonics Found

John B. Stephensen kd6ozh at comcast.net
Tue Jul 17 12:45:59 PDT 2007


I'll move the switching regulator inside the shield for the CAN-Do widget in the next revision. The amplitude is low enough so this should eliminate the problem. One thing to do before the parts arrive is to measure the third-order intercept point for 1 MHz or wider tone spacings.

73,

John
KD6OZH

  ----- Original Message ----- 
  From: Juan Rivera 
  To: 'John B. Stephensen' 
  Cc: eagle at amsat.org ; 'Bill Ress' ; 'Dave Black (Home)' ; 'Dave Black (Work)' ; 'Dave hartzell' ; 'David Smith' ; 'Don Ferguson' ; 'Juan.Rivera (Work)' ; 'Samsonoff at Mac. Com' 
  Sent: Saturday, July 14, 2007 23:44 UTC
  Subject: RE: Source of 70 cm Microphonics Found


  John and all,

   

  As soon as I get my presentation written I'll get started on replacing those caps as well as the parts you think are associated with the ripple.  Here's my list of discrepant items so far:

   

  1) 5 kHz radiated and conducted EMI susceptibility - EMI source is the CAN-Do switching power supply.  No further action needed from me at this time.  Amount of receiver filtering and shielding required is TBD pending top-level EMI review and possible modifications to CAN-Do module and enclosure.  I stand ready to make any modifications or do any testing that the CAN-Do team requests.

   

  2) Excessive ripple in the passband - I'll order parts next week and attempt to dig deeper into this.

   

  3) 157 kHz spur - caused by Receiver switching power supply.  Do you want to try to push the switching frequency higher to get it out of the passband?  Or modify the circuitry to attempt to attenuate this spur? IF we end up with a two-compartment enclosure, with the digital circuitry separated from the analog, then this problem will probably go away with filtering.

   

  4) Microphonics - I'm fairly certain that swapping those 5 ceramic chip capacitors for film will fix this one.  I'll get parts on order next week.

   

  5) Master oscillator frequency jumps - No further action needed at this time.  Revise design on next rev to use analog TCXO instead of digital.

   

  6) Lack of autonomy - No further action needed at this time.  Revise both local oscillators on next rev to fixed tune receiver.

   

   

  Have I missed anything?

   

  73, Juan

   

   

   

  -----Original Message-----
  From: John B. Stephensen [mailto:kd6ozh at comcast.net] 
  Sent: Saturday, July 14, 2007 2:57 PM
  To: juan-rivera at sbcglobal.net
  Cc: eagle at amsat.org; Bill Ress; Dave Black (Home); Dave Black (Work); Dave hartzell; David Smith; Don Ferguson; Juan. Rivera (Home); Juan.Rivera (Work); Samsonoff at Mac. Com
  Subject: Re: Source of 70 cm Microphoncs Found

   

  This is what we suspected earlier but couldn't prove. Those capacitors, plus 

  C54, need to be replaced with film capacitors similar to C23, C24, C52 and 

  C53.

   

  73,

   

  John

  KD6OZH

   

  ----- Original Message ----- 

  From: "Juan Rivera" <juan-rivera at sbcglobal.net>

  To: "John B. Stephensen" <kd6ozh at comcast.net>

  Cc: <eagle at amsat.org>; "Bill Ress" <bill at hsmicrowave.com>; "Dave Black 

  (Home)" <dblack1054 at yahoo.com>; "Dave Black (Work)" 

  <dblack at mail.arc.nasa.gov>; "Dave hartzell" <hartzell at gmail.com>; "David 

  Smith" <w6te at msn.com>; "Don Ferguson" <kd6ire at sbcglobal.net>; "Juan. Rivera 

  (Home)" <juan-rivera at sbcglobal.net>; "Juan.Rivera (Work)" 

  <Juan.Rivera at gd-ais.com>; "Samsonoff at Mac. Com" <samsonoff at mac.com>

  Sent: Saturday, July 14, 2007 18:18 UTC

  Subject: Source of 70 cm Microphoncs Found

   

   

  > John,

  > 

  > The microphonics is caused by C19, C27 and C28 (.047uF 25V) and C50 and 

  > C51

  > (.022uF 25V).

  > 

  > I found the best way to isolate this is to inject a CW signal into the

  > receiver, tune the signal in at the IF output using my TS-2000 in AM mode,

  > and then listen to the audio output while lightly tapping each component 

  > one

  > at a time.

  > 

  > According to my research, barium titanate, the base ceramic material for

  > most ceramic SMD capacitors is inherently microphonic.

  > 

  > Juan

  > 

  > 

  > 

  > 

   
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