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Microphonic film capacitor

I'm developing a frequency synthesizer.

I was worried about ceramic capacitor microphonics, so I used these film capacitors in my loop filter:

Panasonic ECPU1C105MA5 1uF, 16V, 1210, PPS (Farnell 969-4315) AVX CB017D0223JBA 22 NF, 50V, 1206, PEN (Farnell 165-7961)

The 1uF is microphonic. Tapping it with a plastic rod produces FM on the VCO equivalent to ~ 20uV amplitude on the control voltage.

Looks like I'll have to try a ceramic or a tantalum. Leakage shouldn't be a problem as I'm using an AD9901-style PFD which behaves like an XOR gate PD when locked.

Any comments or advice?

TIA Andrew.

Reply to
Andrew Holme
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Don't tap it with a plastic rod?

John

Reply to
John Larkin

Sno-o-o-ort ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
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Reply to
Jim Thompson

Ceramics can be FAR WORSE... If you can find a "film" capacitor where the plastic sheet has been coated by sputtering and/or depositon, that constuctionshould be the least sensitive.

Reply to
Robert Baer

Has anyone compared through-hole film caps vs. SMT as far as microphonics goes?

Reply to
Spehro Pefhany

If possible, avoid having any substantial DC voltage across the cap-- that will minimize voltage fluctuations as a result of transient changes in capacitance.

Reply to
Spehro Pefhany

the

t be a

PD

Thanks for the tip. Unfortunately, these are loop filter integrator caps, and need to have between 1 and 15V across them.

The previous version of my board had through hole filter components (Poly Layer caps) and that is microphonic also I discovered last night.

My options seem to be:

  1. Tantalums
  2. Re-design filter with smaller caps (could try NPO/COG) and larger resistors
  3. Miniature aluminium electrolytics
Reply to
Andrew Holme

"Spehro Pefhany" wrote

Just tried this. At the bottom end of my tuning range, where the cap voltage is small, I see no detectable microphonics. At the top end, where I have almost 15V on the cap, hitting the board produces a large frequency deviation spike. I'm very dissappointed with this Panasonic film cap; it looks nice, with rainbow colours as it catches the light; but it doesn't work.

Reply to
Andrew Holme

Dow Corning do some nice glass caps but they may not have the values you want.

--
Best Regards:
                     Baron.
Reply to
Baron

...

LOL!

("Doctor, it hurts when I do this." "So, don't do that.")

Cheers! Rich

Reply to
Rich Grise

I've now tried:

  1. Film 1210 16V (the original)
  2. Tantalum 0603 16V
  3. Ceramic X7R 1210 50V
  4. Radial aluminium electrolytic 63V (15 years old)

Guess which: a. was better than the film but still a bit microphonic. b. can hear a pin drop next door. c. has no microphonics whatsoever.

Reply to
Andrew Holme

Good idea..the SMTs are more likely to be made by sputtering or depositionof the metallization.

Reply to
Robert Baer

Are they still being made??

Reply to
Robert Baer

Betcha the X7R was the worst, with the Olde Sprague being the best...

Reply to
Robert Baer

Correct :-)

Reply to
Andrew Holme

he

be a

D

Green Xenon will think you have a condenser microphone.

G=B2

Reply to
stratus46

As far as I know ! Someone posted a link to the manufacturers data sheets recently. I belive it was AVX. I have a copy of the pdf if you want it.

--
Best Regards:
                     Baron.
Reply to
Baron

SMT caps have a dreadful reputation in audio. Try substituting with a polypropylene or even polyester type with leads temporarily.

SMT resistors can be pretty crappy too.

Graham

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Reply to
Eeyore

Serious audio uses through hole components for all passives. WE KNOW there's a difference.

Graham

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Reply to
Eeyore

Aluminium electrolytics can be the most microphonic of all.

Graham

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Reply to
Eeyore

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