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#1 2009-02-17 08:26:54

Registered: 2009-01-15
Posts: 1

Capacitor C35 value

The value of C35 is marked as 47pF on BOM, but the Digikey part list specify a 100pF capacitor. Which is the correct value to use? Can anyone enlighten how this value is calculated?


#2 2009-02-17 09:13:26

Registered: 2008-10-20
Posts: 35

Re: Capacitor C35 value

47pF is the correct one becasue it's used for HF antenna.


#3 2009-02-17 11:26:47

Registered: 2008-05-21
Posts: 416

Re: Capacitor C35 value

If there are fixes that need to be made to the part-list, please let's try to sum them up so I can fix them. It will avoid problems for future users wink


#4 2009-09-16 08:05:45

From: Montreal
Registered: 2009-09-15
Posts: 14

Re: Capacitor C35 value

Hmm. Does everyone concur that the correct value for C35 is 47pF (50pF on the schematic?) and not the 100pF listed on the various BOMs?
Also, I've noticed that C37/C38 on the BOMs I've seen are 100uF capacitors whereas the schematics indicated 220uF?

See … ted-v1.pdf as I'm checking things over as I'm considering building one or a few PM3s.

The various parts lists I've seen also have the parts count off by two resistors.
R22, R27, R28, R29, R45, R46 make 6 resistors not 4.

Anyone notice any other discrepancies/fixes?


#5 2009-09-16 12:18:40

Registered: 2009-08-07
Posts: 3

Re: Capacitor C35 value

depends on the antenna impedance, i guess--should be resonant with the tuning cap, which means (ignoring antenna parasitic capacitance, which is significant) that 2*pi*(13.56 Mhz) = 1/sqrt(Lant*Ctuning). in theory you can design for whatever impedance you want, but 47 pF seems better.

C37/C38 don't matter much one way or the other, just bulk decoupling. either should be fine.

if you do R22, R27, R28, R29, R45, R46 all 27 ohm like that bom then you won't be able to almost-open-circuit the antenna (to sense the carrier without loading it too much); would be better to let R45, R46 be 10k. but unless you really cared about the modulation depth when simulating a tag, shouldn't matter.


#6 2014-02-09 10:08:53

From: France
Registered: 2010-06-15
Posts: 444

Re: Capacitor C35 value

I suppose i have the same problem.
According to this thread i can try to make an antenna in order to make it resonant at the right frequency or i can change the capacitor. there is no difference apart the fact that i have to buy/find a 47pF capacitor while i still have the wire to make an inductance.
could we agree on the shape of an adeguate antenna for the 100pF capacitor?
supposing i use a square shape, using the info found on the 6° post of this thread , i should make an antenna where a=2,5cm, b=0,1cm c=0,1cm and Number of turns=3,31. Correct? the diameter of the wire makes difference? i did'n find it the linked datasheet with formula (i read it very fast).
if my problem is not related with the C35 capacitor, could someone share its reference RF measures with an oscilloscope in order to compare my measures with yours?

Last edited by gaucho (2014-02-09 10:25:33)

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