CGW glass-dielectric capacitors

[Fred Bartoli]

Le Sun, 04 Nov 2007 08:45:21 -0800, Tom Bruhns a écrit:

OK, I just answered at least part of the questions I just posted, via
the AVX website. I suppose with a high (+140ppm/C) but stable
temperature coefficient, they could be useful in thermal compensation,
but unfortunately, it's more often that a negative temperature
coefficient is appropriate for that (esp. in an LC circuit).

I'm scratching my head, wondering why I'd want to use a glass cap
instead of a C0G ceramic. Though I don't see a voltage coefficient
listed in the AVX data sheet for C0G caps, my distortion measurements
tell me that it cannot be very large; I suppose even in the glass caps,
it's not truly zero, just vanishingly small.

I did measure it 2 or 3 months ago for some C0G because I had some
concerns about this.
It was for 100pF/500V leaded Philips C0G caps and Johanson's 33p/3kV
leaded C0G.
At 0V and 500V for the Philips one and 0V/3kV for the Johanson one the
change was under the resolution of my 4275A, which is 1 out of 5 digits.

 

[John Larkin]

Le Sun, 04 Nov 2007 08:45:21 -0800, Tom Bruhns a écrit:


I did measure it 2 or 3 months ago for some C0G because I had some
concerns about this.
It was for 100pF/500V leaded Philips C0G caps and Johanson's 33p/3kV
leaded C0G.
At 0V and 500V for the Philips one and 0V/3kV for the Johanson one the
change was under the resolution of my 4275A, which is 1 out of 5 digits.

I have around here, somewhere, a paper where they make a lumped L-C
delay line using some sort of ceramic caps. The caps are sufficiently
nonlinear that they get a shock-line effect that dramatically sharpens
the risetime of an edge as it propagates through the line. It runs at
something like the 2KV level.

John

 

[Fred Bartoli]

Le Mon, 05 Nov 2007 07:43:52 -0800, John Larkin a écrit:

I have around here, somewhere, a paper where they make a lumped L-C
delay line using some sort of ceramic caps. The caps are sufficiently
nonlinear that they get a shock-line effect that dramatically sharpens
the risetime of an edge as it propagates through the line. It runs at
something like the 2KV level.

Certainly not C0Gs.

I've once had the idea of using some Hi-K ceramics to toy with this
but... too much of productive work. Maybe one day.

Hi-K ceramics have some funny behaviour, like ESR dependance on voltage,
time relaxation of capacitance and ESR after temperature or voltage
stress,...

 

[[email protected]]

Consider a 7-pole 40 MHz lowpass filter, 50 ohms or so, running at
levels up to maybe 2 volts RMS. Using C0G caps, do you have any guess
as to how much THD the caps might generate on their own?

We're doing a new DDS synthesizer/arb: lookup table, 14-bit
diff-current-out dac, diffamp, lowpass filter, output amp. It's
clocked at 128 MHz and max output will be half Nyquist, 32 MHz.

We're seeing complex patterns of harmonic distortion versus frequency
and amplitude, but I hadn't thought about the caps as culprits. We'd
like to hold -60 dB to 10 volts p-p, which is optimistic. Commercial
RF signal generators and arbs seem to have distortion specs above
-40dB, some as bad as -20.

John

I haven't done the experiment, but I would guess that inductors are
less ideal than capacitors. Are you attempting to position the
inductors so that they don't couple?

I recall in the dark ages the sine source used on HP ATE (as opposed
to bench instrumentation) was around the -60db THD point. It was a
pain that the bench instruments were not as good, i.e. they required
passive filters to reach the same level. I was in design, not test, so
I don't recall exactly the model of ATE, but I believe it was the HP
9480.