If I am correct, they are nomenclatured as NPO types. Mouser list NPO caps.

ack Schmidling wrote:
I am in need of zero tc caps for my VFO project but can't seem to find
any info on them. Who makes them? What are they called?

Digikey has some with tc's of 200 ppm but that aint zero.

The also have some that are 0 +/- 500 ppm and that makes no sense.

I need two at 1000 pf.

Thanks,

js

I am in need of zero tc caps for my VFO project but can't seem to find
any info on them. Who makes them? What are they called?

Digikey has some with tc's of 200 ppm but that aint zero.

The also have some that are 0 +/- 500 ppm and that makes no sense.

I need two at 1000 pf.

Thanks,

js



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On May 20, 12:44 am, Jack Schmidling wrote:
I am in need of zero tc caps for my VFO project but can't seem to find
any info on them. Who makes them? What are they called?

Ceramic capacitors with nominal zero temperature coefficient are
usually called C0G nowadays, but sometimes the older term NP0 is
used. There's lots in the Digi-Key catalog (mostly surface mount mind
you...if you are looking for through-hole parts).

73,
Steve VE3SMA

"Jack Schmidling" wrote in message
...
I am in need of zero tc caps for my VFO project but can't seem to find any
info on them. Who makes them? What are they called?

Digikey has some with tc's of 200 ppm but that aint zero.

The also have some that are 0 +/- 500 ppm and that makes no sense.

I need two at 1000 pf.

Thanks,

js

Don't forget, a lot of the drift is caused by the inductor in the resonant
circuit, and it will have a positive coefficient. Most VFOs have some
compensation using a cap with a negative temp coefficient to compensate.

Pete

On May 19, 8:44 pm, Jack Schmidling wrote:
I am in need of zero tc caps for my VFO project but can't seem to find
any info on them. Who makes them? What are they called?

Digikey has some with tc's of 200 ppm but that aint zero.

The also have some that are 0 +/- 500 ppm and that makes no sense.

I need two at 1000 pf.

Thanks,

js

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Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silverhttp://schmidling.com


Look for C0G ceramic caps, or silvered-mica caps. They both have low
temperature coefficients. C0G are normally maximum 30ppm/C. As Uncle
Peter suggested, the inductor will normally have a positive
temperature coefficient of inductance -- it's easy to see why for an
air-core inductor, since it expands and gets bigger as the temperature
increases. You can use C0G for much of the total capacitance, and
combine that with a cap with a negative temperature coefficient--if
you can find one--to get a very low net temperature coefficient of
oscillator frequency. These days, it's fairly common to stabilize a
VFO with a phase or frequency locked loop, so it becomes mainly short-
term stability that you care about in the oscillator design.

Cheers,
Tom

On 20 mayo, 01:44, Jack Schmidling wrote:
I am in need of zero tc caps for my VFO project
I need two at 1000 pf.
Polystirene (styroflex) capacitors are a good choice. They have a
desirable slightly negative temperature coefficient, and excellent Q.
1000pF is within the spectrum of usual values.
Or you may choose a bunch of NP0 ceramics in parallel, plus ~1/20th of
the total value made up of a N750 (-750ppm/centigrade) capacitor. Many
paralleled caps are better than a single multilayer unit because heat
(yes, real life caps do heat somewhat in oscillators) is better
dissipated away, thus reducing one of the drift causes.
73s and DXs
Daniel Perez LW1ECP

Polystirene (styroflex) capacitors are a good choice. They have a
desirable slightly negative temperature coefficient, and excellent Q.
1000pF is within the spectrum of usual values.
Or you may choose a bunch of NP0 ceramics in parallel, plus ~1/20th of
the total value made up of a N750 (-750ppm/centigrade) capacitor. Many
paralleled caps are better than a single multilayer unit because heat
(yes, real life caps do heat somewhat in oscillators) is better
dissipated away, thus reducing one of the drift causes.
73s and DXs
Daniel Perez LW1ECP
==================================
I support the above opinion ,having made a simple FET based vfo for
3.5-38 MHz which serves as a training tool for the UK Intermediate
Licence practical assessment ,for which trainees have to calibrate a vfo
against a receiver and/or frequency counter.

The VFO was made (semi-ugly style) on PCB board with 'ground' holes
drilled for upright grounded components ,with the other lead to support
the relevant non-grounded components resulting in the shortest possible
interconnections.
Using polystyrene caps and with the VFO not having any buffer stage the
device fed by a 9v PP battery drifts not more than 20 Herz in 30 minutes.

Frank GM0CSZ / KN6WH

Using polystyrene caps and with the VFO not having any buffer stage the
device fed by a 9v PP battery drifts not more than 20 Herz in 30 minutes.

Frank GM0CSZ / KN6WH

What kind of battery is a 9v PP battery? And, it would likely be helpful to
know what the Inductor is that the negative temperature coefficient of the
poly cap is compensating for. Is that drift specification for the first 30
minutes after turn on or after the oscillator has been on for a couple of
days?

W4ZCB

Highland Ham wrote:

I support the above opinion ,having made a simple FET based vfo for
3.5-38 MHz..........
Using polystyrene caps and with the VFO not having any buffer stage the
device fed by a 9v PP battery drifts not more than 20 Herz in 30 minutes.

Any chance of a schematic and/or pics?

Sounds interesting.

Jack K9ACT

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Using polystyrene caps and with the VFO not having any buffer stage the
device fed by a 9v PP battery drifts not more than 20 Herz in 30 minutes.
----------

What kind of battery is a 9v PP battery? And, it would likely be helpful to
know what the Inductor is that the negative temperature coefficient of the
poly cap is compensating for. Is that drift specification for the first 30
minutes after turn on or after the oscillator has been on for a couple of
days?
-----------
A 9v PP battery is a standard battery pack , alkaline /NiCad / NiMH as
used in all sorts of consumer electronics incl battery operated smoke
alarm devices. Dimensions :45x25x17 mm. Available all over the world.
The battery is 'velcro-ed' to empty space on the board
I did not use dedicated neg temp coeff components ,but used 4 recycled
polystyrene caps from the junk box.

FET is 2N3819 but any other HF FET would do.
The tuning cap is a air variable cap (old style trimmer ,silver plated
plates and ceramic body) with added reduction gear/scale in front panel.
Adjustable inductor with core is from Toko .It is mounted on its side,
soldered to PCB material plane.
There is a compact foil trimmer for fine adjustment

Since double sided PCB board was used , I drilled many holes in the
material and soldered through hole pieces of wire ,such that the 2
copper planes are connected at many places.

The 20 Hz stability is reached after applying power to the circuit for
approx 20 minutes in a room with a stable temp.

The power supply connection to the circuit is via an insulated pin which
sits on a tiny board island (having used a special PCB foil cutter which
removes a tiny ring of copper material) .
Although I also could have used an upright 10 MegaOhms (1/4 W) resistor
serving as pin and 'insulator'.

The unit is built onto an 2mm thick piece of aluminium bent as an L

Summarising : A typical little project using predominently junk box
components, with success resulting from using RF friendly components
and shortest possible connections between frequency determining elements.

I don't have a web site to post to ,but can take a pic and send it to
those interested, by email

You can contact me by e-mail by removing abcxyz from my NG address


Frank GM0CSZ / KN6WH

now almost fully migrated to Linux (using Mandriva 2007.1 distro)