Measuring Load Capacitance In-Circuit

Are there any 10 to 20 pF capacitors connected to the crystal in your circuit?

The obvious thing is to use an oscilator where the circuit capacitance determines frequency, and make it with enough feedback that lossy capacitance doesnt matter. Which leaves me wondering why you cant do that? If you really need to. Is the new xtal malfunctioning?

NT

snipped-for-privacy@care2.com wrote in news:1166220763.581737.87000 @j72g2000cwa.googlegroups.com:

what

The OP needs to buy a replacement crystal, but the OEM no longer supplies them. So he goes to any company that makes custom crystals, and the first thing they ask is, how much circuit capcitance will be loading the crystal? This is because this capacitance changes the crystal frequency, and they want to grind the crystal so it will oscillate at the correct frequency when it's in-circuit.

This is in existing equipment, built in the 80's. The old xtal is shot, physically broken. In order to order a new xtal for it, they need load capacitance specification.

Thanks for the reply.

Steve

No, the design date does not help me.

Crystal resonators can be shifted in frequency with coils and/or capacitors. Many times, a small ceramic trimmer capacitor is used to tweak the crystal to the desired frequency.

Thanks for the replies. This circuit only uses trimmer caps for the tuned amplifier (further down the circuit, not for the feedback loop), and coils for the frequency adjustment. Had this unit used trimmer caps, could you assume the load capacitance would be close to the mid-range value of the trimmer cap, or is that a bad assumption?

I guess it just depends on the circuit.

Are there any general accepted values for xtals in terms of load capacitance specifications?

Thanks aga>

the first thng will ask is it a series resonant or parallel resonant...

If it is series, you don't need to know the load C.

If it is parallel I'd order a crystal for 22 pF load. If it then runs a little high, you could pad it down. Mark

Why not go for a highish C value and add a trimmer. Adjust to correct f. Why not measure the circuit capacitance? You cant do it by looking at part values because other cct elements have capacitances too in the pF range.

So again, I dont understand where exactly youre having a problem with this.

NT

Sorry if I was unclear. I was originally asking how to actually measure the circuit capacitance. I have a handheld meter, however, it runs at a fixed frequency, and when I hook it up to the circuit, it measures negative capacitance, so I don't think it will work in this instance.

Thanks aga>Steve wrote:

Steve wrote in news: snipped-for-privacy@4ax.com:

This is getting interesting! How about if you scan the schematic and post the oscillator circuit for us to see? (Somewhere on the web or in the newsgroup alt.binaries.schematics.electronic)

I'll do that, but it'll be Monday before I get the schematic.

Thanks for all the replies everyone.

Steve

May be right off the wall, but many items back in those days had crystals specified at 30pf parallel resonance. I have had occasions where I have had to have a xtal custom cut, and have not known the specs, and the xtal cutting company has just asked me to send in a copy of the schematic with my order, along with the make and model number of the equipment. It's amazing what they have archived away in dusty filing cabinets, or what they can deduce about a crystal from a schematic. With the experience that they have in these matters, they might even be able to work out specs from the broken xtal.

Arfa

Typical values are between 9 - 32 pF; with 18 - 20 pF being the most common.

Steve: Have you checked any of these references?

Make a small LC oscillator where osc f depends on C. Dont connect a C, connect to the circuit where the crystal goes, so youre using teh circuits capacitance. Note the frequency produced. Now disconnect and put fixed caps on there to get the same osc frequency. The capacitance youve hooked on equals the capacitance of the circuit, and thus the capacitance your xtal needs to be specced for.

Note use a stabilised supply for this osc.

NT

Aaah, I get it. That makes sense. I'll have to try it once I get back in the lab, I'll post what I find.

Thanks for the help, I appreciate it.

Steve

The schematic is up on alt.binaries.schematics.electronics under Schematic for xtal problem

Another thought, should I have the unit powered up and this particular xtal switched on when I attempt to measure the circuit capacitance? It has some diodes which switch the oscillators on one at a time, I don't know how they would effect the measurement if they weren't biased on.