Measuring Load Capacitance In-Circuit

[[email protected]]

I need a replacement crystal, but the MFG no longer helps on these
units. The Xtal is 122.6667MHz, HC49U package, but I have no idea what
the load capacitance of the circuit is. Is there an easy way to
measure this in circuit? All thoughts appreciated.

Thanks
Steve

 

[Charles Schuler]

I need a replacement crystal, but the MFG no longer helps on these
units. The Xtal is 122.6667MHz, HC49U package, but I have no idea what
the load capacitance of the circuit is. Is there an easy way to
measure this in circuit? All thoughts appreciated.

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

 

[[email protected]]

I need a replacement crystal, but the MFG no longer helps on these
units. The Xtal is 122.6667MHz, HC49U package, but I have no idea what
the load capacitance of the circuit is. Is there an easy way to
measure this in circuit? All thoughts appreciated.

Thanks
Steve

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

 

[Jim Land]

[email protected] wrote in @j72g2000cwa.googlegroups.com:

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?

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.

 

[Steve]

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

 

[Steve]

I'll have to look at the schematic again, but IIRC it had an inductor
across the xtal. These circuits were designed in the late 70's, if
that helps.

 

[Charles Schuler]

I'll have to look at the schematic again, but IIRC it had an inductor
across the xtal. These circuits were designed in the late 70's, if
that helps.

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.

 

[Steve]

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 again,
Steve

 

[Mark]

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

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

 

[[email protected]]

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.

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

 

[Steve]

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 again,
Steve

 

[Jim Land]

I have a handheld meter, however, when I hook it up to the circuit, it
measures negative capacitance.

This circuit uses coils for the frequency adjustment.

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)

 

[Steve]

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

Thanks for all the replies everyone.

Steve

 

[Arfa Daily]

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 again,
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

 

[Charles Schuler]

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?

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



Steve: Have you checked any of these references?

http://www.maxim-ic.com/appnotes.cfm/appnote_number/726/
http://www.raltron.com/cust/tools/appnotes/Crystal Mini Primer - Part 1.pdf
http://www.ecliptek.com/crystals/glossary.html
http://rfdesign.com/mag/radio_piercegate_oscillator_crystal/

 

[[email protected]]

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 again,
Steve

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

 

[Steve]

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

 

[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.