Clock crystal: heatsink or grounding required?

[ianw]

Hi,

I have been having intermittent problems with the clock on my video
recorder.
When on standby the clock fails (displays ----) and hence will not
autostart or record from standby.
On opening the box it appeared that the clock crystal had been secured
to a copper pad on the pcb with a white silicon-type substance (heat
sink? But not sticky). The crystal had 'popped-up' and was no longer
in contact with the pcb.
Since re-securing the crystal (by pushing it down) the clock has
behaved itself.
I was not aware that a clock crystal needed either heat dissipation or
the 'can' grounding?
Is this a red herring?

The PVR is a Humax PVR9200T and the crystal is ...

Manufacturer: AEL CRYSTALS
Order Code: SCC4065
WATCH CRYSTAL 32.768KHZ
Frequency, output:32.768kHz
Accuracy, frequency +@:20ppm
Accuracy, frequency -@:20ppm
Temp, op. min:-10°C
Temp, op. max:60°C
Capacitance, load:12.5pF
Temperature stability +@:50ppm
Crystal case type:Watch A
Temperature stability -@:50ppm
Thanks,
Ian
Perthshire

 

[Arfa Daily]

Hi,

I have been having intermittent problems with the clock on my video
recorder.
When on standby the clock fails (displays ----) and hence will not
autostart or record from standby.
On opening the box it appeared that the clock crystal had been secured
to a copper pad on the pcb with a white silicon-type substance (heat
sink? But not sticky). The crystal had 'popped-up' and was no longer
in contact with the pcb.
Since re-securing the crystal (by pushing it down) the clock has
behaved itself.
I was not aware that a clock crystal needed either heat dissipation or
the 'can' grounding?
Is this a red herring?

The PVR is a Humax PVR9200T and the crystal is ...

Manufacturer: AEL CRYSTALS
Order Code: SCC4065
WATCH CRYSTAL 32.768KHZ
Frequency, output:32.768kHz
Accuracy, frequency +@:20ppm
Accuracy, frequency -@:20ppm
Temp, op. min:-10°C
Temp, op. max:60°C
Capacitance, load:12.5pF
Temperature stability +@:50ppm
Crystal case type:Watch A
Temperature stability -@:50ppm
Thanks,
Ian
Perthshire

Crystals are often 'fixed' to the board to improve vibration resistance,
although they should, of course, be firmly soldered as well Anything
that helps to keep the can temperature stable, is also no bad thing, as they
are quite temperature-sensitive for long term frequency stability. You will
also quite often see a crystal can soldered around its skirt to a topside
groundplane, or with a wire strap soldered from the can to some other nearby
piece of metal, or in the case of an HCxU package, layed on its side and
soldered down to ground. In the case of a VCR, the accuracy of the clock
crystal for the RTC generator, is neither here nor there - within reason -
and I don't think that you should bother yourself too much about the gunge
blob that was originally there, as long as you now have good soldered
connections to it.

Arfa

 

[petrus bitbyter]

"ianw" <[email protected]> schreef in bericht
Hi,

I have been having intermittent problems with the clock on my video
recorder.
When on standby the clock fails (displays ----) and hence will not
autostart or record from standby.
On opening the box it appeared that the clock crystal had been secured
to a copper pad on the pcb with a white silicon-type substance (heat
sink? But not sticky). The crystal had 'popped-up' and was no longer
in contact with the pcb.
Since re-securing the crystal (by pushing it down) the clock has
behaved itself.
I was not aware that a clock crystal needed either heat dissipation or
the 'can' grounding?
Is this a red herring?

The PVR is a Humax PVR9200T and the crystal is ...

Manufacturer: AEL CRYSTALS
Order Code: SCC4065
WATCH CRYSTAL 32.768KHZ
Frequency, output:32.768kHz
Accuracy, frequency +@:20ppm
Accuracy, frequency -@:20ppm
Temp, op. min:-10°C
Temp, op. max:60°C
Capacitance, load:12.5pF
Temperature stability +@:50ppm
Crystal case type:Watch A
Temperature stability -@:50ppm
Thanks,
Ian
Perthshire

The construction of crystal oscillators is often underestimated. A not well
designed oscillator may look like good in a prototype and in 90% or more of
the production. The problems appear in the 10% or less. Ever had a similar
problem in a keyboard in which the crystal was not fixed to the pcb and had
no correct groundplane. It also was placed too far from the processor. In
that case the problem was solved by soldering a short, thick (stranded) wire
to the metal can and the nearest GND. (In that days keyboards were still
pretty expensive so retrofit on failure was an option.) In your particular
case, the best option is the one that works. So press the crystal to the pcb
using some (hot melt) glue or kit to keep it firmly fixed in place. Don't
solder if you don't need to.

petrus bitbyter