Radiated Emission from quartz

Hi guys,

I'm new here and I am asking your suggestion to solve a radiated emission problem that's afflicting a device I am working on.
I am facing high radiated emissions from a quartz running at 20 MHz frequency (checked that the quartz was the emitter using a sniffer antenna) and the frequency the device "over-emits" at is 160 MHz. I tried increasing the capacitors value connected to the quartz as to smooth rise time but, apparently, it hasn't worked the way I wanted to. Is there any other trick I could count on (consider that I tripled tha capacitors value without affecting device functionality)?

Do you think it could work if I continued to increase the capacitors value? What if I placed an emission absorber tape over the component? what do you think it could be the most cost effective solution?

Thanks in advance for your suggestions!

Giulio

Your talking of a quartz, not a quartz oscillator? The observations sounds very unlikely in several regards:
- a quartz respectively the oscillator circuit won't show even harmonics, unless the oscillator is operated in very unusual (or "defective") way
- the drive level of the oscillator circuit should be controlled in a way, that no high order harmonics can be observed, at best a small amount of 3rd.

I would rather expect a different source of radiation. But you can check the quartz waveform to verify, if it's operated regularly. If the emission is actually caused by the oscillator circuit, you can try to reduce the drive level, e.g. by inserting a series resistor at the oscillator output, see common circuit suggestions from crystal vendors.

Hi and thanks for your reply,

I recognize I have been quite approximative in my first explanation... the quartz output feeds a DSP which uses the 20 MHz frequency for its purposes (PLL fundamental input I guess). So I recognize the quartz should not emit no other than the 20 MHz tone but, as the PLL part is integrated into the DSP, the only thing I can work on is the quartz itself and its circuitry, am I right?

With a "home-made" setup (spectrum analyzer and antenna) I saw that by means of increasing the load capacitance I reduced emissions (around 7-8 dBm) while the same variation wasn't detected when I performed the test in a proper anechoic chamber.

Hope that explains a little bit more, anyhow thanks in advance for your reply

So the radiation must be expected to be completely originated from the DSP (I guess, it's using 160 MHz core clock). Changing the crystal circuit doesn't change anything, you have to care for the DSP power supply filtering, bypass caps, ground layout. If it's using a databus or periherals operated at high data rate, they may also be part of the problem.