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Microphonic problem in Frequency Synthesizer unit

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Newbie level 4
Oct 9, 2009
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Hi I bought Frequency synthesizer unit and I found if i applied vibration to that unit waveform comming out form synthesizer is not remain good .I can see lots of spur during vibration.

Can anyone suggest me what shoud I do?

Don't vibrate it!

Failing that, I would try to isolate WHY it has increased phase noise under vibration.

Most frequency synthesizers use a stable low frequency oscillator (such as a 10 MHz crystal oscillator) to generate higher frequencies. They do this in various ways, such as using phase locked loops, etc. But the net effect is that that 10 MHz frequency standard gets "multiplied" up to several GHz. So if the frequency standard has noise under vibration, the noise at the higher frequency will be 100's to 1000's of times worse. So I would try to improve the frequency standard's stability. Maybe buy a better one to use externally, or mount it on rubber flexible stand-offs to reduce some of the vibration.

There can be many other reasons, though. A cover can be moving mechanically, which is pulling a VCO frequency, and the PLL does not have sufficient open loop gain to compensate. You would want to stiffen such a cover to vibrate less.

There could be a poor connection, like between a substrate and a housing ground, and mechanical vibration is making/breaking the ground contact--thereby causing big jumps in VCO frequency. You would want to fix this poor mechanical design to allow sufficient stress relief.

You could have a loose comonent, like a coil or wire inside of a VCO, that is moving under vibration, and causing frequency modulation.

And on and on.

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Hi Rich,

Is it enough if I cushion the crystal in the synthesizer?


The crystal clock is important, because in most cases the noise that shows up on the crystal oscillator gets multiplied by 500 to 10,000 times before it comes out as a microwave frequency! In dBs, that is 20 * LOG (500) = 54 dB.

So if the clock has a sideband+/- 500 khz from its oscillation frequency that is say 100 dB down, at the microwave frequency that sideband will now be only 46 dB down!

One way to improve things is to put the crystal oscillator on its own board/modules with some mass, and to support it on 4 rubber grommets to allow it to move somewhat. The mechanical scheme acts as a lowpass filter to the mechanical vibration. You would need a flexible coax cable from the crystal to the main board.

Failing that, there are some types of crystals that behave better under vibraton than others.

But there are plenty of other sources of trouble. In a PLL, the VCO is locked to the crystal thru a loop amp and various digital dividers. The open loop gain of the loop filter is there to keep the VCO locked to the crystal. If the VCO is very vibration sensitive, it is going to want to jump around at the mechanical vibration frequency. If you had, say, 20 dB of open loop gain in the loop filter at the vibration frequency, then the VCO phase noise will only be reduced by the 20 dB under vibration. If your VCO has a -50 dBc sideband at Fvib when free running, closing that loop will drop the sideband to -50 -20 =-70 dBc. If you needed lower noise sidebands, you would need to increase the loop filter gain!

There are other things to look at, like covers bouncing around under vibration. The moving cover looks like a big varying capacitor to the microwave circuitry, and can cause problems, like VCO load pulling.

You can also have minor components that are microphonic, like certain types of capacitors, causing trouble.
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It's likely that the problem originates in one of 3 places:

1] Microphonic crystal reference oscillator.
2] Microphonic VCO.
3] Microphonic loop filter components.

You can diagnose which of these by watching the signal on a modulation analyzer or similar and just tapping the components with a pen until to you localize the problem.
A very good source for information on microphony in VCOs is:
Vibration-Induced Phase Noise
Reducing Vibration Induced Phase Noise in Crystal Oscillators - New Ideas

Microphonic VCO's are much rarer now as many VCO's use planar resonators but earlier, cruder and cheaper designs oftened used to use unsupported wire resonators which could be terribly microphonic.

Loop filter components are critical to achieving low microphony and fast settling; always use either NPO or PPS capacitors as many other varieties of capacitor are both microphonic and also suffer from dielectric absorption which will result in a long settling tail as the synthesizer hops from channel to channel.
If the loop time constant that you need dictates very large capacitor values then consider using an active capcitance multiplier implemented with a low noise op-amp and an NPO or PPS capacitor.
Here is a reference:Capacitance Multiplier

I hope this helps...
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