The main filtering there is the 100nF from the drain of the fet, Personally I would add a ferrite bead in series with the 12V rail, or maybe a few tens of ohms of series R?
The tant is useful mainly because it is predominantly resistive at HF (They are notoriously poor RF caps), which helps to damp any resonances in the supply wiring.
I assume the thing is Colpitts or some sort of Hartley design, with the output taken from the source?
Regards, Dan.
The parallel capacitors do little at the moment because they are shunted by the impdedance of the power lines. As Dan points out, the real filtering is done by the RC network of 100R and the 100nF capacitor at the drain pin of the FET. It would make more sense to move the tantalum so it is across the other 100nf capacitor, that shouldn't change the operation of the circuit but the longer RC time constant will help to filter out lower frequency interference from the supply.
In general, always connect a ceramic capacitor across a tantalum type (or any other electrolytic) in RF situations. A good ceramic capacitor has low ESR at high frequencies while electrolytics get higher as the frequency increases. Use an electrolytic to achieve the value you need and a ceramic across it to keep it filtering at HF.
Brian.
The ferrite bead may help but the effect may not be dramatic. If you need better RF isolation a choke would be better. The real issue with the supply rails is that they inevitably go to all parts of the circuit and are therefore an easy path for interference to travel along from one stage to another. The physical location of the power wiring will to a large extent decide how much signal is coupled along it. For example, if your oscillator is inside a screened box and interference can enter the box along the supply wire it will work like an antenna, introducing noise and allowing oscillation out as well. In a case like that you need two stages of filtering, one as mentioned in the earlier post and another as the power enters the screened area.
Brian.
I would go with the same RC filter at each stage, as long as the current is low enough that a reasonable value of R can be used without causing too much voltage drop. Additionally, add a ceramic capacitor across the incoming power rail at the point where it physically enters the circuit. That should help to minimize the coupling along the supply line as well as isolate the individual stages.
Brian.
Thank you!Yes, that is correct. You can still leave the ferrite bead there if you want to, it does no harm and may be beneficial but it's effect would be very small compared to adding the extra capacitor.
Brian.
It would probably be better placed on the incoming supply before the first capacitor as long as it is physically close to the capacitor and not further along the wiring. It would turn the circuit into an LC-RC filter giving more rejection of power line RF.
The electrolytic capacitor will do almost nothing at frequencies higher than a few hundred KHz, it's job is primarily to reject LF from the supply and maybe increase stability of the amplifier stage if it's prone to LF oscillation. If you increase the value it will give better supply noise rejection but at the expense of making the oscillator slower to start up because of the RC time constant with the 100R resistor. If the supply is stable it should be fine with 4.7uF.
Brian.
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