Is this a gate/drain capacitance problem?
- From: Dave <nospam@xxxxxxxxxxx>
- Date: Thu, 11 Aug 2005 23:16:24 +0100
I have a noise generator consisting of a reversed bias "zener" diode (8.2V), a 28V supply and a resistor of about 2 kOhm. There is a commerical bias T that has and L and C in it.
As the 28V is witched on/off, so the diode avalances or not, and so noise is produced. So the circuit is something like this, although the L & C are are bit more than just simple compoenents, as this bias T worked from 20 kHz to 8GHz.
+28V | | R | | L | | Zener ---||-------noise out. | | ground
I wanted to add a method of TTL control of the noise. So stuck the drain of an n-channel fet between the R and L, and grounded the source. As the gate is driven, it basically crowbars the supply to the zener. That seems to work OK.
What I find odd is that if there is no 28V supply at all, but the gate of the FET is drive, so "spikes" of noise appear on the output as the TTL signal changes state.
Do you think I am just injecting charge ito the gate, some of that gets to the drain, and so the drain develops a voltage on it, that is sufficient to cause the diode to generate noise? I can't see how, as the diode will not generate much noise unless it avalacnhes, and the TTL signal is less than the breakdown voltage of the diode. Perhaps the L's that are around are messing things up.
I'm using a power FET in a TO-220 case, simply because that was all I had, but I'm wondering if swapping to a low power FET will cure this?
In normal operation, there is no reason to use the TTL drive unless there is the 28V supply present. But I'm concerned that since I can see this effect with no 28V supply, it is probably there (but not so visable) when the supply is there.
.
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