Re: Is this a gate/drain capacitance problem?
- From: "Andrew Holme" <andrew@xxxxxxxxxx>
- Date: Thu, 11 Aug 2005 23:49:55 +0100
Dave wrote:
> 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?
It'll have a lower drain-gate capacitance. Do you get the same thing if you
replace the FET with a small capacitor?
Two alternatives that spring to mind:
1. PNP BJT switching +28V on/off
2. NPN BJT with base to +5V; emitter (via series resistor) is TTL input;
collector to R / L junction
.
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