Re: Is this a gate/drain capacitance problem?
- From: "Zigoteau" <zigoteau@xxxxxxxxx>
- Date: 12 Aug 2005 03:06:05 -0700
Hi, Dave,
> 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.
>
>
> 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.
Not terribly elegant. Apart from your problem of spikes, both with and
without 28V, there are issues of quiescent power dissipation.
With 28V present, the spike will be much bigger because you're
essentially changing the current through the Zener.
>
> 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.
Does your noise generator have a specification? You have done it the
cheap and cheerful way. Does it need to be any better?
Yes, a small-signal FET will clearly have lower gate-drain capacitance,
but not zero. If the spike amplitude is still too high, then you might
have to go to a reed relay.
Cheers,
Zigoteau.
.
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