Re: Is this a gate/drain capacitance problem?
- From: "Zigoteau" <zigoteau@xxxxxxxxx>
- Date: 14 Aug 2005 08:37:27 -0700
Hi Dave,
> I have a commercial (10 MHz to 18 GHz) noise source that works this way,
> but the noise is produces is too low (its about 15 dB above thermal
> noise). This is not enough for what I need.
> > Does your noise generator have a specification?
> Not a formal one. I am doing this for a lab experiment, and to have a
> source around of high noise source I need one again.
>
> > You have done it the
> > cheap and cheerful way. Does it need to be any better?
>
> Since I intend keeping it for a while, and perhaps use with a lock-in
> amplifier, which will be messed up to a small extent by the spikes, I
> would rather get rid of them if I can.
>
> Applying the TTL without the 28V is not a mode it will be used in. That
> is for sure.
>
> But it will be used in
>
> a) 28 V switched on/off at about 10 Hz by a commercial noise figure
> meter, with the TTL open (i.e. the 28V works to switch the noise on/off).
>
> I assume the noise figure meter gives the transients before it makes
> measurements.
>
> b) Me applying 28V constantly and using the TTL drive from perhaps a
> lock-in amplifier for some tests. The TTL was very much an afterthought
> I added "just in case I have a need for it".
>
> But I am concerned the noise the TTL it is causing. This might be an
> issue if it caused a massive spike when it switched the 28V supply. With
> zero current in the diode, the diode will generate the same noise power
> as that of a resistor at room temperature (about 295K). With the 28V on,
> it will generate the same noise power as a resistor about about
> 100,000K. I hope the transients are not causing it to generate the same
> noise power as a resistor at 1,000,000 K at the time of the transient.
> > If the spike amplitude is still too high, then you might
> > have to go to a reed relay.
> I want to avoid a mechanical unit. The delays induced by them will
> probably mess things up a lot more than the transients.
I am not familiar with handling 18GHz. Do you need special transmission
line techniques? I can see that components might not work the same way
at 18 GHz as they do at 1 kHz, and this explains why you must switch
the zener diode on and off, rather than switching after removal of the
DC component.
It sounds as if you do not really need to have a transition between
noise-on and noise-off faster than a millisecond, and it also sounds as
if the source does not have to produce noise all the way down to
baseband. The easiest way to get rid of the switching transient is to
filter your TTL output to the FET gate so that it is no faster than you
need, perhaps with an RC filter.
Cheers,
Zigoteau.
.
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