Re: Is this a gate/drain capacitance problem?
- From: Dave <nospam@xxxxxxxxxxx>
- Date: Tue, 16 Aug 2005 08:47:19 +0100
Zigoteau wrote:
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.
Yes, at 18GHz, you would use transmission line techniques. The commercial source will definitely use it.
My homemade source is part homemade, and part commercial, in that the biasing arrangement (the bias T, consisting conceptually of just and L and C, but in practice a lot more), is a commercial bias T, that was lying around. That is speced from 30 kHz to 8 GHz. At the time purchased, it was state of the art, but no longer is.
It sounds as if you do not really need to have a transition between
noise-on and noise-off faster than a millisecond,
The commercial noise meters (such as the HP 8970A I am expecting will arrive today) will I am sure give any transients time to settle before making a measurement.
and it also sounds as
if the source does not have to produce noise all the way down to
baseband.
I want it as low as possible. The biasing arrangement for the diode will limit the -3 dB point to 30 kHz at the low end.
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.
Actually, thinking about it, I think that will make matters a lot worst! The reason being, that the amount of noise produced by the diode at low frequencies just at the point of avalanching is very large indeed. The diode is not operated at that voltage, as it is too unstable, but keeping the diode voltage in an intermediate state for any longer than necessary will make matters very poor.
If you look at the noise output on a spectrum analyszer, whilst adjusting the diode current with a DC power supply, you will see what I mean.
.
- References:
- Is this a gate/drain capacitance problem?
- From: Dave
- Re: Is this a gate/drain capacitance problem?
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- Re: Is this a gate/drain capacitance problem?
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