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help!some like oscillating.what's the it?

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berryfan

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when I don't add signal to my chip,It shows about -50dbm signal at 1.8G.why?is my chip is osillating ,but it's so small.It's a differential amplifier.
who can tell me what's that?thanks.....
 

It must be just the noise picked up by your setup. Reduce the loop area of your connection probes to the oscilloscope. You should see this figure reducing.
 

berryfan said:
when I don't add signal to my chip,It shows about -50dbm signal at 1.8G.why?is my chip is osillating ,but it's so small.It's a differential amplifier.
who can tell me what's that?thanks.....
Click to expand...

Is it a clear sinewave of 1.8 GHZ ?
In this case, your circuit indeed oscillates. But that´s no surprise as each amplifier is able to oscillate if it incorporates some unwanted feedback path´s.
 

Some offset voltage in the circuit is not surprising at all, unless it has a nice sine shape, in which case, it might be unstable.
 

LVW,I don't have oscillagraph,.so I can't make sure it's a sinwav

Added after 4 minutes:

I want to make sure the factor in my sch.so ,anybody has any suggestion in my sch.thanks all in advance
 

Is any oscillator signal is only -45dbm?it is a oscillating?
 

An amplifier that oscillates will have its output at max level, not way down at -45dB.
I think it is picking up interference or noise.
 

Audioguru said:
An amplifier that oscillates will have its output at max level, not way down at -45dB.
I think it is picking up interference or noise.
Click to expand...
Parasitic oscillations at very high frequencies occur very often with rather small amplitudes (due to slew rate or other limiting effects).
 

berryfan, you must have spectrum analyzer when you know the level and frequency of parasitics. They can be sine wave or increased noise level.
 

It can be picking up GSM 1800 MHz signal! Don't bring your cellphone into the lab.
 

LVW,"Parasitic oscillations at very high frequencies occur very often with rather small amplitudes (due to slew rate or other limiting effects). "
my slew rate is not good.it can cause oscillations?
 

berryfan said:
LVW,"Parasitic oscillations at very high frequencies occur very often with rather small amplitudes (due to slew rate or other limiting effects). "
my slew rate is not good.it can cause oscillations?
Click to expand...

Yes, it can. A bad slew rate causes phase lag and this can give reason to oscillations.
 

thanks seriously LVW.
1.what kind of slew rate can be considered as bad slew rate.
2.is it difficult to identified in simulation about the oscillations of this reason (slew rate)

Added after 15 minutes:

and this signal is changing between -40~-50dbm.it's weird.
 

berryfan said:
thanks seriously LVW.
1.what kind of slew rate can be considered as bad slew rate.
2.is it difficult to identified in simulation about the oscillations of this reason (slew rate)
Added after 15 minutes:
and this signal is changing between -40~-50dbm.it's weird.
Click to expand...

In fact, I really don´t believe that the slew rate is responsible for the oscillations in your case. Instead, it could be any kind of unwanted feedback.
Some questions are important in this context:
How did you measure the spurious level and its frequency ?
Is your circuitry with lumped elements or is it an IC ?
For which purpose (frequency) is it designed ?
What kind of transistors do you use (transit frequency) ?
 

berryfan wrote:
Enquiry: Re: help!some like oscillating.what's the it?
thanks.I have used spectrum analyzer ,and the I see in 980M and 1.8G there can be signal changing between -40--50dbm.I think if it's a oscillation ,it can be one power.not changing so much.what do you think

Your situation is clear. You have bat test setup and you are picking-up GSM bands. Your circuit is stable. Make tests in Faradays cage or use metal enclosure for test setup.

Added after 5 hours 39 minutes:

To clarify this conect a small 5cm antenna to spectrum analyzer or touch center conductor and you will see the same spurs.
 

hai,LvW and Borber
when I around the chip ,but not let it in.I found the signal is less than-58dbm.it become small.so I think it's not oscillation.am I right?
if yes,why other chip in lab doesn't have the signal as my chip.
from this phenomena ,can we get the result that the chip isn't oscillating.
thanks seriously.
 

Make test mentioned in last sentence of my previous post. If result is similar to what you see no your circuit then stop guesing.
 

If after shielding the amplitude becomes small, it is definitely external pickup and not in-circuit oscillations.
For your chip to oscillate at 1.8GHz, it requires atleast one of your feedback loops in the circuit to have UGB close to 1.8GHz. I don't think your diff amp allows so much of bandwidth.
 

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