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berryfan
Joined: 10 Jul 2007
Posts: 30
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 20 Nov 2008 9:24 help!some like oscillating.what's the it? |
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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.....
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saro_k_82
Joined: 17 May 2007
Posts: 266
Helped: 55
Location: India
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20 Nov 2008 12:51 Re: help!some like oscillating.what's the it? |
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| 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.
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LvW
Joined: 07 May 2008
Posts: 1466
Helped: 242
Location: Germany
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20 Nov 2008 13:32 Re: help!some like oscillating.what's the it? |
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| berryfan wrote: |
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..... |
Is it a clear sinewave of 1.8 GHZ ?
In this case, your circuit indeed oscillates. But thatīs no surprise as each amplifier is able to oscillate if it incorporates some unwanted feedback pathīs.
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Sadegh.j
Joined: 29 Nov 2006
Posts: 803
Helped: 33
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21 Nov 2008 0:08 Re: help!some like oscillating.what's the it? |
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| 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.
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Audioguru
Joined: 19 Jan 2008
Posts: 1148
Helped: 95
Location: Toronto area of Canada
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21 Nov 2008 1:45 help!some like oscillating.what's the it? |
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| It is probably amplifying a 1.8GHz signal it is picking up.
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berryfan
Joined: 10 Jul 2007
Posts: 30
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21 Nov 2008 5:54 help!some like oscillating.what's the it? |
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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
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berryfan
Joined: 10 Jul 2007
Posts: 30
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23 Nov 2008 6:50 help!some like oscillating.what's the it? |
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| Is any oscillator signal is only -45dbm?it is a oscillating?
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Audioguru
Joined: 19 Jan 2008
Posts: 1148
Helped: 95
Location: Toronto area of Canada
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23 Nov 2008 14:51 help!some like oscillating.what's the it? |
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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.
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LvW
Joined: 07 May 2008
Posts: 1466
Helped: 242
Location: Germany
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23 Nov 2008 15:13 Re: help!some like oscillating.what's the it? |
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| Audioguru wrote: |
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. |
Parasitic oscillations at very high frequencies occur very often with rather small amplitudes (due to slew rate or other limiting effects).
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Borber
Joined: 01 Jan 1970
Posts: 1561
Helped: 117
Location: Slovenia
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23 Nov 2008 15:28 Re: help!some like oscillating.what's the it? |
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| berryfan, you must have spectrum analyzer when you know the level and frequency of parasitics. They can be sine wave or increased noise level.
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psmon
Joined: 04 Jun 2007
Posts: 173
Helped: 22
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24 Nov 2008 0:21 help!some like oscillating.what's the it? |
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| It can be picking up GSM 1800 MHz signal! Don't bring your cellphone into the lab.
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berryfan
Joined: 10 Jul 2007
Posts: 30
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24 Nov 2008 8:14 help!some like oscillating.what's the it? |
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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?
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LvW
Joined: 07 May 2008
Posts: 1466
Helped: 242
Location: Germany
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24 Nov 2008 10:19 Re: help!some like oscillating.what's the it? |
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| berryfan wrote: |
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? |
Yes, it can. A bad slew rate causes phase lag and this can give reason to oscillations.
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berryfan
Joined: 10 Jul 2007
Posts: 30
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24 Nov 2008 10:47 help!some like oscillating.what's the it? |
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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.
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LvW
Joined: 07 May 2008
Posts: 1466
Helped: 242
Location: Germany
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24 Nov 2008 11:00 Re: help!some like oscillating.what's the it? |
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| berryfan wrote: |
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. |
In fact, I really donīt 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) ?
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24 Nov 2008 11:00 Ads |
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Borber
Joined: 01 Jan 1970
Posts: 1561
Helped: 117
Location: Slovenia
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24 Nov 2008 16:48 Re: help!some like oscillating.what's the it? |
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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.
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berryfan
Joined: 10 Jul 2007
Posts: 30
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25 Nov 2008 7:44 help!some like oscillating.what's the it? |
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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.
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Borber
Joined: 01 Jan 1970
Posts: 1561
Helped: 117
Location: Slovenia
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25 Nov 2008 8:12 Re: help!some like oscillating.what's the it? |
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| Make test mentioned in last sentence of my previous post. If result is similar to what you see no your circuit then stop guesing.
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saro_k_82
Joined: 17 May 2007
Posts: 266
Helped: 55
Location: India
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25 Nov 2008 8:27 Re: help!some like oscillating.what's the it? |
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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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