why my oscilloscope change my circuit behaviour ?

Thread Starter

clowzer

Joined Jan 15, 2020
1
i made a transmitter and receiver circuit, but when i use the oscilloscope in my receiver circuit all its OK, i receive the frequency, but when i remove it, i can't receive the frequency ! i think maybe a oscilloscope give to my circuit a hight ground ... i don't know ...
any ideas ?
 

crutschow

Joined Mar 14, 2008
24,910
The probe connection adds capacitance in parallel with resistance to ground.
10:1 probe typically adds 5-10pF and 10 megohms.
1:1 probe adds 50-100pF and 1 megohm.
 

Janis59

Joined Aug 21, 2017
1,174
Oscillo changes ANY RF circ. By two ways - the C from oscillo gnd to Earth and 2) with Probe input C. So, when it is needed much there are few ways to diminish this impact - using the divider probes 2) using the lightguide active probes [while damn expensive] 3) using the battery driven DSO if only factor (1) is the problem. 4) sometimes the milder problem may be solvd by means of 1:100 probe (China makes such for 10-20 $). Yet NO ONE EVEN MOST prestigeous and most expensivest probe NEVER to NO-ONE may not be exploited at RF over about 10% of it`s nominal Voltage. I have a large box of good probes marked for 3 kV burnt at 200 Volts only because it was 100 MHz. The input cap is those detail all producers fail to calc the REACTIVE POWER. I have no doubt it is easily standing those 3 kV at DC, but as soon the cap have the disipating RF power, it warms upon xploding. That box contains the most cheapest China pobes, the old good russian probes, the Siemens probes, the most costy Rhode&Scwatz probes, all them sleeps together in their grave.
 
Last edited:

AlbertHall

Joined Jun 4, 2014
9,820
Any test instrument connected to a circuit might affect the operation of that circuit. They all add resistance and capacitance. It is something you need to beware of when probing things that might be sensitive.
 

KeithWalker

Joined Jul 10, 2017
877
i made a transmitter and receiver circuit, but when i use the oscilloscope in my receiver circuit all its OK, i receive the frequency, but when i remove it, i can't receive the frequency ! i think maybe a oscilloscope give to my circuit a hight ground ... i don't know ...
any ideas ?
Either the oscilloscope probe is acting as an antenna to pick up the transmitted signal or it the capacitance of the probe is changing the resonant frequency of a tuned circuit in the receiver to the correct frequency. Without knowing more about your circuit and the type of frequency and modulation you are using, we can only guess.
Regards,
Keith
 
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