Series resistor sets impedance?

[neazoi]

Hi, I have a 50 ohm output HF (1-30MHz) signal generator.
What if I put a series 10k resistor on it's output (center lead of the coaxial)?
Will it increase the impedance seen by external devices connected to it, to about 10k?

For the time being please ignore the effects of signal attenuation, just focus on the impedance set.

 

[volker@muehlhaus]

What if I put a series 10k resistor on it's output (center lead of the coaxial)?
Will it increase the impedance seen by external devices connected to it, to about 10k?

yes

 

[KlausST]

What if I put a series 10k resistor on it's output (center lead of the coaxial)?

A resistor has two legs.
Your description only tells where you connected one leg .. but what did you do with the other leg?

And if you say "increase" ... this means there are two situations: one with the resistor, one without the resistor. You should give clear informations for both situations..
Where exactly are the "external devices" connected?

As so often... I tiny effort to take a pencil and a paper .. would be so helpful.

Klaus

 

[neazoi]

A resistor has two legs.
Your description only tells where you connected one leg .. but what did you do with the other leg?

And if you say "increase" ... this means there are two situations: one with the resistor, one without the resistor. You should give clear informations for both situations..
Where exactly are the "external devices" connected?

As so often... I tiny effort to take a pencil and a paper .. would be so helpful.

Klaus

1st case: 50 ohm generator connected to a filter
2nd case: 50 ohm generator connected to a 10k series resistor and the other resistor lead to this filter.

Will the input of the filter "see" a 10k input impedance?

 

[volker@muehlhaus]

Will the input of the filter "see" a 10k input impedance?

Your previous question was clear enough, the answer has not changed: yes, the filter will "see" 10kOhm source impedance (10 kOhm + 50 Ohm in series)

 

[albbg]

This is completly true only in theory, because any connection, even short, between the 10k resistor and the filter will alter the impedance, since you system is not matched.
The worst case is the resistor directly connected at the output of the generator, then a coax to go to the filter.
The best case is the coax cable connected to the generator, then the 10k connected as close as possible to the filter.
Furtermore any parasitics capacitance to ground at filter side will play a role.
Of course low frequency behaves better.

What's about the purpose of this added 10k resistor ?

 

[FvM]

Which filter has 10k input impedance? Intended frequency range?

 

[KlausST]

Hi,

Your previous question was clear enough, the answer has not changed: yes, the filter will "see" 10kOhm source impedance (10 kOhm + 50 Ohm in series)

you surely have more HF experience than me ... so usually I rely on your informations.

I´m totaly fine with your answer .. in an idealistic environment ... or in a simulation.

But the OP most probably talks about a real circuit.
Thus - according my idea - the wiring plays a big role. Even which wires he uses and where he places the resistor.
In a coarse simulation you don´t simulate the wires.

Tell me if I´m wrong: in a real circuit the 10k impedance the OP talks about ... is rather easily overruled by the impedance of the wiring.
Thus to really get 10k impedance ... the wiring needs to be done carefully. (and the selection of the resistor itself)
(it´s rather easy to get the 10k impedance when generator and filter is on one small PCB, but still there the GND plane will have influence. I mean talking about 10k as source impedance is rather high for HF).
If I´m not mistaken a stray capacitance of 0.5pF causes a parallel impedance of 10k @ 30MHz (making a total impedance of 7k). And I think 0.5pF are more than realistic on a real circuit.

--> Thus I would vote for the OP to show a photo of the total circuit including generator, wiring, filter... for a more detailed discussion.

Klaus