how to AC couple

N

notwist

New Member

• Dec 4, 2007

• #1

i have a signal i want to input into a spectrum analyzer, but i have to AC couple this signal to avoid damaging the analyzer. How can I AC couple this signal? The signal is being generated by a TIMS module so i can just connect the cable to the analyzer but how do I ac couple it?

Thanks in advance.

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Dec 4, 2007

• #2

You connect it via a capacitor, but I would have expected a spectrum analyser to be AC coupled anyway?.

 

Overclocked

Member

• Dec 4, 2007

• #3

Use a Capacitor in Series with the AC Signal, I think the value Oscilloscopes use is 10pF Or So.

 

JimB

Super Moderator

Most Helpful Member

• Dec 4, 2007

• #4

Overclocked said:

I think the value Oscilloscopes use is 10pF Or So.

Click to expand...

Er, no.

My scope (Telequipment D75) uses 100nF.

You may be thinking of the parasitic parallel capacitance at the input connector, 29 pF in my scope.

JimB

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Dec 4, 2007

• #5

JimB said:

Er, no.

My scope (Telequipment D75) uses 100nF.

Click to expand...

Yes, it's got to be large enough to pass low audio frequencies.

You may be thinking of the parasitic parallel capacitance at the input connector, 29 pF in my scope.

Click to expand...

In parallel with 1Mohm no doubt?.

 

RadioRon

Well-Known Member

• Dec 4, 2007

• #6

Spectrum analyzers are typically dc coupled.

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Dec 4, 2007

• #7

So why would you DC couple an RF device?.

Or do you mean it has a 50 ohm or 75 ohm load directly across the socket, and (presumably) capacitor coupled from that?.

 

RadioRon

Well-Known Member

• Dec 4, 2007

• #8

Well, you have a good point. What I mean is that it looks like a 50 ohm resistor to anything connecting to it. Ironically, the signal processing that follows does indeed have a high-pass filter in most cases, so you don't get the benefit of DC coupling on the display.

 

Leftyretro

New Member

• Dec 4, 2007

• #9

And of course a transformer can be used to AC couple a signal to a device. Not as common as a cap because of costs but has an advantage of step up or step down voltage ratio or even center-tapped output for balanced signal processing.


Lefty

 

K

kchriste

New Member

Forum Supporter

• Dec 5, 2007

• #10

Some HP spectrum analyzer's inputs will be damaged by having DC applied to the input. Says it right on the unit in red letters.