a signal generator has impedance of 600 ohm

[PG1995]

Hi

What does it mean when it says that a signal generator (or, some other equipment) has an output impedance of 600Ω?

For example, theoretically, an op-amp has infinite input impedance and zero output impedance. To me, this means that an op-amp needs extremely low current from the signal applied to its input but it can deliver, ideally, infinite current to anything connected on its output.

So, perhaps, it means that a signal generator always has its own internal impedance of 600Ω in series with any load connected to its output.

Please help me with it. Thank you.

Regards
PG

 

[MikeMl]

My HP 651B Sine Generator has two BNC outputs, one labelled 50Ω and the other labelled 600Ω. You can think of it as being an ideal voltage source in series with 50Ω or 600Ω, respectively.

An opamp with closed-loop feedback is a voltage source, but it can only supply a finite amount of current. This prevents you from driving a 741 into a loudspeaker, for example.

 

[JimB]

Most signal circuits connecting various items of equipment together are designed to have a well defined impedance.

Most RF circuits are 50 ohm.

Most audio circuits are 600 ohm.

There are variations and exceptions.

I dont know if you have come across the "Maximum Power Tranfer Theorem" yet, but it states that maximum power is transferred from source to load when their impedances are the same.
This is all good stuff when considering small signal circuits, but when you get to power circuits you will find that it is not very practical.

In an RF circuit, it is usual for the source (signal generator), the load (input circuit of a recever), and the cable connecting them together to all have the same impedance, usually 50 ohm.
Such a circuit is said to be (impedance) matched.

At RF* if a cable is not terminated in the correct impedance, there can be a higher or lower voltage at the output end of the cable than there is at the input end, depending on the length of the cable and the frequency.
This effect is due to standing waves on the cable, and the mismatch between the cable and the load is often expressed as the VSWR (Voltage Standing Wave Ratio), the rario between the highest and lowest voltage on the cable.

So after all this waffle, getting back to your question, good test equipment has well defined output and input impedances in order to prevent such problems.

* At audio frequencies also, if the cable is long enough (many miles long enough).

JimB

 

[MrAl]

Hi

What does it mean when it says that a signal generator (or, some other equipment) has an output impedance of 600Ω?

For example, theoretically, an op-amp has infinite input impedance and zero output impedance. To me, this means that an op-amp needs extremely low current from the signal applied to its input but it can deliver, ideally, infinite current to anything connected on its output.

So, perhaps, it means that a signal generator always has its own internal impedance of 600Ω in series with any load connected to its output.

Please help me with it. Thank you.

Regards
PG

Hi,

Yes that's pretty much it as the other posters have noted too.

If you think of it as having a 600 ohm resistor in series with an ideal voltage source, then when you load it with a circuit that has 600 ohms input impedance you'll measure 1/2 of the open circuit voltage at the generator output.
In other words, before you connect the load the generator outputs say 10 volts, but after you connect the load it drops to 5 volts because half of the voltage is dropped by the non ideal generator internally by it's own 600 ohm resistance.

 

[KeepItSimpleStupid]

yea, 50, 75, 120, 192, 300, 600, 10K, 1 Meg in parallel with 20 pf, 10 Meg are common numbers.

Some of these are related to "transmission lines" where the characteristic of the medium has to match the termination.
Scopes are typically 1 M||20 pf and that allows x10, x100 etc probes to work properly. They can also be 50 ohms or switchable.

When the input and output Z are matched, a square wave will be reproduced faithfully since a square wave, in theory, requires infinite. bandwidth.

Let's not forget ideal voltage and current sources. For the ideal voltage source you would like it to be zero and for an ideal current source, you would like it to be infinite.

Input and output Z shows up nearly everywhere. You need to know when to treat it as an Impedance or a resistance.

There is a spec for amplifiers called "damping factor". This is the ratio of the rated driven load to the output Z of the amplifier. 100 is a common number.

Professional audio uses 600 ohms. Consumer audio generally uses 10,000 for an input Z. Why? Cost.

Connectors have an impedance too. BNC's are usually 50 ohms, but 75 ohms can be had.

At low frequencies think of input Z as a resistance. At high frequencies composed of R, L and C. In the case of a scope and a x10 probe, there is a variable capacitor on the probe that forms a divider between the scopes Z and the probes Z. The object is to 'compensate' the probes capacitance such that the network acts purely resistive. That's done by attaching the probe to the "cal" point on the scope and adjusting for a square wave.