# scope signal adapter

#### automagp68

Joined Nov 13, 2011
81
Hi guys quick question

Im a first year EE student and i picked up this old Tektronix 2205 20 mhz scope from an estate sale

In the manual there is some procedure I'm suppose to go through (not named by the manual) for some adjustment purpose assuming that you have the originally included "signal adapters" I can't seem to find a picture of the "signal adapters" anyway and or what they do.

Can someone explain what a signal adapter is for (this scope) and its purpose. I can't seem to find any information on it online.

Also, i noticed that the probe impedance is 1MΩ

All the ones i have seen have been 50Ω

Not sure how this effects the probes. I have about 5 different probes it came with.

Any ideas would be greatly helpful as I'm trying to learn this device and also test its current accuracy.

Thanks

#### SgtWookie

Joined Jul 17, 2007
22,210
Im a first year EE student and i picked up this old Tektronix 2205 20 mhz scope from an estate sale
Hard to go wrong with a Tektronix.

In the manual there is some procedure I'm suppose to go through (not named by the manual) for some adjustment purpose assuming that you have the originally included "signal adapters" I can't seem to find a picture of the "signal adapters" anyway and or what they do.

Can someone explain what a signal adapter is for (this scope) and its purpose. I can't seem to find any information on it online.
Tek 2205 Service Manual: http://www.danielvg.be/2205_SM.pdf
From page 2-7:
CONNECTING SIGNALS
The signal adapter supplied with the instrument is
usually the most convenient way to connect a signal
to the 2205. These signal adapters are shielded to
prevent pickup of electromagnetic interference.
When connected to the 2205 input, a signal adapter
presents 1M Ohm and about 100 pF impedance to the
circuit under test. If this capacitance is disruptive to
the circuit being tested, use the optional 10X probe
Basically, they are talking about the entire scope probe, from the bayonet base to the tip of the probe.

Also, i noticed that the probe impedance is 1MΩ

All the ones i have seen have been 50Ω

Not sure how this effects the probes. I have about 5 different probes it came with.
The impedance is 50 Ohms at the input to the O-scope, and a 1x probe is 1 MEG at the tip; a 10x probe is 10MEG, a 100x probe is 100MEG. You can frequently find switchable probes that will go 1x-10x or 1x-10x-100x.

Don't forget to adjust the compensation for your probes. That procedure is documented on page 2-9. The PROBE ADJ output is located on the bottom of the front panel, just to the left of EXT INPUT (see page 2-4).

#### automagp68

Joined Nov 13, 2011
81
Hey Sgt!

thanks for all the great info and that link. That is great. I could not find that thing anywhere. And that seems more helpful then the one i have.

So the signal adapter is part of the probe? Am i understanding that correctly.

Also, the other thing i may have messed up mentioned is that the scope actually says 1MΩ input impedance.

Like my old leader scope says 50Ω

Im not exactly sure how that effects measurments.

#### SgtWookie

Joined Jul 17, 2007
22,210
Thanks for all the great info and that link. That is great. I could not find that thing anywhere. And that seems more helpful then the one i have.
That is the service manual. It is a really, really good thing to have. After you gain some experience with building things, and save up some \$, you should consider replacing all of the electrolytic capacitors in your O-scope. It's a good 'scope, but it's getting old - and electrolytic capacitors are usually the most troublesome item in older test equipment. The best thing to do is to replace them all, so you will get another 15-20 years of service out of it.

If you fail to replace the capacitors before they start dying, you risk damage to the adjacent components when the caps start failing - they frequently take out a number of components when they go. You really don't want that to happen.

So the signal adapter is part of the probe? Am i understanding that correctly.
It's informally called an oscilloscope probe, or o'scope probe. "Signal adapter" was more proper, but antiquated - if you ask someone for a signal adapter, they will probably look at you as if you had two heads. If you say "O'scope probe", or "oscilloscope probe" they will know that you are talking about this:

Also, the other thing i may have messed up mentioned is that the scope actually says 1MΩ input impedance.

Like my old leader scope says 50Ω

Im not exactly sure how that effects measurments.
1MEG can certainly load a signal. 10MEG will load the signal 1/10th as much. You use x1 and x10 at different times. It's getting late to describe when you'd use each mode, but you'll pick that up as you use it.

The Navy NEETS module 16 has an intro to oscilloscopes and various other pieces of test equipment. It's dated due to the equipment in the document, but you will have many of the same controls on your O'scope:
http://www.phy.davidson.edu/instrumentation/NEETS.htm

You should go through the rest of the modules when you have time. I went through a predecessor course back in the 70's.

#### crutschow

Joined Mar 14, 2008
24,701
..............
The impedance is 50 Ohms at the input to the O-scope, and a 1x probe is 1 MEG at the tip; a 10x probe is 10MEG, a 100x probe is 100MEG. You can frequently find switchable probes that will go 1x-10x or 1x-10x-100x.
............
Most old oscilloscopes have a 1meg input. Obviously that is required if a 1x probe is 1meg and a 10x probe is to have a 10meg impedance.

Many newer scopes have a switchable 50Ω/1megΩ input impedance. The 50Ω impedance is used when you want a matched termination for 50Ω cables and source. The 1megΩ input is used for 1x and 10x probes.