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WANTED: Cheap/free oscilloscope (Baltimore/Washington area)

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kfitch42:
I am a software guy that has recently developed an interest in electronics, I would love to get a hold of an oscope to further my new hobby. In the videos Dave keeps saying that people give these things away, so I thought I would try asking. I don't need anything fancy. A 10Mhz dual channel analog scope would be awesome. Heck, 1Mhz would probably be enough for me for quite a while.

I am in the Baltimore/Washington area and would be willing to pick up.

Galenbo:

--- Quote from: kfitch42 on May 14, 2012, 03:56:29 am ---I don't need anything fancy. A 10Mhz dual channel analog scope would be awesome. Heck, 1Mhz would probably be enough for me for quite a while.
--- End quote ---

I think you forget something. There's a huge difference between "looking at 1Mhz signals" and "needing a 1Mhz scope"
A scope is always lying to you. The higher the max. frequency, the less it lies.

If you look at (for ex.) a 10% AM modulated signal, but with the carrier above max.frequency, you will see the unmodulated signal.
Signals that disturbe your circuit will not be visible.
Even in the most basical setups, you won't see/learn/detect much.


--- Quote from: kfitch42 on May 14, 2012, 03:56:29 am ---I am a software guy that has...
--- End quote ---

Why don't you buy a digital one, with a decent library for the sofware world?
It will let you use your software skills to develop purpose-built measurements.

vk6zgo:
"If you look at (for ex.) a 10% AM modulated signal, but with the carrier above max.frequency, you will see the unmodulated signal."


I'm sorry,but that doesn't make sense!
Firstly,the frequency response of an Oscilloscope is not a "brick wall" LPF.
For instance,a 10 MHz 'scope will have a response which is only down 3dB ( to 0.707 of midband in voltage terms) at the quoted frequency,so a signal of higher frequency will be displayed at a reduced amplitude.
I have looked at a 27MHz CB radio output with a "10MHz" analog 'scope.

 In your example,if,as is normal,the modulation frequency is low compared to the carrier frequency,you will either see a reduced amplitude signal,but with the correct percentage modulation,or if the carrier is very much higher than the "max frequency",you will see nothing!

If the modulating frequency is fairly high,you may have a reduced upper sideband,but the lower sideband will be reproduced,so you will still see modulation.

sorin:
If you look at (for ex.) a 10% AM modulated signal, but with the carrier above max.frequency, you will see the unmodulated signal."
I think this only applies to digital oscilloscope, and the key point is the samples/second not the bandwith directly.

vk6zgo:

--- Quote from: sorin on May 14, 2012, 04:26:56 pm ---If you look at (for ex.) a 10% AM modulated signal, but with the carrier above max.frequency, you will see the unmodulated signal."
I think this only applies to digital oscilloscope, and the key point is the samples/second not the bandwith directly.

--- End quote ---

Sorry,that doesn't work,either! ;D
As an example,let's consider a 10MHz carrier fc,modulated by a 1kHz signal,fm

At the output of the modulator will appear,: The carrier,=fc
                                              the lower sideband (LSB)=   fc-fm
                                              the upper sideband (USB)=  fc+fm
                                              fm is also present,but is removed with a high pass filter.(HPF)

The frequencies now present are: carrier at 10MHz
                                                   LSB at    9.999MHz
                                     &            USB at   10.001MHz
An Oscilloscope,being a wideband device, sees the result of this as the familiar modulation envelope.

If we now sample this signal at an 8MHz rate,for example,we are effectively mixing our three  signals with a new signal ,which we can call
f(sample),or for short,fs.

The result which goes to the digital part of the 'scope,is:

fc-fs

(fc-fm)-fs,
and
(fc+fm)-fs

The sum signals are also produced,but are removed by the LPF effect of the sampling circuit

The signal which is displayed on the screen is a 2MHz carrier,with sidebands at
1.999MHz & at   2.001MHz .
The 'scope display therefore still shows the classic envelope shape.

If the frequency fs is brought very close to fc,the resulting carrier frequency becomes small compared to that of fm,& the classic envelope is no longer evident.

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