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

[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]

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 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.

I am a software guy that has...

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]

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.

Sorry,that doesn't work,either!
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.

[vk6zgo]

But wait,there's more!
If the Oscilloscope has very poor memory depth,& we want to display the modulated signal at the 1kHz rate,the sampling rate in some old or cheap DSOs may reduce to something like 10k/samples/sec.
In this case, fc-fs & the other products for the LSB & USB won't be reproduced correctly,&  the displayed signal may appear as an unmodulated carrier,but of a very much lower frequency than fc,etc.

In no case,will fc be displayed at the correct frequency with the sidebands stripped.

[alm]

vk6zgo: note that these are all RF examples. RF signals have a very limited bandwidth, since having a signal that spans the spectrum from 1 MHz to 100 MHz would make antenna and tuner design quite complex, not to mention pissing off your neighbors. A digital signal, however, can easily have a 100 kHz fundamental, with rise times down in the ns range (just run an I²C signal through a fast logic gate like the 74HC series everybody uses these days). It may also contain very short glitches. These glitches may not be visible on a slow (analog or digital) scope, but can screw up your receiver if it's fast enough to detect them. In that case, you would need a scope faster than your receiver.

[vk6zgo]

vk6zgo: note that these are all RF examples. RF signals have a very limited bandwidth, since having a signal that spans the spectrum from 1 MHz to 100 MHz would make antenna and tuner design quite complex, not to mention pissing of your neighbors. A digital signal, however, can easily have a 100 kHz fundamental, with risetimes down in the ns range (just run an I²C signal through a fast logic gate like the 74HC series everybody uses these days). It may also contain very short glitches. These glitches may not be visible on a slow (analog or digital) scope, but can screw up your receiver if it's fast enough to detect them. In that case, you would need a scope faster than your receiver.

Indeed!
I was addressing the original remarks about a modulated RF carrier.
If they had been about the well known requirements for a digital signal,(or any old complex wave for that matter),I would have not have felt any need to comment.

The major point I was making was about the similarity of sampling & mixing with modulated signals.
In for instance,a Superheterodyne HF Receiver,if you were receiving a 10MHz signal,& you were using an IF centred on 2MHz,you could use a local oscillator on either 12Mhz,or 8MHz into  the mixer LO input,giving an IF output of 2MHz in either case.*
The 2MHz IF in each case is complete with sidebands,just as in the Oscilloscope case,& in the time domain exhibits the classic envelope.

*
(Obviously,in a real radio,being able to tune the same RF frequency at two different LO settings is undesirable & other means are used to minimise this possibilty).

[Galenbo]

... you will see the unmodulated signal...

Replace the "will" by "possibly will"

I had this fenomen, don't know anymore what frequencies.

Please let me know if this is never possible, give some more info in that case.

.

[vk6zgo]

... you will see the unmodulated signal...

Replace the "will" by "possibly will"

I had this fenomen, don't know anymore what frequencies.

Please let me know if this is never possible, give some more info in that case.

.

In the example where the sample rate is very close in frequency to that of the carrier,you may see a sinewave,but that would be the modulating frequency,rather than the carrier.
In the other case,where the sample rate is extremely low compared to the carrier frequency,you may again see a sinewave.
In each case,they will not be the original carrier frequency.

A good test may be to remove the modulation--in both the above cases,the display should change.

[Deutoplasm]

You could check with the colleges/universities and labs for old analog CROs.  I got mine through my electronics instructor at the college. He bought a new digital scope and gave me the old one.  I almost lost my shit when he showed up and gave it to me.  Perfect condition Tenma 75-3055 of 1993 vintage.  It's my first scope and it kicks ass.  I'm sure with a little poking around, you can find an old scope for cheap/free.

[Galenbo]

...you may see a sinewave,but that would be the modulating frequency,rather than the carrier.

Of course, this is what I said in my first post. The carrier frequency is much higher, and will not be seen due to the limited bandwdth of the scope.

[vk6zgo]

...you may see a sinewave,but that would be the modulating frequency,rather than the carrier.

Of course, this is what I said in my first post. The carrier frequency is much higher, and will not be seen due to the limited bandwdth of the scope.

It may have been what you meant,but it is not what you said!

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

The normal reading of that sentence is that the"unmodulated signal" is the carrier.

In the quoted scntence,you will notice that I said  "may"!

I'm not altogether sure that this would work,but it seems to me that if the sample rate was exactly the same frequency as the carrier,they would "zero beat"like a direct conversion radio receiver & the modulation frequency would be visible.
I don't have a  DSO to test this hypothesis on,but it seems it would work.
If the sample rate is not the same, some other frequency components will be present.
You may have seen "aliasing"----Did you check that the apparent frequency was that of the modulation?

Note: None of this will happen with an analog 'scope!