measuring signals above 1MHz

[dcac]

Hi, I bought recently a second hand oscilloscope (Hameg 1005 100 MHz) which came with 100MHz Textronix probes. I noticed that displaying waves that gets closer 1MHz becomes almost impossible task. I use 'hold' variable to 'freeze' a wave normally, but when for the waves with hight frequencies the variable becomes not very sensitive and even a small variable turn will cause the trace run faster until impossible to see the actual wave.

I have checked if the probe are 'calibrated' by connecting the probe to the internal square wave generator of the oscilloscope and then inspecting if the edges of the waves were straight.

Could that be a fault of the oscilloscope or the probes have to be from the Hameg too?

 

[BobK]

You need to use internal triggering and set a trigger level that will make the display stable.

Bob

 

[KrisBlueNZ]

The probes don't have to be the same brand as the scope. The compensation trimmer capacitor must be adjusted as you described. Your problem could be as Bob says, the trigger level is set too far from the centre of the wave, so when the amplitude drops, the scope no longer triggers from it, and the display loses sync with the waveform.

 

[(*steve*)]

At lower frequencies do you have a stable display of the waveform when the oscilloscope is free running (i.e. no use of hold etc.)?

The probe adjustment is not so much that the edges are straight, but that the corners are "square". The adjustment will allow you to trim out any undershoot or overshoot on the leading and trailing edges.

 

[dcac]

I think that there is a problem with the oscilloscope. I choose channel 1, AC triggering and mode automatic and still need to fiddle hold off variable to get a stable trace even for a low frequency sinusoidal wave. According to the service manual of the oscilloscope, hold off have to be used only in cases such where a wave have many potential triggering points in one cycle. I can get way with a sort of stable wave without using hold off if I choose 'line' triggering. However, AC triggering according the manual should be used as the main option normally for non complex waves.

 

[(*steve*)]

What do you see with low frequency signals and poor triggering? Do you see signal all over the place, or do you see two distinct copies of the trace (typically they are time shifted, but it can also appear as inversion if you're using a sine wave)?

 

[dcac]

The problem is I cannot get a stable trace only a band that is a size of the real signal amplitude and the band looks like everything is squashed into one. When I start playing with hold off I do get sometimes two waves that have phase difference 180 and playing further with the hold off I get a trace which almost stable and does not move forth and back for a second.

here is the video of illustrating the problem:

 

[shrtrnd]

Does your scope have an internal 50 ohm/ 1Meg ohm input impedance selector?
Do you need an external 50 Ohm feedthru?

 

[BobK]

Sounds like your scope is not triggering reliably.

You appear to have the signal generator hooked up to channel 2. Is triggering set for CH2?

Bob

 

[dcac]

nope signal generator is hooked up to channel 1

 

[BobK]

Well, then is triggering set to internal and CH1?

Bob

 

[dcac]

I have no idea how to set internal triggering, is it external triggering button have to be off?

 

[BobK]

I would expect so. And there should be a selector or CH1 or CH2 as well. And a triggering level control.

Bob

 

[dcac]

when I go on normal triggering by adjusting level control the trace appears 'stable' for like 1 milliseconds on CRT then disappears suddenly (the screen becomes empty), so by moving back and forth the variable in milisteps the trace keeps flashing on CRT (but there is no such level on the variable that would keep displaying the trace). I mentioned 'stable' because changing time base variable the change can be visible and no need for hold off adjustment. However, changing time base variable on automatic triggering the trace become unstable.

 

[BobK]

Then I would say that there is a problem with the triggering circuit in your scope.

Bob

 

[dcac]

Do you think if I bring it to repair shop would it be costly? Worth thing to do?

 

[Arouse1973]

There is a little push button next to CH1 scope input which is channel select. As Bob said have you checked this, it looks like it is out which might be channel 2. Press it in or out and see it that helps.
Adam

 

[dcac]

I have done quite a few times. This button selects and also triggers a signal. Thanks guys for the help

 

[dcac]

Pressing the button did not change anything - did not solved the issue. So I 've just received another hameg 203-6 20M and it works perfectly no issues with triggering on normal and automatic mode. I think I am gonna just sell the scope for spares which is a shame because the scope has 100M and 3 channels. Probably, before I go to sell I am gonna try looking at repair costs.

 

[Arouse1973]

I think that's a good idea, I suppose it depends on how much you paid for the scope in the first place.
Adam