Scoping a W2AEW Common Base Transistor Circuit

[eev_carl]

Hi,

I'm scoping a common base transistor circuit from a W2AEW tutorial and was wondering if someone could explain to me the odd waveform in the center of the attached image.  It's the yellow sine wave.

My schematic follows the scope image.  The input is a 1MHz sine wave with an amplitude of 100mVpp.  My transistor is a PN2222A.

Thanks in advance,
Carl

Scope of Common Base Transistor Circuit



Schematic

[w2aew]

I suspect that you're getting some low frequency noise coupling, maybe from AC line pickup or a switching power supply, etc.  You have your trigger level set pretty high on the waveform, so it's likely only going to pickup the momentary peaks when this interference occurs.  If you adjust the trigger point down, I'd bet that you don't see this because it will be very infrequent compared to your 1MHz input signal.  Can the scope tell you the Trigger frequency?  That might give you a clue too. 

[eev_carl]

Lowering the trigger did the trick.  The input sine wave looks a lot cleaner.

Thanks for replying and for the source video.  Your channel is outstanding.

[w2aew]

Glad to help, and happy to know that you enjoy my channel.

FYI - it is generally poor practice to make the waveforms so "small" on the scope.  I realize that you likely did this to visualize the gain better by having both channels at the same vertical scale.  Just so you know, this practice reduces accuracy because the scope's ADC is not being fully utilized, and can lead to triggering difficulties.

[bd139]

If your signal/function generator has a sync output, use a BNC cable to connect that to channel D on your scope and set it up to trigger from that channel instead of triggering on the actual signal you are monitoring. That will give you clean trigger with no jitter and obsure glitches like that regardless of signal level. Once you've set the trigger up, you can turn off the channel D display and just concentrate on the input and output waveforms then.

[eev_carl]

Thanks.  Does that tip apply to digital scopes?  I thought the sampling would occur at the maximum value regardless of the zoom.

[bd139]

It does. I have a DS1054Z as well and use this method with my DG1022Z generator.

It's not actually a zoom vertically speaking which is the problem. It's an attenuated signal within the dynamic range of the DAC in the scope. The input is attenuated and for the sake of triggering you really need as much dynamic range as possible so the larger the signal on the display, the more resolution you're getting.

[eev_carl]

I have a DG1022Z generator too.  The trigger out from the DG is a great tip.  i think I'll keep that hooked up.  Thanks

[bd139]

Excellent. Yeah mine lives plugged in all the time. Annoyed that that sync out is on the rear though

Also have a play with your DG1022Z’s sweep function. The sync out doubles as a frequency marker so you can see which part of the sweep is being executed. Turns the pair of devices into a respectable 25Mhz scalar network analyser.