3205B - Is Ext Trigger The Same As Horizontal Input?

[steffen]

I am trying to do IF alignment with a sweep/marker generator. My understanding is that I need to be able to connect the sweep generator to the horizontal input of an oscilloscope. In the case of the 3205B, is that the same as the External Trigger port? In other words, is the external trigger the same as the the horizontal input on a bench scope? (Please pardon such a basic question, but I have searched this forum and not been able to draw a definitive conclusion.)

[ziko]

It looks to me like you are used to a analogue phosphor scope, due to the triggering. Our scopes use digital triggering, you don't need to use the external triggering you can infact use another channel as a trigger.

So in your case from the little I have read about IF alignment, I would have your sweeped signal fed into channel A and on channel B your sweep generator should have an output that can either give a a trigger or synch output or gate voltage or ramp voltage which relates to the start and end of the sweep. You would set channel B as a trigger and setup an appropriate time base to see the entire sweep.

Hope this helps, let me know if this made any sense as I know very little about RF.

[steffen]

Thank you very much for your response Ziko. I will try this and report back to you. I will need a couple of days due to a pressing project commitment. By the way, you are right - I am used to the analogue scopes.

[steffen]

Your reply was much appreciated. I watched an excellent Pico video that explained what I needed to do: http://www.picoscope.tv/general/spectrum-mode.html

However, I have come to the conclusion that there is indeed something wrong with my PicoScope 3502B. I followed the video and was able to set up a sweep, but it was of incorrect frequency and amplitude! I connected my scope's A channel lead to the generator port with a 10X probe. (I also tried it with the probe in series with a 0.1uf mylar cap.) Here is a photo:

Then, I set up the wave form generator as in the tutorial. The generator sweep frequency is set between 100KHz and 500KHz. Please observe that the wave displayed is sweeping at double the frequency (between 200KHz and 1MHz)! Also, the amplitude should be 1 volt and is instead arbitrarily jumping.

Can you please confirm if my scope is defective or in need of alignment. I really hope that there is still something I am doing wrong as opposed to the scope being defective.

Thank you,

Steffen

[steffen]

I ran some tests that cleared up much of my confusion. It's always best to start simple and take it from there. Here is what I discovered regarding sweeping on the PicoScope 3502B:

1. I should have used a sine wave and not an arbitrary wave as in the tutorial.

2. The span was indeed too wide. The span needs to be as close as possible to the stop frequency for an accurate frequency display.

3. It seems that only logarithmic scaling on the Y axis is accurate in spectrum mode. The voltage scaling is always 0.5 the set value. I thought I knew why. I thought maybe the "Amplitude" setting on the sweep generator configuration window referred to the entire amplitude of the waveform (+ and - or peak-to-peak). When viewing the sweeping wave on the scope view in spectrum mode, only a positive waveform is shown moving across the screen. Therefore, the amplitude measurement in the spectrum view would be 0.5X the peak-to-peak value set on the sweep generator configuration window.However, when I switch back out of spectrum mode, each positive and negative peak alone was the expected amplitude. Therefore, I think this is a bug with the scope or its software. Can you please confirm if this is a bug?

4. If you change any sweep or view type setting, be careful that the span does not inadvertently change. If it does, then reset it. Moreover, there are times when you may have to close the application and reopen it again in order to refresh the view with new settings. Can you please confirm if this is a bug?

The above indicates to me that the scope and sweep are basically functioning normally. However, this scope is a bit touchy in some ways and has its limitations. Personally, I think this is more suited for digital application which was the reason I originally purchased it. Now that I am doing vintage television and radio restoration, I think I need a different scope.

The whole issue of being able to align an IF stage with a spectrum view is the next question for me to resolve conclusively. Unfortunately, the built-in sweep generator has a max of 1MHz. You cannot generate 20+MHz sweep signal for the sake of a television alignment - even though the scope has a 100MHz bandwidth. I don't know why they fell down on that feature.

[ziko]

Hi Steffen,

I didn't realise you were going to use the signal generator on the 3000 I thought you had a separate signal generator designed for this application.

Just having a quick glance it looks like the signal is attenuated possibly because the probe is on the x 10 settings, although your image seems to show the probe on the x 1 settings. The automatic switching could be because it is on Auto range, choose a fixed voltage range and this will resolve any switching.

Could you send me your data or file.

Regards