wobbulator trouble

[Bero]

Which level of pulse output of the sawtooth generator
would be sufficient to triggering the scope?
I think I’ve measure about 0.6V DC there.
Another thing, is this signal must be AC or DC coupled to scope trigger?
In article he is a DC coupled, there isn’t any capacitor into this line drawn.

[dominicbeesley]

Hi,

If you've got a 10x or 100x probe better to sample the output of the IF transformers direct or at least before all the filtering. If you don't do that then you'll get a very funny trace. Effectively your passband hopefully looks like a squareish pulse, then this is being filtered by all the AVC and AF filters around the EABC80!

If you can sample the IF output direct with a 10x or 100x probe then best to keep the scan rate low, as low as you can! One sweep a second is just about comfortable on my scope. However if you can't then try increasing it to 50 or 100Hz, this will put you in the audio pass band and you might have more luck - though it is still best to sample before all that filtering.

The other useful thing I added when using X-mod input was to add a "blanker". This blanked the screen during the "flyback" by adding a small capacitance and diode to provide a negative pulse during the flyback, this was added into the Y trace so that the trace was off the screen during the flyback. I'd get the normal scan going first before adding that though...

Dom

[Bero]

Jim, did you think like this – red line marked ?

[jimmc101]

Yes, but the trigger pulse output does not need to be connected.
(You are not using the timebase of the 'scope so no trigger is required.)

Jim

[PJL]

I would agree with Dom.

Loading of the demodulator from the scope probe is going to be a problem. I would suggest you take the output from the anode of the EABC80.

For the sweep not to be impacted by the various filters in the demodulator, the sweep rate needs to be in the audio range. That is not the sweep generator rate but the rate of change over the FM bandwidth of the receiver.

Jim's solution will produce a more accurate scan but varicap's are very non-linear so I am not sure how much you will gain.

[MichaelR]

From your diagram you have not connected it up as advised by jimmc01. You need to be in x-y mode connecting the sweep to that input. You are then not using the internal sweep of the scope but using your sawtooth to sweep the wobbulator and the scope together..

PJL has also raised a valid point about the varicap..... anyway one thing at a time.

Mike

[jimmc101]

Sorry Bero, I got it wrong.
On the 9020 'scope in X-Y mode, CH1 becomes the X input and CH2 the Y input so those inputs need to be swapped.
As I said, the Trigger cable is not needed.

By using X-Y mode the X axis of the 'scope tracks the output of the external timebase generator avoiding the need to adjust the 'scopes timebase to exactly match the external sweep. Thus the effect of changing the sweep time may be tried without having to keep adjusting the 'scope to match.
(I suspect that the trace in Bero's original post was caused by the external sweep terminating just to the right of centre screen. The abrupt of slope at this point is not a symptom of too fast a sweep.)

Using X-Y mode, the linearity of the voltage sweep is not very significant as the 'scope tracks the input to the wobbulator.
I agree the the frequency linearity of the wobbulator is a separate matter.

Jim