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 Post subject: Crystal oscillator question
PostPosted: Nov Mon 27, 2017 2:52 am 
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Found a 1.000 MHz xtal in the parts stash, decided to build a marker oscillator. I based it on the second oscillator for a double conversion superhet from an article in a 1960s issue of QST. See illustration below. However, per the schematic, it is to be coupled to the second mixer grid via a small cap connected to the grid of the oscillator.

Although I get a nice strong waveform (8 volts p-p) on the plate, my scope shows no signal on the grid of the oscillator tube. So, if it is intended to be coupled to the second mixer from the grid, why am I seeing no waveform on the grid?

Thanks for any guidance,

Tom

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 Post subject: Re: Crystal oscillator question
PostPosted: Nov Mon 27, 2017 3:13 am 
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Are you using a 10X probe?

The grid is a very high impedance point and the scope probe could be loading the oscillator and preventing it from oscillating.


Last edited by LM386 on Nov Mon 27, 2017 2:17 pm, edited 1 time in total.

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 Post subject: Re: Crystal oscillator question
PostPosted: Nov Mon 27, 2017 3:14 am 
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I suspect the scope probe is stopping the oscillator. If you have a 2 channel scope you can lookat the grid and plate at the same time. Are you using x10 probes with low capacitance?

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 Post subject: Re: Crystal oscillator question
PostPosted: Nov Mon 27, 2017 7:20 am 
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Hmmm. I don't have a 10x probe. I think one could be rigged up, but I'll have to research that. Thanks for the suggestion.

Tom


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 Post subject: Re: Crystal oscillator question
PostPosted: Nov Mon 27, 2017 2:12 pm 
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The x10 probe will reduce the capacitance at the tip of your probe to around 20pf, without it the capacitance of the probe will be around 100pF. The x10 probe will also protect your scope from damage from the several hundred volts DC found in tube equipment. Making a x10 probe is possible you will need a frequency compensation trimmer cap across the high value resistor or the probe will act like a low pass filter. I recommend you buy a x10 probe it will be small and have a clip on probe tip, you can get them for less than $10.

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 Post subject: Re: Crystal oscillator question
PostPosted: Nov Mon 27, 2017 5:26 pm 
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For now add a 100K to 1 meg resistor at tip of your probe. This will reduce capacity loading without dropping voltage very much.

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 Post subject: Re: Crystal oscillator question
PostPosted: Nov Mon 27, 2017 7:54 pm 
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I think I might try this one: https://cromwell-intl.com/radio/probes.html

Tom


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 Post subject: Re: Crystal oscillator question
PostPosted: Nov Mon 27, 2017 8:24 pm 
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I don't think that one would work well at 1MHz, there needs to be a cap across R1 to compensate for the cable capacitance and scope input capacitance. If you don't have the compensation cap the probe will behave like a low pass filter and attenuate the 1MHz signal. If you were to look at a 10V 1kHz signal it would be 1V at the scope, If you were to look at a 10V 1 MHz signal it would be 3mV on the scope.

This should help
https://www.youtube.com/watch?v=oxJQr4dKnys

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 Post subject: Re: Crystal oscillator question
PostPosted: Nov Mon 27, 2017 9:39 pm 
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For RF use, what you should do is first measure your probe's cable capacitance (disconnected).
Add about 15 pF to that which is typical scope input capacitance.

Then simply add a small 1kV mica cap to the tip of the probe, in series with it.
A mica cap value of 1/9 of the above value is a 10:1 probe, 1/19th is a 20x probe, etc.

Typical added cap values will be accurate above about 100 kHz with typical scopes.


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