[DSP-10] Problems with 19,68 MHz oscillator - amp chain - C72 - need measurement help

[Bob Larkin]

Hi Marius,

I can't help with the color, but here are my waveforms:

Q4 side of C72:  Pretty much a sine wave, except the top is pushed down
slightly and has some slope to it. Top is 3.2 Volts and bottom is -0.8V.

U8 side of C72:  Not a square wave at all. the bottom looks a bit like a
sine wave, but the top is flat. Top 1.6V. bottom 0.5V.

This was with a 250 MHz scope, set for DC coupling,  so it shows pretty good
detail.

Also, I remember that to make an osc, like Q4 work, the value of C72 must
have quite a bit of reactance. If you load Q4 heavily by putting a big cap
at C72 (like .01 uF) the oscillator will not start.  The output power from
Q4 is quite adequate to overdrive U8 (even with 22 pF at C72) with the
resulting clipping of the U8 input.

I hope that this helps.  Good luck!!

73,
Bob   W7PUA



At 09:49 PM 8/7/02 +0000, you wrote:
>
>I thought I had grilled U8 and replaced it with a MAR6 that has
>very similar specs (that was what I had in stock). Had to do a slight change 
>in bias setting resistor.
>Still the waveform looks very distorted. I have measured on the oscillator 
>side of C72 and the waveform looks good. On the MMIC side of C72 the 
>waveform looks very distorted. Even with the new MMIC in place.
>This leads me to suspect C72 ! Can somebody help me with placing a scope 
>probe on both sides of C72 so I can get a report on how your waveform looks. 
>Does it look good on both sides of C72 with respect to ground ?
>Which color does your C72 have (I have the TAPR kit) ? My has a light brown 
>color.
>
>73 de la9eea
>Marius
>
>
>
>
>
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