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Author Topic: 2 GHz Oscillation on RFconcepts rfc 2-317 2 meter amp  (Read 2760 times)
WA7KGX
Member

Posts: 104




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« on: March 19, 2012, 10:12:52 PM »

My second hand unit shows an oscillation at about 2000 MHz in the preamp.

Does this affect the gain or noise figure on two meters?

How can it be eliminated?
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KM3F
Member

Posts: 519




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« Reply #1 on: March 23, 2012, 07:54:35 AM »

How did you determine it's oscillation frequency?
Without seeing the ckt design it's difficult to tell what might be done.
Also what device is used. Bipolar, Jfet or Gasfet.
Is the layout a tuned coil or stripline config.?
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WA7KGX
Member

Posts: 104




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« Reply #2 on: March 23, 2012, 09:06:55 AM »

I measured the 2 GHz oscillation with a spectrum analyzer that goes up to 3 GHz.
I did not see any other oscillations up to the 3GHz limit of the analyzer.

The circuit consists of a CF300 dual gate Ga-As-Fet feeding a U309 grounded gate buffer.
The schematic shows LC circuitry for the preamp, strip line for the power amp.
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WA7KGX
Member

Posts: 104




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« Reply #3 on: March 23, 2012, 06:44:26 PM »

I opened up the amp and touched various parts of the preamp circuit.
Touching some spots quenched the oscillation.  Finally, I bent two
caps horizontal to the board and the oscillation stopped.

I tweeked the trimmers and replaced the cover.
I connected the antenna and turned the preamp on.
Little lines started marching across the spectrum analyzer's screen.
I am now DXing power line noise.
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KM3F
Member

Posts: 519




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« Reply #4 on: March 24, 2012, 10:33:33 PM »

There ya go.
A fluke situation that you took care of very easy.
Sometimes that's the only way is to see how it reacts.
Good luck.
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