Power Amplifier Philips BLF369 Oscillations.

[sutheesh]

BLF369 application reference circuit is used to make pulse amplifier at 200MHz fre.
but the O/P pulsed spectrum is not symmetric.The first lobes at leftside and right side differ in amplitude by 3 dB. Can it be a possibility of oscillation in the device. How can i confirm the oscillation in the device?

pls help.

 

[jiripolivka]

BLF369 application reference circuit is used to make pulse amplifier at 200MHz fre.
but the O/P pulsed spectrum is not symmetric.The first lobes at leftside and right side differ in amplitude by 3 dB. Can it be a possibility of oscillation in the device. How can i confirm the oscillation in the device?

pls help.

While looking for the output spectrum, connect a matched termination to amplifier input. Then introduce a mismatch- this may force the amp into oscillation.
The easy way to reduce such instability is to reduce gain by a matched attenuator at input. Try to find a suitable value by trial. Also make sure the output is matched, another way to force oscillations.

How well is DC power line blocked? Use 100 uF plus 0.1 uF plus 1 nF in parallel. Use a metal enclosure for the board, maybe a partition is needed to separate input and output. Use a recommended number of grounding vias, possibly more.

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Is the input spectrum symmetrical?
For oscillation test, use an ANALOG ammeter in DC circuit, and wave your hand above the board with and without the signal. If the DC current varies with your hand, you have an oscillator.

 

[chuckey]

Surely the asymetrical lobes are due to a poor group delay response across the band ( 200 MHZ -> 1GHZ). This means that the harmonics of your 200 MHZ have unequal time shifts so do not reconstitute the original wave form.
Frank

 

[sutheesh]

I have checked the I/P Spectrum to the BLF369 Amplifier stage (which is class AB CW mode)and output pulsed spectrum also.
The input pulsed spectrum to amplifier stage is purely symmetrical, but output pulsed spectrum is not symmetrical.(Measured with Agilent E4446 PSA). The first lobes at left side and right side compared to main lobe, differ in amplitude by 3 dB.