Question about the amplifier spurious/noise

[antenna_abc]

Hi I have a question about my RF amplifier design.

When I feed it with a single tone CW signal, the output frequency spectrum has a strange response, as shown in the attachment.

Basically, the spurious noise is about -30 dBc below the carrier frequency.

The RF carrier frequency is at 6 GHz band, and the spectrum analyzer span is 1 MHz, with 100 kHz/division horizontal and 10 dB/division vertical. The resolution BW is 10 KHz, and the video BW is 100 KHz.

What will happen if I feed this amplifier with the real modulated signal? Will it degrade the BER?

What will be the possible reason that would cause the problem?

Thanks.

 

[FvM]

An amplifier shouldn't be expected to generate spurious signals at all. You didn't tell the absolute signal level however, and you should also show
the generator spectrum for comparison. I would rather suspect a bad performing PLL generator than the amplifier sourcing the spurious. It
should also be clarified, that the generator output spectrum can't be affected by connecting a load. (In a PLL design, it can, of course).

The analyser shows a span of 1 MHz, not 100 MHz as said in the post. As another point, the display looks like from a TTI PSA
analyzer. I'm not aware of a TTI instrument covering the 6 GHZ range. Is there other, unsaid equipment involved in the
measurement setup?

 

[antenna_abc]

Sorry about the mistake in the original post, the spectrum analyzer (Agilent E4440) setting is:

The RF carrier frequency is at 6 GHz band, and the spectrum analyzer span is 1 MHz, with 100 kHz/division horizontal and 10 dB/division vertical. The resolution BW is 10 KHz, and the video BW is 100 KHz.

The input level is -20 dBm, pretty low compared that with the P1dB of the LNA.

My curious is that whether the spurious would be the results of my drain supply voltage, since I am using active-bias to control the LNA.

 

[vfone]

From the picture you posted most probably your amplifier is oscillating when the input signal is present. Usually the cause of this is poor biasing design and layout design (both together).

 

[biff44]

What is the difference in test conditions from the blue to the yellow trace? Could just be a low frequency oscillation that could be solved with a 10 uF cap in the right place on the bias line.

 

[BigBoss]

The amplifier either oscillates or there is an intereference signal near to testbench.

 

[Element_115]

Two questions:
1)What is the Output Power?
2)What is the input source and its power?

Aside from not seeing any "Gain" from the Yellow trace, the Blue trace could
be normal if this is the Spectral regrowth of your Amp.
But you would need to give the Amps Spec's before anyone can really
give you advice.

Gain
P1dB
IP3
VSWR
NF