Gain in Cascaded Gain Blocks

[aliahmedrauf]

Hi. I'm using two gain blocks BG14A in cascade in my RF board. I have included a pi attenuator between them. The problem is my first in block is providing proper gain of 10dB but the second one isn't giving any gain. Although both gain blocks consuming the required currents. I thought second gain block was in saturation so i had attached attenuation before it but it didn't resolve my problem. So i explored further i have tested both gain blocks seperately by removing one of the gain block. 1st gain block was giving 10 dB gain where as i was getting 4 dB gain from secong gain block. But in cascade i was getting only 10dB gain. I have checked biased network working fine. Biasing voltages are also fine. Can anyone suggest what should i do to resolve this problem??

 

[FvM]

Schematic? Operation frequency, signal level? How do you measure gain?

 

[biff44]

lots of possibilities:

2nd gain block is blown up

short circuit in the 2nd gain stage

Open circuit in one of the series caps in the 2nd gain stage

The system of both gain stages OSCILLATING at some out of band frequency, which you do not notice, which is compressing the output stage.

 

[vfone]

According to the datasheet, BG14A should have more than 15dB o gain up to about 3GHz.
Which means, both your BG14A chips could be dead or you didn't bias them properly, as is recommended in the datasheet.

 

[aliahmedrauf]

Schematic? Operation frequency, signal level? How do you measure gain?

I measured gain on VNA. Or spectrum analyzer by passing sweep signal and observing output at each block using probe.

 

[aliahmedrauf]

According to the datasheet, BG14A should have more than 15dB o gain up to about 3GHz.
Which means, both your BG14A chips could be dead or you didn't bias them properly, as is recommended in the datasheet.

Still it works up to 6G and provides gain up to 10dB.

 

[volker@muehlhaus]

I measured gain on VNA.

What output power? You tried to reduce the power level for your VNA measurement?

 

[aliahmedrauf]

Yes. I did. SSG was improves at low power. Earlier you mentioned that there is a chance that system is oscillating at some out of band frequency. Do you have idea how can i checked my board is oscillating?

What output power? You tried to reduce the power level for your VNA measurement?

 

[volker@muehlhaus]

Let me ask again: what was the VNA output power? How much dBm?
You report that gain increased at low power, so obviously there is some saturation effect.

To check for oscillation, you could use a 50 Ohm looad at the input and a power meter at the output. Power meter would measure wide band, so you don't miss possible out of band oscillation.

 

[aliahmedrauf]

Well i first tested it about 0dBm then i continue to decrease it till - 10 dBm. In my troubleshooting i considered the saturation power of the second gain block so i had attached a pie attenuator between them. But apparently it didn't solve my problem.
Thanks for your reply tomorrow i'll check oscillation problem as you suggested.