Using attenuator for RF power amplifier

[Mabrok]

Hi,

I am going to do measurements of RF power amplifier, and I have some questions as this is my first time doing the real measurements. The design based on PCB and discrete components, output power capability of about 40 dBm (10 Watt)

1. Regarding the stability, Is k-delta test using simulation in ADS enough to verify the transistor stability? What will happen if the amplifier being unstable during the measurements? Are there any risk whether on PA or VNA?

2. The effect of decoupling or bypass capacitors especially on the transistor?

3. If the 50 ohm input and output are not matching perfectly or poor matching (due to load and source impedance for the real transistor is different from that obtained during the simulation), what will happen for VNA if have a high reflection whether on input or output? How can protect the VNA? Is using attenuator between output of port 2 and input of VNA is sufficient to protect VNA?

 

[doctorworm]

Many VNAs are rated to about +30 dBm (1 W) input power so you will definitely need an attenuator if you plan to measure the S-parameters under large drive conditions. It would be best to try and keep the output power below +10 dBm for peace of mind.

Generally, it's acceptable to measure S-parameters at small-signal, with an input of around -30 dBm so, depending on the gain of your PA, you may not need an attenuator at all. If K > 1 and you don't see any oscillations at small-signal, that's usually a good enough indication that your PA is stable.

After you're satisfied with the small-signal S-parameters and stability, then you can go on to measure your PA at large-signal, using a signal generator at the input and attenuator and spectrum analyser/power meter at the output. Before you run any automated power sweeps, have a look at the output spectrum and see if any unwanted spurs appear as you gradually turn up the input power.

 

[Mabrok]

Many VNAs are rated to about +30 dBm (1 W) input power so you will definitely need an attenuator if you plan to measure the S-parameters under large drive conditions. It would be best to try and keep the output power below +10 dBm for peace of mind.

Generally, it's acceptable to measure S-parameters at small-signal, with an input of around -30 dBm so, depending on the gain of your PA, you may not need an attenuator at all. If K > 1 and you don't see any oscillations at small-signal, that's usually a good enough indication that your PA is stable.

After you're satisfied with the small-signal S-parameters and stability, then you can go on to measure your PA at large-signal, using a signal generator at the input and attenuator and spectrum analyser/power meter at the output. Before you run any automated power sweeps, have a look at the output spectrum and see if any unwanted spurs appear as you gradually turn up the input power.

My VNA is 26 dBm, PA expected to give around 13 dB gain (small signal gain) based on simulation using ADS. So, output power from PA suppose to be 39 dBm which is excessive the VNA power max. I will use attenuator of 20 dB. Then, 39-20= 19 dBm. So, VNA protected as i can handle upto 26 dBm. What do you think?

 

[doctorworm]

Hi. Yes, that should be ok. Personally, I would use more attenuation at the output and keep the VNA input level around 0 dBm, because the VNA signals tend to be cleaner between -20 dBm and 0 dBm. The VNA probably won't be able to drive your PA hard enough to make it reach 40 dBm anyway - not without the use of a pre-amp... and of course, that would have to be included in the calibration.

Judging by the amount of gain, I'm guessing this is a single-stage PA, which means you won't need to worry about the stability of individual stages. I'd recommend that you use a -20 dBm input signal and just look at the stability at small-signal. If K > 1 and you don't see any spikes in the S11 or S22 that look like they are close to going > 0 dB, then that's probably enough to suggest your PA is stable.

For large-signal power measurements, I'd always use a signal generator and a power meter and measure the output power at several frequencies and only use the VNA for small-signal testing. Of course, it's up to you how you test your design - just make sure that your output power will always been attenuated well below the maximum rated input level of the VNA, spectrum analyser or power meter. Oh and make sure your attenuator is also rated for at least 40W!

 

[Mabrok]

Hi. Yes, that should be ok. Personally, I would use more attenuation at the output and keep the VNA input level around 0 dBm, because the VNA signals tend to be cleaner between -20 dBm and 0 dBm. The VNA probably won't be able to drive your PA hard enough to make it reach 40 dBm anyway - not without the use of a pre-amp... and of course, that would have to be included in the calibration.

Judging by the amount of gain, I'm guessing this is a single-stage PA, which means you won't need to worry about the stability of individual stages. I'd recommend that you use a -20 dBm input signal and just look at the stability at small-signal. If K > 1 and you don't see any spikes in the S11 or S22 that look like they are close to going > 0 dB, then that's probably enough to suggest your PA is stable.

For large-signal power measurements, I'd always use a signal generator and a power meter and measure the output power at several frequencies and only use the VNA for small-signal testing. Of course, it's up to you how you test your design - just make sure that your output power will always been attenuated well below the maximum rated input level of the VNA, spectrum analyser or power meter. Oh and make sure your attenuator is also rated for at least 40W!

Thank you for very detailed explaintaion. As i only have 20 dB attenuator. So, can i use two attenuators connected in series to get 40 dB attenuation for VNA's safety.

 

[Mabrok]

I want to use 40 dB attenuator for my high power power amplifier. In my lab only have 20 dB attenuator. So, my question is Can I use two attenuators connected in series in order to get 40 dB attenuation?

 

[stenzer]

Hi,

yes. Keep the power handling capability of your attenuators in mind as they can handle only a certain input power.

BR