Hi,

Working on an Anritsu 2711A and have identified problem as being damage due to overloaded input. A SMD N-FET in an SOT-23 package (3 pin) identified with AF3 is blown and I need to replace it. The input attenuator is in the input of a 100kHz to 3GHz Spectrum Analyzer, so the N-FET in question must work with little loss in this circuit. Anyone have any ideas on the identifity of this AF3?
Any help would be greatly appreciated!

Mitch

 

https://ea4eoz.blogspot.com/2017/09/repairing-anritsu-ms2711a-handheld.html

 

https://ea4eoz.blogspot.com/2017/09/repairing-anritsu-ms2711a-handheld.html

Thanks for that but I had seen it already. He was not able to ID the N-FET either and just harvested unused ones off the RF generator attenuator. My Generater is activated, so no spare N-FET for me to harvest.
Any ideas of proper specs/sources for AF-3?

 

Thanks for that but I had seen it already. He was not able to ID the N-FET either and just harvested unused ones off the RF generator attenuator. My Generater is activated, so no spare N-FET for me to harvest.
Any ideas of proper specs/sources for AF-3?

Seems you have a choice to make for a temporary fix. Use ones from the generator to fix the scanner and then test possible replacements on the generator stage that's likely to be less critical as you can normalize the RF levels over a given frequency band usually.

https://www.nxp.com/packages/SOT23.html

 

https://www.rfmw.com/skyworkscatalog.pdf
Page 35.

 

From the linked repair blog, that's not the corect part.

Resistors are easy to find, all of them were 1% in 0603 size. But what about the pin diodes? They all have AF3 marking on top. A quick Google search show the only SMD device in SOT23 package with AF3 markings is a SMV1249-004 hyperabrupt varactor diode from Skyworks. A RF switch with a varicap diode? I don't think so. Also SMV1249-004 is common cathode, and these diodes should be common anode.

A clue come from the switching signals, A, B, C and D in my schematics. When the analyzer wants to bypass an attenuator (for example, the second one) it puts 0 volts on A, and -5 volts on B. When it wants the attenuator to be active, it puts -5 volts on A, and 0 volts on B. These voltages does do not make sense with pin diodes. +5 and -5 volts makes sense, but zero volts? Maybe there was a missing +5 volts supply line...
...
The answer was easy, they couldn't because they are not pin diodes. I realized there is another electronic device, usually used to switch signals, that could work with these 0 / -5 volts control signals: The FET transistor.

Suddenly everything had sense. They were not pin diodes, they were N-FET transistors. I was happy., but the joy lasted only a brief moment. What N-channel FET in SOT-23 has AF3 marking? I really have no idea. Even Google had no idea. Except for the small number of 0603 resistors (easy to find), I had six unknown FET transistors, and an unknown DC blocking capacitor. How to replace them if I don't know what components are they?

 

I can't see any FET at the attenuator input of the Spec A. However, a PIN diode, or even a zener diode could be up there. Just remove the device and see if the SA operates normally (without overloading it).

Why aren't you trying to get your part from Anritsu?

 

Hi SLK001,
Thanks for the response. Actually, in the article referenced above, the poster determines that the AF3 device is actually an GaAs N-FET. I am just trying to determine if anyone has an idea of the actual part number of this N-FET. Looking at the photo of the mixer section, there is a Skyworks component there so perhaps the AF3 is a Skyworks part as well. If so, an Alpha AF002C1-39 (GaAs IC Control FET Series DC-2.5GHz) looks like it might be that device? Not sure, comments welcome!

 

Hi NSASPOOK,

Thanks for your interest and comments! I agree that a swap of an AF3 from the generator section is a good idea, but I would like to have a stock of AF3 parts in hand before I start de soldering and soldering in the attenuator and generator sections. Just gives me comfort in case something goes sideways!

Any thoughts on AF3 being an Skywards (Alpha) AF002C1-39?

Thanks,

Mitch

 

Skyworks (Alpha)! Sorry for the typo!

 

Hi NSASPOOK,

Thanks for your interest and comments! I agree that a swap of an AF3 from the generator section is a good idea, but I would like to have a stock of AF3 parts in hand before I start de soldering and soldering in the attenuator and generator sections. Just gives me comfort in case something goes sideways!

Any thoughts on AF3 being an Skywards (Alpha) AF002C1-39?

Thanks,

Mitch

With a proper hot air gun transferring those parts won't be a problem. No idea about that part being the right one. It's seems to be rare too.

 

I would prefer to act simply soldering it into elementary common emitter circuit and applying the Nano-VNA. Will read the Gain, frequency-gain poduct, phase margins, resonant points if any; Nano VNA is able for tetrapole opeartions, to be different from 1201 VNA what allows only dipole operations.

 

By the way, all smd parts have the codes. And code-books are zillions over the Goooooogle.

 

By the way, all smd parts have the codes. And code-books are zillions over the Goooooogle.

I looked up AF3 in SOT-23 case and did not find anything other than the part that Danko referenced above:




which the repair blog author did not believe was the correct part because, as he stated:

"A quick Google search show the only SMD device in SOT23 package with AF3 markings is a SMV1249-004 hyperabrupt varactor diode from Skyworks. A RF switch with a varicap diode? I don't think so. Also SMV1249-004 is common cathode, and these diodes should be common anode. "

Can you suggest another site where I can determine the part number for the part inscribed with AF3 in this spec analyzer? If you could I would be most grateful!

Mitch