Need Help to identify two RF-FET and cracked component - Page 1
Forum section: electronics / rf-microwave

author: Storm
author_rank: Contributor
timestamp: December 22, 2022, 04:18:30 pm
content: Hello RF-Members,can anyone identify these two FET, which are part of a 4GHz-Oscillator used in a Wandel & Goltermann SNA-23 Spectrum Analyzer?I suspected Hewlett-Packard/Agilent but was not able to find any type with 401 or 501 within their type designators…The second question addresses the cracked component on the bottom left of the first picture… I think that this component is a capacitor to GND because of the two bonding wires connected to one plate jumping on and off again. I have never seen such capacitor types before but they seem to be very delicate (cracks)… Due to the cracks, the conductive path is interrupted, which explains the non-working condition of the oscillator.I wonder if anyone out here has some experience with such problems, especially because I would want to try to use conductive glue to bridge the cracks. But I am not sure if the capacitor below the top plate is also cracked…Hoping for some expert opinion…Storm

author: eb4fbz
author_rank: Regular Contributor
timestamp: December 25, 2022, 11:21:33 pm
content: That’s a single layer capacitor. No idea about the Fets.

author: coppercone2
author_rank: Super Contributor
timestamp: December 26, 2022, 02:32:35 am
content: The SLC capacitors have better parameters, not uncommon to see them in RF circuits based on literature

author: Storm
author_rank: Contributor
timestamp: December 26, 2022, 01:33:49 pm
content: Yes, it is a ceramic PCB within the Microwave Module that is responsible to mix the input signal to the first IF of 4.422GHz and to the second IF of 421.99MHz. Now, after some days of repair trials, I know that the unknown RF-FETs are OK but the three cracked SLC are the problem. The cracks prevent the DC-current flow to set the correct operating point for the FET, which results in a non-oscillating circuit. Since I have THREE such cracked capacitors within this circuit, I wonder how this can happen? Thermal overload? Aging? Mechanical stress from the gold wire bonding process?Anyway, I tried to bridge the cracks by carefully applying conductive adhesive. Looked good, but the resulting resistance was in the 300-700 Ohms range which was still way too high for the circuit. The resulting OP-Point was still much too high (-4.8V instead of the nominal -1.9V)Then I decided to use soldering, although this was very delicate because I needed to work under my microscope with the smallest solder iron tip I have. It worked out in the way that now the operating point was OK and the 4GHz-oscillator started working but unfortunately the oscillation was not stable… The PLL around this oscillator seemed to have problems maintaining a fast startup and stable oscillation.My soldering seems to have changed some parameters on the ceramic circuit which lead to this kind of behavior… Next step is to try to cleanup my soldering…

author: coppercone2
author_rank: Super Contributor
timestamp: December 27, 2022, 04:34:42 am
content: Hmm can soldering really effect it? Are you sure the caps are not damaged?

author: Hamelec
author_rank: Regular Contributor
timestamp: December 30, 2022, 06:43:22 pm
content: if you have vertical cracks in the capacitor you will also have some horizontal one and you will never achive the same capacitance as before..