| Electronics > Repair |
| Anritsu ML2438A power meter calibration problem |
| (1/1) |
| Aiy:
Hello everyone, My Anritsu ML2438A with power sensor MA2474D has an offset when calibrating against the internal 50MHz source. When I enable the RF-output from the calibrator after Zero/Cal cycle (no error) the readout is -2.55 dBm. No Calfactor is set. Does anybody have experience with these meters and power sensors and their function? How should I approach the troubleshooting? Best regards |
| Aiy:
So I ended up taking the power sensor apart. Here's my reverse engineering so far. The detector is built with three sections, 1. RF section Directly after the K-connector I see what I assume is a DC-block After that I see what I think is dual diode detectors and 50Ohm termination, possibly positive/negative mounted. After that is a mystery chip (see attached picture) which I think is a Maxim or Macom chip. Possibly this is a crosspoint switch. One of the control signals going to the RF section is switching 0 to 5V at low power levels (less than -30dB). I believe this could be to eliminate the detector bias? 2. DC amplification and signal conditioning, the signal chain is: Two stages of amplification, differential, one for each diode and a gain selection. As the attached PCB top and bottom photo shows the layout is symmetrical for the two diodes. 2.1. is a TI TLC2202AI dual OPAmp (PCB Top) 2.2. is a Max 393 Quad SPST switch (PCB Bottom). Single 0/5V control signal from the Power meter. Two switches are used to change the gain between Unity and an amplification ratio. 2.3. is a Max 410 dual OpAmp (PCB Top) There is also a 24AA64/24LC64 64k I2C eeprom for model and calibration coefficients, neat compared to older HP models I've used. The bottom side holds a bunch of ESD protection diodes aswell. 3. 12-pin interface to cable Pinout coming when I find my notes, but from the top of my head there's the following: Ground +-5V Two control signals - Gain, Crosspoint switch? I2C - SDA, SCL Differential detection signal - makes sense for long cable lengths It seems something is broken in the RF section, possibly one of the diodes in my detector is dead. I see the output on the differential amplifiers chain react to RF signals, and when I inject negated DC signal with the same magnitude as the working side, the readout is very close to the correct RF amplitude. Best regards |
| Aiy:
I replaced both diodes in the detector with Macom MSS10 beam-lead diodes from Mouser. The sensor now does the Zero-0dBm calibration correctly. I have swept the detector side-by-side with two other Anritsu detectors and the frequency response is nearly identical. I also took a photo of the mystery Maxim chip. It is a Maxim AC642-EZ 72A (C) 1993. Best regards |
| Jabi:
Good morning: Where are the dual diode detectors in the photos? I can't quite see them to replace them in my Power meter... Regards de Jabi, ea2aru. |
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