| Electronics > Repair |
| Aeroflex IFR-2398 / calibration / repair help |
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| G0HZU:
Note: It's risky to make (fairly precise) conclusions about stage gain when probing 'in circuit' especially up at 3.5GHz. If you are using the 470R probe then you are measuring voltage gain so you can get confusing results if the amp has different source and load impedances. You could try probing/comparing at the low impedance tap points of each of the little printed BPFs in the first IF. But you can probably still get confusing results when probing here. Have you ruled out the possibility of gain loss due to instability somewhere? There do appear to be lots of little bits of (what looks like) RF absorber stuck on the signal path in various places. I assume these are there to suck up unwanted LO harmonics at extremely high frequencies in order to try and reduce the amplitude of internal spurious terms caused by the undesired meeting and mixing of 1st and second LO harmonics at extremely high frequencies. But they might also be there to improve stability in some cases. Have any of them fallen off? Are there any more of them stuck inside the lid? |
| andy_silicon:
The cover is full of shielding and black RF absorbers. Also the 1st LO is 3.4-6.2GHz :-DD I made an improved probe which is what allowed me to do *any* probing. ( the best / most reliable results were achieved by probing the BPF strips ) Nope - I'm not 100% sure that instability somewhere isn't causing the loss. |
| G0HZU:
In case you haven't already done this, I'd strongly suggest you take a decent photo of the inside of the lid in case any of the absorber bits fall off the lid during the faultfinding :) |
| KJDS:
--- Quote from: andy_silicon on July 02, 2014, 08:49:36 pm ---The cover is full of shielding and black RF absorbers. Also the 1st LO is 3.4-6.2GHz :-DD I made an improved probe which is what allowed me to do *any* probing. ( the best / most reliable results were achieved by probing the BPF strips ) Nope - I'm not 100% sure that instability somewhere isn't causing the loss. --- End quote --- The 470R resistor is great in a 50 ohm system to give a 20dB ish loss. However as you work your way along those filter resonators you get further and further away from 50 ohms, and at the non grounded end then you'll see much higher voltages. I'd disconnect the bias to each amp in turn and measure the level with the 40MHz cal signal again. If it drops further then you know the amp is ok. If it doesn't change much then that amp is likely to be the issue. |
| G0HZU:
--- Quote ---about 30-40dB down at 2.7G compared to 40Mhz --- End quote --- I think you need to focus on this issue rather than poke around the amps in the first IF because the fault above is most likely to be something wrong in the first mixer or a LO1 drive level issue into the mixer or a fault in the RF signal path leading to the minicircuits mixer (or it could be linked to instability somewhere). Can you trace/probe the RF path through the attenuators up to the first mixer RF input port across 40-2700MHz to look for 30dB loss here at the high end? Note: If there's lots of absorber stuck to the underside of the lid then this is probably there to prevent (GHz) signals propagating around the unit by reflecting off the underside of the lid. This can cause a feedback path for oscillation so maybe the unit hoots if you try and just fit an alternative bare metal lid (with no absorber fitted). |
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