Electronics > Repair
Rohde & Schwarz - IQ Modulator A240 (1084.9800.02) - Level Problem
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rg58:
This one doesn't have a heat sink. It looks like the one on the right in your photo.
Yes, you can tell that some rework/modifications have been done here. It might be a refurbished one, who knows...

Makes sense that the heat will also be dissipated through the source terminals.
Harry_22:

--- Quote from: rg58 on May 14, 2024, 06:56:31 pm ---It's also interesting to see that they use those pin diodes for a controlled attenuation (as it seems), but maybe I am wrong?

--- End quote ---
I found following:

"A PIN diode is a semiconductor device that operates as a variable resistor at RF and microwave frequencies. The resistance value of the PIN diode is determined only by the forward biased dc current. In switch and attenuator applications, the PIN diode should ideally control the RF signal level without introducing distortion which might change the shape of the RF signal. An important additional feature of the PIN diode is its ability to control large RF signals while using much smaller levels of dc excitation."
Astralix:
Hi!

This seems to be a revision 6 board, as all earlier boards did not have any heat-sink screwed in.
The early boards all had the chamber shielding leading over the SHF output driver and massive ground areas with many through-holes into the other layers.

That made changing the output driver nearly impossible without damaging the PCB and surrounding parts.
In a later revision, that bridge from one side to the other of the casing was removed. There was also a change in the BIAS settings, probably lowering the BIAS voltage a bit.
And they used SHF-0286K instead of SHF-0186K sometimes.

But as we have not yet tested and repaired enough boards, we cannot say for sure, what to change and set to make the thing more reliable.
SHF-0186K is available from several sources as "used", sometimes "NOS". SHF-0286K is not available.

Wonder if there is a good hint how to remove the bridging shield on the old units and improve cooling after a repair without interfering with the electrical parameters. Bringing a metal plate in may influence the signal by pure existence... The Revision 6 PCBs have a clearly different layout compared to the older units.
rg58:
Thanks Harry! Just learned something new, something I should have known actually. They are doing the AM (amplitude modulation) and additionally the output level attenuation in a range of 0...-5dB with them.

I ordered the SHF-0186K from China. Let's see how it performs (if it's original?).

About the thermal pad, it could probably introduce some attenuation at higher frequencies, sitting right on top of the tiny FET packages (=>capacitance). But then again, the unit does a self-calibration and should even this out I believe.

Will try the get an ordinary, sticky, white thermal pad with a thickness slightly bigger than the old one which is 1.1mm.

On the bottom of the board's A side, besides the socket, there's a circle with a "97" inside and on the other side "-04-". Is this the revision number you are talking about? It suggests 04/1997.
rg58:
The other board is part number 1084.9800 and has "00" (as in 2000) and "-10.01-" on it - as if this was a "later" revision. Although the part number suggests this to be the FIRST version. Confused  ???
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