EEVblog #470 - Agilent N9344C 20GHz Spectrum Analyser Teardown

[mikeselectricstuff]

I’m wondering what these bodge wires are for. What kind of mistakes in the board layout remain unnoticed up until the board is already in production? And in which kind of situations is it even possible to still being able fix it with hacking in small wires afterwards?

It isn't always mistakes - for example it can be to adapt to use different parts if the original isn't available.

[EEVblog]

Did you reassemble this insturment? Does it work again?

Yes, seems to work fine.

[Dongulus]

Top-notch session of PCB and systems engineering porn.

I was confused by Dave's comment about how the 10 Mhz reference oscillator would not work after being turned over. I've never worked TCXOs before so I had no idea that they were sensitive to orientation. How can a robustly designed instrument intended for potentially rough field work be expected to operate containing such a component?

[c4757p]

I suspect it was a joke, similar to his usual "the electrons will fall out!" 

[EEVblog]

I suspect it was a joke, similar to his usual "the electrons will fall out!" 

 

[edpalmer42]

I suspect it was a joke, similar to his usual "the electrons will fall out!" 

Not to mention the 'right tongue angle'.

Wait, I mentioned it, didn't I?   

Ed

[SeanB]

I suspect the bodge wires connecting ground planes had more to do with a faulty via only detected during final test as being either open or intermittent, probably going open during the IR reflow of the board. Would have been better to have used a thicker wire, but hopefully the voltage regulator is happy with the added ground inductance.

[ftransform]

unsolder that gold shielding you coward, I would be proud of you then, like I am proud of war journalists.
put it on a hot plate... agilent said to take it apart......... common I know you want to....

[SeanB]

The copper tape over the hole is a vent for reflow soldering , so the case does not collapse from vacuum after soldering, or blow out the solder during reflow. The copper adhesive is most likely a conductive adhesive.

[bitwelder]

What's the purpose of the arrow on top of the gold shielding? Reflow direction?

[Watermelon]

Does anyone has a datasheet available for those odd 4 leaded RF amplifiers? Cannot find any at first glance 

Look for "4-Micro-X" as the Package / Case.  There are a few on DigiKey that do 0-12 GHz amplification from RFMD: http://www.digikey.com/product-detail/en/NBB-310-T1/689-1001-1-ND/1302337

This one has "N4" on the package like in the picture above.

Interesting read, thanks! Now i understand why they don't need an external supply, it's just 2 transistors and some resistors...

[pomonabill221]

I suspect it was a joke, similar to his usual "the electrons will fall out!" 

Or the electrons will all roll to one side of the can when the board is rotated...

[lewis]

What's the purpose of the arrow on top of the gold shielding? Reflow direction?

So you know which way to orientate the units so the electrons don't fa..... bugger it.

[SeanB]

Interesting read, thanks! Now i understand why they don't need an external supply, it's just 2 transistors and some resistors...

Some have a lot more than the 2 transistors inside, you can make a pretty complex amplifier if it needs to be level from near Dc to light, and has only 2 pins available. Those square ones must be a lot more, even if they have the same power arrangement.

[KJDS]

Having once worked at a test gear manufacturer I can give some insight into why there'll be a few bodges on the boards.

The early demonstrator models are part of the initial pre-production run. These are built and tested using the normal production facilities, but with design support. Some of these will then be used for the qualification testing, full temperature tests, both operational and storage, vibration, EMC and a whole load more detailed testing that would be carried out in production. Sometimes these tests bring up issues that require minor mods to a board and this will often be replicated across most of the batch. It's not unusual for the first couple of units to have components changed by designers who are not expert solderers just to prove that the mod works as intended. Once everyone is happy then some of these units become demo models, others will be used for accelerated and highly accelerated life tests and production will be allowed to ramp up, sometimes carrying on with minor board mods and sometimes waiting for a board respin and any associated requalification.

[ftransform]

I figured out how to remove the can nicely. Wrap it with resistance wire and heat it. Probably more safe for the board then a hot plate.

Aint anyone else dying to know what the fuck is under those cans?

dave should take some risks... he has it made.....

[larry42]

Under those cans? Local oscillators and the front end attenuator.

[ejeffrey]

Some examples, there is more top notch quality showing all over those boards, it seems really smart having expensive controlled dieletric pcb's and then just apply some dabs of random flux all over the impedance controlled traces, must be the secret sauce 

Flux on the PCB has essentially zero effect on the impedance.  For a microstrip geometry the capacitance is almost entirely due to fields inside the dielectric not above it -- which is the whole point.  A small fraction of the capacitance does come from the field above the board, but those field lines are relatively long, so a thin layer of dielectric crap won't really affect it.

[jancumps]

so a thin layer of dielectric crap won't really affect it.

If it would affect it, it wouldn't be there.

[kfitch42]

Did you reassemble this insturment? Does it work again?

Yes, seems to work fine.

But, how many parts were left over?   IIRC you said there were 2 million parts ... so if you had two 'extra' screws at the end that would be 1 PPM, right? That sounds pretty good when you put it that way

[thewyliestcoyote]

I love the tear down! Really shows all the RF design magic that is hard to come by.

I have a couple of 4 port +50GHz PNA's, older VNA's, and about 2 dozen field foxes at work and would look to see what they are in the insides and not get fired for destroying a instrument that costs more then I make in a couple of years.

Dave maybe you could do a teardown of a PNA, field fox, or the PXI unit for a 26.5 GHz VNA?

[rfbroadband]

at 20:25 its is a 1:1 125 MHz RF balun not a relay...

very nice tear town.