Levelling in RF sig gen.

[CJay]

OK, I tried here a few months ago and got no replies so here goes again.

I have an Aeroflex IFR 3413 sig gen which displays an error, the error is unlevelled.

After running the unit for a period of time the RF module gets very hot, output from it is very low and distorted.

I suspect the output drivers are toasted, the heat appears to be coming from two linear regulators which supply power to the output devices.

Lifting the inductor that feeds power from said regulators to the output devices stops the heat generation and measuring from supply connection to ground shows a very low resistance for both output devices.

I've had no luck sourcing or even identifying the devices and the manufacturer does not publish, to my knowledge, schematics so I can't rely on a part number to find an alternative.

I need to draw out the schematic of the output stage but I think it varies the level of amplification by changing the applied voltage to the push pull final devices, does this sound like a reasonable method to the highly experienced RF guys here?

[PA0PBZ]

I need to draw out the schematic of the output stage but I think it varies the level of amplification by changing the applied voltage to the push pull final devices, does this sound like a reasonable method to the highly experienced RF guys here?

By applied voltage you mean the power supply or the RF into the final? Normally the RF level is controlled and the final has a fixed gain, you can't just lower the supply because you change the characteristics of the amplifier, and thereby changing chances of oscillating, distortion and so on.

Pictures?

[CJay]

I need to draw out the schematic of the output stage but I think it varies the level of amplification by changing the applied voltage to the push pull final devices, does this sound like a reasonable method to the highly experienced RF guys here?

By applied voltage you mean the power supply or the RF into the final? Normally the RF level is controlled and the final has a fixed gain, you can't just lower the supply because you change the characteristics of the amplifier, and thereby changing chances of oscillating, distortion and so on.

Pictures?

That would be the way I would think it worked too, push pull amplifier with variable drive but isolating the output from supply and altering the output level gives me a stepped drive signal, I think it may be AGC action but I haven't got the knowledge of this sort of equipment to say with any certainty.

I've attached two pics, showing where I've isolated the power in the second by lifting a lead from a choke that feeds the failed devices.

The H1681 devices seem to be short to ground or low ohms and the bad soldering is how I found it, seems someone else has had a go at it already.

[PA0PBZ]

Oh dear, someone has been there before, I hate that!

I found a block diagram and indeed the RF level to the final is ALC controlled and then the final is followed by a step attenuator, the usual setup.
The level detector must be somewhere in the bottom left corner of your first picture. Now to find out what these H1681 devices are, a first search didn't find anything even remotely close.

[CJay]

I have a feeling this one has been assembled out of parts, the seller has a pile of 'spares' he offered me after I bought this but he doesn't have any RF modules.

I don't mind taking a chance on faulty kit but this one was expensive and I'm not afraid to admit I don't feel completely confident repairing it.

I've been looking for that manual and the H1681 devices for a few months and found nothing, your Google Fu is strong

The attenuator board isn't fitted to mine, it's an option.

[rfeecs]

The H1681 devices seem to be short to ground or low ohms and the bad soldering is how I found it, seems someone else has had a go at it already.

H1681 might be Hexawave HWF1681RA:
http://www.hw.com.tw/PDF/FET/Ceramic/HWF1681RA_V3.pdf

If so, they are MESFETs.  Depletion mode and the gate looks like a diode.  So if you measured Drain-Source with no bias applied, they would look like about 1 ohm.

[CJay]

The H1681 devices seem to be short to ground or low ohms and the bad soldering is how I found it, seems someone else has had a go at it already.

H1681 might be Hexawave HWF1681RA:
http://www.hw.com.tw/PDF/FET/Ceramic/HWF1681RA_V3.pdf

If so, they are MESFETs.  Depletion mode and the gate looks like a diode.  So if you measured Drain-Source with no bias applied, they would look like about 1 ohm.

Ah, now that looks like the part, package is right as well and I knew in the recesses of my mind there were parts that behave like that but couldn't recall what they were.

Wonder if the bias voltage is good and correct then, I'll have to do some reading about how to operate them and try to work out the bias arrangement.

I suspect one device at least is faulty though, they measure slightly differently in circuit.

[CJay]

Interesting, MESFETs need negative voltage on the gate, so next step is to verify it's presence or lack of.

Any idea where to source the MESFETs or a suitable equivalent from?

I (very optimistically) tried the sample request on the Hexawave site, it throws an error, I've also sent an email to their UK distie but they don't even seem to list Hexawave as a supplier now.

[rfeecs]

I don't know if Hexawave is even still in business. 

Transcom maybe makes a similar part TC2676:
http://www.transcominc.com.tw/product_download_file.php?sfn=userdata/file/43_508505c364624.pdf&dfn=TC2676.pdf
But 8V bias instead of 10V, so that may be a problem.

Amcom may have something AM032MH4-BI-R:
http://www.amcomusa.com/downloads/datasheets/FET-series/AM032MH4-BI-R.pdf

Other alternatives are similarly sized GaAs pHEMTs or GaN HEMTs.  US companies like MwT, Wolfspeed (formerly Cree), MACOM (formerly Nitronex).  Japanese companies like Sumitomo, Toshiba, Mitsubishi.  But not likely in the same package and will have different performance.

Scrounging some parts off a spare unit might be a better option.

[rickells]

hi CJay,
I purchased an Aeroflex IFR 3416 that has No RF output & an ERROR # 522.
I want to repair my unit, and think that we have similar problems.
I need some guidance, you are well ahead of me !

thanks,