Looking for help with an HP 8554B spectrum analyzer plugin

[Eric F. Richards]

Greetings...

I don't know where to ask this, so I'm shooting for multiple groups.

I purchased an HP 140T mainframe plus spectrum analyzer plugins,
including the 8554B mentioned in the subject, at a hamfest. It turns
out that it is working but is quite "deaf." From asking around I have
been told that this is a common problem for this plugin and the
culprit is the first mixer.

Well... the first mixer is an upconverter to 3 GHz, and one of those
magical microwave "cans" that make no sense to those of us used to
thinking in lower frequencies. The only semiconductors in the can are
a pair of diodes, back to back, with the designation "385" on them.
HP weenies have suggested that it could be short for 1901-0385, but
that part number turns up very few references, and none with the
nature of the diodes and their operating characteristics.

So I'm wondering if anyone out there knows what type of diodes these
are likely to be and what kind of characteristics I should be looking
for in replacement parts.

Does anyone know anything about this dinosaur? Any help would be
appreciated.

Oh, BTW, the rest of the analyzer and frame work fine. I'm using a
borrowed 8553B plugin with it now and getting the results I'd hoped
for. While I don't plan on doing any critical work with it, the 8554B
is a more capable plugin and I do have to return the borrowed one...

Thanks much!

 

[Pete KE9OA]

You might be able to substitute them with 1N5711s. It looks like somebody
must have accidently transmitted into this unit.
You can find these plug-ins at hamfests in the 50 to 100 dollar range now,
but the 5711s might at least get you started.

Pete

 

[A E]

Greetings...

I don't know where to ask this, so I'm shooting for multiple groups.

I purchased an HP 140T mainframe plus spectrum analyzer plugins,
including the 8554B mentioned in the subject, at a hamfest. It turns
out that it is working but is quite "deaf." From asking around I have
been told that this is a common problem for this plugin and the
culprit is the first mixer.

Well... the first mixer is an upconverter to 3 GHz, and one of those
magical microwave "cans" that make no sense to those of us used to
thinking in lower frequencies. The only semiconductors in the can are
a pair of diodes, back to back, with the designation "385" on them.
HP weenies have suggested that it could be short for 1901-0385, but
that part number turns up very few references, and none with the
nature of the diodes and their operating characteristics.

These *might* be microwave mixer diodes, like the 1N416D, for example. The thing
to look for is noise figure. I have a 1L20 spectrum analyzer plug-in for my Tek
scope, from the same era as your SA (but much much cruder). I have never seen
your SA, but all I can say is that the first mixer in my 1L20 contains a single
diode.
Do yours look like this?
http://www.dfpresource.org/1n416d.jpg
If so, you might try a kludge like this:
http://www.dfpresource.org/modded1n5711.jpg
Worked for me.

 

[Eric F. Richards]

These *might* be microwave mixer diodes, like the 1N416D, for example. The thing
to look for is noise figure. I have a 1L20 spectrum analyzer plug-in for my Tek
scope, from the same era as your SA (but much much cruder). I have never seen
your SA, but all I can say is that the first mixer in my 1L20 contains a single
diode.
Do yours look like this?

Not even remotely. Here's a pic of the assembly, out of the "can:"

http://www.dim.com/~efricha/mixer-diodes.jpg

 

[Fred Abse]

Schottky barrier diodes like this are used in mixers all the time
because of their speed (no charge storage), but from what I understand,
weren't used back then because, well, they didn't exist.

They've existed since the 1950s.

The principle was postulated in 1938.

 

[Fred Abse]

Not even remotely. Here's a pic of the assembly, out of the "can:"

http://www.dim.com/~efricha/mixer-diodes.jpg

AHHHHH... They're wire-ended.

Go to agilent.com and have a look through their range of Schottky diodes.
There's almost bound to be something there.

 

[Eric F. Richards]

AHHHHH... They're wire-ended.

Go to agilent.com and have a look through their range of Schottky diodes.
There's almost bound to be something there.

Unfortunately I already did that, and Agilent's Big Book of
Semiconductors doesn't have a listing for it.

Don't take this wrong, but Usenet is kinda my last option. Or second
last -- I plan on getting a photocopy of the service manual.

Eric

 

[A E]

They've existed since the 1950s.

Yes, surface barrier transistors and such used the Schottky effect in those
times, but as a collector of some 60s vintage stuff, Schottky diodes are absent
in these units. Every other kind of diode, Schockley, tunnel, snap, varactor,
back, GaAs, Ge, etc... but not a single Schottky diode. In my 1L20 the 1N5711
performs the same as the 1N416D. So why did they use a complicated to build unit
like the 416D if they had had 1N5711-style devices back then? I suppose they
must have been used in the 60s because analog samplers with Schottky diodes are
certainly in the literature, so maybe they were too new, expensive?
Really, I'd like to know.

The principle was postulated in 1938.

But as a commercial product, when did Schottky diodes arrive on the market?

 

[Leigh W3NLB]

Oh well.


Sweet. Looks like a single balanced mixer with the LO injected in the coil on top? I'm
way out of my league here, I'm hoping that someone in the know jumps in here, I'd like
to know myself.
But, I'm guessing you tested those diodes by lifting one lead and determined that they
indeed are dead...

Testing a microwave mixer diode using an ohmmeter will guarantee that
it's dead. They make special low-current meters for testing these
animals.


73 de Leigh W3NLB

 

[Fred Abse]

So why did they use a
complicated to build unit like the 416D if they had had 1N5711-style
devices back then?

Many possible reasons. One is that their biggest customer (guess who)
already had them on inventory.

Old, proven, technology.

Who it was that made them comes into the equation, too. Use an
already-approved supplier rather than have to go through the paper chase
for a new one.

Look at any older Tektronix parts list, for example, and you'll find that
some of the biggest names in the industry back then are missing from the
supplier code listing. They seemed to like Motorola and RCA best.

 

[Eric F. Richards]

You might be able to substitute them with 1N5711s. It looks like somebody
must have accidently transmitted into this unit.
You can find these plug-ins at hamfests in the 50 to 100 dollar range now,
but the 5711s might at least get you started.

Pete

Pete,

I'm probably going to have to go this way since other suggestions have
appeared to be dead ends for now. Thanks for the suggestion. So...
is a '5711 a high-speed Schottkey? (I know, I can look it up, but my
connection is very slow right now...)

Thanks,

Eric

 

[A E]

Testing a microwave mixer diode using an ohmmeter will guarantee that
it's dead. They make special low-current meters for testing these
animals.

True, I use a 1.5V cheapie analog meter for testing tunnel diodes, but in my limited
experience, diodes like the 1N416D can be tested with modern DMM with no damage. But you're
right, I should have recommend testing with low current anyways.

 

[Steven Swift]

AHHHHH... They're wire-ended.

Can you get a close up of the numbers? I have a junk box full of odds and
ends of HP diodes.

Go to agilent.com and have a look through their range of Schottky diodes.
There's almost bound to be something there.

 

[Eric F. Richards]

Can you get a close up of the numbers? I have a junk box full of odds and
ends of HP diodes.

Not soon. The pic was taken by an experienced tech who took a look at
it as a favor. I'll have to *gently* get it apart myself or ask him to
do so again. He knows his way around RF but was uncomfortable working
on this thing; I'm almost scared spitless.

....the joys of microwave...

 

[Eric F. Richards]

Should have made not of this in the previous message. The markings
appear to be HP 385. Likely part number is 1901-0385

 

[Fred Abse]

It looks like somebody
must have accidently transmitted into this unit.

You don't need to transmit into most older analyzers. Much more than 10dBm
will croak some of them. You could do that with some signal generators.

BTDT

 

[Fred Abse]

I've been told that finding a contemporary replacement should be no
problem, but without specs, I'm really in the dark here.

Look for devices specced as mixers in the right band at the right power.
There should be info somewhere about local oscillator power for this
beast.

It's probably only going to be a few dBm, maybe a few tens of dBm.

 

[Roger Glover]

Should have made not of this in the previous message. The markings
appear to be HP 385. Likely part number is 1901-0385

Eric,

I can confirm that this is/was the right part number. In the '70's a matched
pair was ~UKP 150.

You might try something like a pair of BAR28, if you can get them, (Shottky
barrier - useful to 10GHz) at UKP 0.5 each IMO they would be worth trying.

Good luck,

 

[Pete KE9OA]

I haven't seen them blow out from signal generators, but I have from some of
the high output synthesizers that would put out +23dBm. It's too bad that
they couldn't make a more robust mixer....................you would figure
that they would be using at least a Level 17 mixer.

Pete

 

[Leigh W3NLB]

You don't need to transmit into most older analyzers. Much more than 10dBm
will croak some of them. You could do that with some signal generators.

BTDT

My 8554B says "Maximum 10 dBm" on the front in big red letters.

There's probably a reason why they put that on the panel.


73 de Leigh W3NLB