trimpot guts

John Larkin hath wroth:

Does it need to be a rotary, user adjustable, potentiometer, or can you use a "slide" type of pot for factory adjustments? See:

Allen-Bradley (and probably others) make these.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Why pots? I never use pots, creates service headaches and adds cost in production. What's that shiny surface you placed them on?

Another option can be dual-gates such as the BF998. My favorite used to be dies such as the SD5400 with several on there because you can DC-servo them. But then they became expensive and hard to find boutique parts :-(

Why don't they make something like the CD4007 in high-speed logic? Depending on your circuit you might be able to use an unbuffered high-speed inverter/buffer.

--
Regards, Joerg

http://www.analogconsultants.com/

PIN diodes?

Nope, John needs to regulate all the way down to DC. Of course, one could use PIN diodes plus a CD4007 but then it would really become unorthodox.

--
Regards, Joerg

http://www.analogconsultants.com/

Oh yeah, "step". My bad.

Is there any way to treat the internal structure like a transmission line? Put a ground plane behind the pot and use stripline to match the unit to the rest of the circuit?

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what\'s the reason?
For if it prosper, none dare call it treason."

"Follow The Money"  ;-P

And then a guy at Bourns improves design reliability and ...

--
Regards, Joerg

http://www.analogconsultants.com/

Are you trying to make 10 units or 10,000 units? If you can afford the extra time, you could use two parallel SMT resistors, one installed and one not, and use 1% resistors (glued to toothpicks for handles) to trim it. When you find the one you need, solder it in. With the right test rig you could measure the un-trimmed amplitude and compute the value to insert.

--
Ben Jackson AD7GD

http://www.ben.com/

Hi John,

"John Larkin" wrote in = message news: snipped-for-privacy@4ax.com...

How about Bourns 3329H? Note that the leads come straight down out of the pot and are only

0.1" apart.=20

How many dB range do you need? Are you doing L pad, T pad, pi pad, or ?? Or changing gain-determining resistors in an amp?

--=20 Regards, Howard snipped-for-privacy@ix.netcom.com

I actually have some of them, and will try them. I'd prefer surface mount, to avoid via capacitance, but I guess we could surface-mount them, too, if we bent the leads.

It's just

opamp output----------+ | | p o

1.2GHz with a potmeter?

How about these?

Ok, a bit noisy and sets you back five bucks in qties. I you go with the pot I'd buy tons of them so you don't get a black eye when Bourns "optimizes" the potmeter design and all your calcs go out the window.

--
Regards, Joerg

http://www.analogconsultants.com/

Sure, why not? The 3314G looks like it may work OK at that frequency, although it would be cool to find something faster. Hey, I could add a lowpass filter in front of my scope and see how that looks. I have an

870 MHz MiniCircuits gaussian filter around here somewhere. This pot definitely bleeds through around 3 GHz or so.

The DC situation looks dicey, too. All you RF guys ignore anything below a gazillion Hertz.

1 GHz design is easy; DC-to-1GHz is harder.

John

Or use a TV amplifier. Most of those roll off rather quickly around a GHz.

True. That's why I sometimes split DC from the RF path early on. You mentioned that it is to accommodate photodiode tolerances. Since those rarely exceed +/-15% couldn't you just use a resistive divider where the resistor to ground is a series combo of a real resistor and the DS path of a zippy RF FET? The BF998 comes to mind.

--
Regards, Joerg

http://www.analogconsultants.com/

Yesterday we were playing with a circuit (on a whiteboard) that would use two gaasfets and an opamp. We would dump some dc current into the drain of one fet and the opamp would servo its gate voltage to get the saturated drain voltage to some target. Apply that same gate voltage to the other fet, and it becomes a stabilized Rds-on to ground, part of an attenuator. Since the fets on one reel tend to match very well, that should work.

The little NEC fet we're using elsewhere (NE3509) make a nice variable resistor. It behaves just about like a super-hot jfet. Rds-on is about

6 ohms at zero gate voltage, 10 at -0.1, 30 at -0.2. Drain capacitance is almost constant at about 0.35 pF, about the same as a surface-mount resistor. Helluva part for 85 cents.

But that's a lot of junk to replace one trimpot.

Hey, just for fun while I was testing trimpots, I soldered two hardlines to a piece of copperclad, with a small gap between their center conductors, which I bridged with a 47 ohm, 0805 resistor. Then I did a TDR/TDT test on the mess. The thru signal had a 30 ps risetime (which is about what the scope is good for) and looked almost perfect, with a tiny bit of ringing, a few per cent maybe.

John

That's how I usually do that. But I won't rely on reel tracking. In cases where it's not all that hot you might want to consider these, under 10c:

Of course the 9pf start to hurt at a few hundred MHz but sometimes when you only need a dB or two you can set it in series with a resistor. Man, I wish the SD5400 hadn't become boutique-ware. Stuff was so easy when you could buy them for around two bucks.

However, you have to factor in the field returns where something changed after the unit got banged or the fighter pilot turned freighter pilot nailed it to the runway again.

Sound like the electronics version of shooting a can down the road ;-)

--
Regards, Joerg

http://www.analogconsultants.com/

Apologies for using google groups...

Have you looked at this attenuator?

In an ancient (paper) HP data book I found a DC-coupled (but specced to 26GHz) analogue variable PI-attenuator made from GaAs MESFETs, the part number was HMMC-1001. There are some interesting ideas in the datasheet. There is a replica attenuator on the same die and they use op-amps to servo the gate voltages so that the replica attenuator has the right attenuation and input impedance at DC, and therefore (assuming reasonable device matching across the die), so does the RF attenuator. I don't think Avago will admit that these devices ever existed so that part is mostly just interesting for the datasheet.

Chris

Again, these RF guys like to take liberties with the word "DC". The test circuit has 100 pF caps at the chip input and output, yet the attenuation accuracy spec is "DC

John Larkin a écrit :