picosecond test points

Per MHz, actually.

--

John Larkin, President Highland Technology Inc

jlarkin at highlandtechnology dot com

Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators

John Larkin a écrit :

SD14?

--
Thanks,
Fred.

something like this:

think mouser have them

-Lasse

Looks good. Another approach would be something like a BFP650 follower, with the 50-ohm coax forming the emitter load when the probe is connected. You'd need a very small base inductor to keep it from oscillating.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

I have a couple, but the tip is nasty and it's hard to ground.

John

--

John Larkin, President       Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

OK, that leads to...

What I don't understand is that all the inner probes seem to be flush with the outer shell, which doesn't look like it would make a reliable connection to the board.

The PCB layout for these would be an outer ground ring and an inner via, with the signal trace on an inner or bottom layer. Or maybe an outer broken/c-shaped ring for grounding, and the signal/pad on top.

--

John Larkin, President       Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

both outer and inner are spring loaded, from the datasheet:

Spring Force: Signal Conductor: 2.3 oz. @ .107 (2.72) travel Shielding Plunger: 5.8 oz. @ .107 (2.72) travel

yes something like that, maybe just a standard round test point with a big enough void in the soldermask to get to the ground plane

btw. seen this?:

-Lasse

.pdf

It's a spring loaded plunger...

,

I did something like that on the little up-grade board that put ECLinPS into the critical bit of an old TTL-based pulse generator for electron spin resonance - that would have been back around 1996.

The original designer wanted buffered test points so that he could probe the critical signals without loading the trace, so I buffered the relevant traces with BFR92A emitter-followers. We didn't bother with a base-stopping inductor - something like an 0603 33R hard up against the base did the job perfectly adequately.

-- Bill Sloman

Phil Hobbs wrote: : Looks good. Another approach would be something like a BFP650 follower,

Phil, I was about to suggest that if you've ended up using the BFP650 by following my example, you might be better off with the BFP640. Or more modern ones with a higher hFE (I'm fond of the NESG3031 and NESG4030).

Namely, my choice of BFP650 (and not the then more widely available BFP640) was driven by the search of the lowest possible R_BB, because I was deemed to encounter lower source resistances than the 50ohms for which SiGe devices are generally optimized. I reasoned that the base geometry of the BFP650, as a medium power device, is more likely yield a low R_BB. In your applications BFP640 may work better with the "more ordinary" impedance levels and ambient temperatures.

The "I was about.." part means that I'm somewhat puzzled now that I looked up the recent Infineon data sheets. The BFP650 is now listed as a SiGe:C part (the datasheet 2010-10-22) and its Gummel-Poon model reads RB=6.376 . The preliminary datasheet I have, dated Aug-16-2004, lists it as an ordinary SiGe part with RB=1.036 . So, I'm wondering how much the device has changed from the versions I've been using. I seem to recall that also in some other more recent data sheet versions it was listed as a SiGe, not SiGe:C .

Given the fact that the most recent BFP640 datasheet (dated 2007-05-29) reads RB=3.129, the BFP650 does not seem to have any noise advantage any more, either.

In fact, there was a recent paper in the RSI where the Jena group claim to have achieved 15 pV/rtHz with BFP640 in LHe, as compared with my 75 pV/rtHz with BFP650. This is a puzzling result, actually, because it is better than one would expect in the picture where u_N originates from the collector shot noise acting on the r_E - provided that the thermal voltage saturates at ~5..7 meV even when the ambient temperature keeps going down. According to my data it does saturate in BFP640 just like it does in all other SiGe's I've tried, when measured by the transconductance. I'm wondering whether my transistors have always been oscillating so high that I cannot see it (ie. >27 GHz) and the V_T saturation is an artefact due to it...

Regards, Mikko

... a typo, sorry, they actually claim 35 pV/rtHz . And they seem to have regenerated the emitter inductively, so the instability hypothesis may indeed be true in my case.

Regards, Mikko

pdf

Yup, I was going to suggest a connector if you've got room.

Maybe just a ground ring on top for the hardline.. and some sort of springie contact for the signal on the bottom.

gnd-----. .----gnd-- | | |.-- | -----*| *------ ----+ ^ +-springie contanct (phospur bronze?)

George H.

I started by taking your advice, which was pretty valuable--thanks again. I noticed the change in specs as well, when I ordered another batch a couple of months ago. They changed the type number to BFP650H instead of BFP650E when they changed the process. (The dogs.)

I'm using it as a cascode stage for a SKY65050 pHEMT, and it works great. One of the best things about it is that it has effectively infinite Early voltage, so you can get a lot of voltage gain out of a single stage. Even for situations where you don't care so much about ultralow noise, the combination of a gigantic f_T with a very high V_A is unique in my collection. It does want to oscillate at 14 GHz if you look at it crosswise, but a nice 5-ohm bead in series with the base cleans that right up.

I'm going to be using it in a new front end design, as a bootstrapped-bootstrap wrapped round a couple of parallelled BF862s. DC coupling the bootstrap and using a current source load gets rid of the thermal tails on the BF862s by keeping their dissipation constant. (Running them just slightly above I_DSS gives them a zero tempco anyway, but this one needs really good dc stability.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Funny you should say "cascode." I was just about to post the following:

If I want a fast, low-capacitance, accurate, positive current source, I could build a fairly slow active current source with a voltage reference, a resistor, an opamp, and a p-fet. But it would have a lot of capacitance and would be fairly slow. So I could cascode that with a microwave-type PNP transistor, with a bit of base resistance or a ferrite bead to keep it stable. But then I'd have the base current error, and the good fast PNPs, what few there are, have mediocre betas.

A PNP darlington is too slow.

So, how to correct for the base current error?

Two ideas so far: make a Darlington, but add a lowpass filter from the higher-current transistor base, into the emitter of the second transistor. The main transistor needed a base resistor anyhow.

Or sense the base current of the PNP, with opamps and such, and increase the current of the precision/slow source to make up for it.

--

John Larkin, President       Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

John Larkin a écrit :

Try this :

Ic = 3mA

1K | ___ |/ 5V>-----|___|----+------| | |>

.-. | | | | 1K | | | '-' | | | |\ | ||-+ 3V>--|+\ | ||---------|-----||-+ .-|-/ | | | |/ | ___ | '--------------+--|___|-+ | 1K .-. | | | |1K '-' | === GND

Delta Ic < 3nA for 45

Fred Bartoli a écrit :

Cute!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

If you use a BFP650, VAF is huge (or even negative), even with a 40 GHz f_T. See Figure 3 on the datasheet,

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net