trimpots at speed

I'm thinking of using three current-mode opamps: tia stage, inverting amp, output buffer. I could tweak the gain anywhere, probably the middle stage.

I've used supply-current trimming (with a dac) to set the input impedance of a mmic. One of the Sirenza SiGe parts can be rimmed to exactly 50 ohms input impedance. It changes the gain a bit, of course.

That is an issue. Just replacing a fiber cable with an "identical" one can change things a few dB; just unmating/remating one connection can do the same thing, especially on the laser end.

I guess I'd look pretty silly selling a "calibrated" detector if the cal spec was +-50% or something.

John

One pF in odd places can do a lot of strange things in a 200-MHz TIA, if it's inside the feedback loop. Getting modestly well-calibrated dc response is pretty simple, but having the ac gain be the same is the trick. The transimpedance bandwidth is roughly sqrt(GBW*f_RC), where f_RC is the 3 dB corner of the feedback resistor and the total capacitance on the summing junction (C_diode + C_in), so to get 200 MHz from an 800 MHz op amp, you need 1/(2*pi*Rf*C) > 50 MHz.

I'd probably put the pot in a low-impedance part of the circuit following the TIA. Maybe use a voltage divider with a loaded pot as the shunt element (e.g. 500 ohm pot with 50 ohms from each end to the wiper). That way the problem is more like scope probe compensation and less like Q-multiplier design.

Back in the palmy days, Bourns used to make 50- and 75-ohm variable attenuators that worked up to > 100 MHz.

Rene's comments about accuracy are also worth thinking about, though I'm not quite as pessimistic as he is. Still, you might need to think about how to control etalon fringes in the fibre. An APC fibre stub between the input connector and the PD would be a good possibility--it would reduce the backreflection from the fibre end significantly, and give you a clean and repeatable transition from fibre to photodiode.

Cheers,

Phil Hobbs

So call it "calibratable" or "adjustable." Almost every time something has changed in a CATV system, I've had to rebalance or sweep.

Ever coil yer fiber around a pencil?

John, you mean to use the pot as feedback in a transimpedance or opamp. Considering that going with 50 Ohms becomes more important the higher the frequency is, I see some problems with the pot to get a decent impedance. Otherwise is is just a few additional nH and thus RF-fitting.

I once did a gain adjustment by using a pot in the supply path to set the gain of MAR6 amplifiers. This worked at the expense of VSWR and such. But it worked. Now I'd perhaps use a voltage controlled attenuator, such as the RVA2500 plus some excess gain.

BTW, the calibrated version ... photodiodes are not necessarily linear when the pulses are shorter than the bandwidth of the diode, just in case. Exact optical power measurements are a nightmare. Being 50% off is common.

Rene

I've seen power meters of the expensive, bolometer type to differ by factors of two when illuminated by the identical beam. All claimed to be calibrated. Upon closer thinking I found calibrating a light source myself to be rather tricky. Compare to a blackbody ? The unknown emissivity comes in. Attenuate and count the photons ? What about the lost photons ? Apparently the industry can live with a uncertainity of 3dB, the problem is known I was told.

The APC connector is a good idea to reduce backscatter. Is it polarisation dependent ?

Rene

Trimpots have been used for a long time in oscilloscope vertical amplifiers.

What follows will probably be obvious to you: if you use them in a low-impedance path, the inductance associated with the path length will get you eventually; on a high-impedance node, it will be the stray capacitance that will be problematic.

Be careful with design, part selection, and layout and you will almost certainly be able to achieve >200MHz bandwidth. It's been done, and with larger parts than the tiny parts now available.

-frank