n-channel mosfet rise-fall time comparison

You have discussed making a simple 20dB resistive attenuator probe for low microwave frequencies in the past. I believe you recommended using a Caddock resistor for the series element. This would work fine at low microwave frequencies.

A thin film sheet might be also suitable for these frequencies, perhaps up to 5GHz or so. But have you see it used in a 40dB attenuator for 18GHz?

The series resistor would have low stray capacity between the ends, but it would still have capacity to the shield. Win's analysis shows this might give problems at 18GHz. Also, depending on the length, the stray inductance might become a problem at 18GHz.

So you want the length to be as short as possible to reduce the inductance, but this increases the capacitance betwen the ends.

Alltogether, high values of resistance are difficult to use at these frequencies.

OK, put four 10dB sections in cascade. The resistors are cheap, and the values remain the same.

Mike Monett

Darn, now you've got me curious. I wish people wouldn't do that.

ftp://66.117.156.8/VAT-20.zip

That's a Mini-Circuits VAT-20, a cheap 6 GHz, 20 dB attenuator. The 18 GHz parts are a lot smaller.

John

[...]

Why? How can you consider possibilities without curiosity?

What's the schematic look like? The black rectangles look like thick film resistors. But you can barely see the outline of something else on the surface. Is that thin-film?

Also, it looks like a single section 40dB attenuator for 18GHz may be difficult for a number of other reasons besides the ones I gave above. I did some research on microwave attenuators.

Agilent states their attenuators use thin film:

"Agilent attenuators achieve flat-frequency response and high accuracy through the use of thin-film attenuator cards. These cards are composed of high-stability tantalum nitride resistive film, deposited on sapphire or alumina substrates."

Vishay sells microwave thin-film resistors that go up to 2kohm:

"Industry's Smallest Thin-Film Resistors for Microwave Applications Offer High Performance in Chip as Small as 0.010" by 0.020""

Hittite recently announced three wideband attenuators for the DC to 25 GHz frequency band. The maximum attenuation is 20dB:

Barrie Industries offers a thin-film chip attenuator. The maximum attenuation is 30dB:

IMS offers thin-film attenuators for DC to 20 GHz. The maximum attenuation is only 10dB. The higher attenuations have a much lower frequency range:

MSI offers thin film attenuators to 24dB at 20GHz:

So the maxium attenuation I could find in a single section attenuator for 20GHz is 30dB or less. If a 40dB attenuator exists, it didn't show up in an extensive search.

I did find another on-line attenuator calculator. It handles PI, T, Bridged-T, and Balanced attenuators. It doesn't require javascript:

Benjamin Lewis has calculators that allow different input and output impedances. They also don't require javascript:

Microwaves 101 has an excellent page on microwave thin-films. They explain that 2 microinches of tantalum nitride (TaN) has a DC resistance of about 50 ohms per square. They go on to explain:

"This thickness of TaN is less than 1% of a skin depth at X-band, so the RF sheet resistance is very nearly equal to the DC value. The plot below shows how the RF skin depth varies over frequency; the error is only about 1% all the way up at W-band, less at lower frequencies. Nothing to concern yourself with."

So thin film is a necessity at these frequencies due to skin effect.

Here's the catch of the day. Triquint Semiconductors show how to calculate the attenuation of the evanescent wave in microwave cavities. The following paragraphs are especially interesting:

"Design Guidelines for Microwave Cavities"

"These equations provide a good first order approximation to the problem and can sometimes highlight serious radiation issues before the design is frozen. Due to the many other variables which can add to, or subtract from, the radiating or propagating signal (bondwires, substrates, microwave structures, filters, passive components, etc), it is best to stay as conservative as the design will allow. At higher frequencies such as 40 GHz it becomes difficult to build a channel below the waveguide cutoff frequency (b=0.147 inches at 40 GHz) and still support the circuit element sizes. To achieve a 3x ratio at 40 GHz would require a channel width of 0.049 inches and a height from module floor to lid less than this value."

"If the design dictates that active components, such as MMIC amplifiers, be placed in a propagating waveguide channel, it is prudent to limit their gain to 20 or 30 dB maximum. The use of absorber on the lid in this case will almost always be required and some gain ripple due to radiative feedback of the output signal can be expected. The best course of action is to keep everything very close to the ground plane. This reduces to a minimum the radiation of components such as bondwires and other transitions. It is not uncommon for a MMIC amplifier with 15 or 20 dB of gain to lose about 1 dB when a lid with absorber is placed above the MMIC. This is an indication that the radiative signal level is not negligible."

inesApNote.pdf

This indicates you might want to stay at or below 20 dB in single-section precision attenuators, as well as wideband amplifiers.

For critical high frequency work, you might want to look at chip bonding. Luis Cupido has started a Yahoo group to discuss the issues. The registration is free.

(Thanks to John Miles KE5FX for mentioning this group in one of the mailing lists.)

Luis Cupido's web site is a cornucopia of information for anyone interested in high frequency work. He even describes a Corner Cube Harmonic Mixer for 411GHz:

We are surrounded by a wealth of information that is available instantly for little or no effort. But the answers to one question immediately create more questions.

How can you not be curious?

Mike Monett

Thickfilm on alumina; screened and fired conductors, screened and fired cermet resistor elements, laser trimmed.

in >-----+-----R2-------R3------+------< out | | | | R1 R4 | | | | | | gnd>-----+----------------------+------< gnd

The R2-R3 node is a pretty big hunk of conductor, essentially a C to ground, probably to kill capacitive shoot-through, maybe to deliberately limit the bw to 6 GHz.

John

A la Winfield's high voltage probe. But the thick film will also have a built-in frequency limit due to skin effect, as described in the Microwaves

101 web page.

You said the 18 GHz parts are smaller. What do they look like?

Mike Monett

Ask Mr Google.

John

Thanks, John. That's money in the bank.

Mike Monett

Not sure what your up to with this Jamie, but here is a tip. After you finish your circuit don't discount the fact that the drain voltage can oscillate in production models. Just to pass this on we had to use a snubber on a simple switching amp to damp oscillations. There was no reason that we could see for this to happen but it did. So we loaded the drain with a series resistor and cap to ground and dumped the unwanted frequencies. They were are arcing the boards like an rf burn. A very tiny hole straight out of the body of a resistor we used for feedback that was directly connect to drain. You can tailor the rise and fall times of the fet by sourcing and sinking the gate drive with different impedance on each driver.

Jamie, so what, who says you have to drive the fet with a single gate resistor. Use a current sink and a current source with independent impedance to control the fet timing.