Gain equalizers
Updated September
2, 2008
Click
here to visit our main page on attenuators.
Click
here to go to our page on parabolic equalizers.
What's a gain equalizer (or "equalizer"
for short)? It's a special attenuator that has a frequency response
that is intentionally not flat. There are many reasons you
might want to use an equalizer. Perhaps the most common is to fix
the problem of wide-band microwave systems. Most stuff has a negative
gain slope (it has less gain, or more loss, the higher in frequency
you go) This includes not only amplifiers, but passive stuff like
coax cables and microstrip transmission lines as well. The antidote
to negative gain slope is adding a linear gain equalizer that has
a positive gain slope to your lineup.
A second type of gain equalizer
provides a "parabolic" response, which is used to counteract
the gain of wide-band traveling wave tubes. New for November
2005! We just added a page on parabolic
equalizers!
There are not many microwave
vendors that advertise equalizers as part of their wares, many of
the techniques for this remain trade secrets of companies doing
wideband work. Sorry, we're not gonna provide any links to equalizer
vendors here, unless they show us the money!
Equalizer
example 1: C-band equalizer
Here, you want to straighten
out the gain of M/A-COM's MAAM37000-A1G surface mount LNA. The gain
plot below shows that it tilts about one dB from 3 to 7 GHz.
Fixing this is an easy assignment,
we're going to show you how to do it with a conventional surface-mount
attenuator chip. All you've got to do is mount a 3 dB chip attenuator,
with some added inductance to ground. Just move the ground via a
few hundred mils from the chip with some transmission line, and
you're styling. This is certainly the poor-man's equalizer!
We need to take a photo of the
configuration, and generate some plots, check back later for more
details.
Equalizer
example 2: wideband equalizer
Here's your next assignment.
You have to build a wideband module, 1 to 40 GHz, with 20 dB of
flat gain. Don't ask why, just do it! You decide to use a TriQuint
MMIC, because you feel sorry for everyone that has stock options
in TQNT, and you are hoping that by purchasing a few hundred amplifiers,
you will put them one more wafer away from bankruptcy. A good choice
is the TGA4830, below we have pilfered its gain response from TriQuint's
on-line data sheet.
You can cascade two of these
puppies and get more than 20 dB gain at 40 GHz, but you will have
four dB of gain tilt, which won't go away using a conventional attenuator.
Now what?
Let's design an ideal equalizer
that provides four dB positive slope over the same frequency band,
using ADS. One way to create an equalizer is to start with an ideal
attenuator network such as a tee, and add capacitors to "short
out" the series resistive elements with frequency, and inductors
to "open" the shunt resistive elements with frequency.
Like in the ADS diagram below. This is very much an ideal equalizer,
we have left out all parasitics that would be present in a physical
model.
A "perfect" equalizer
would be a matched attenuator with zero loss at 40 GHz, and 4 dB
loss at DC. However, you will have to accept some loss at the high
end, perhaps 0.5 dB. So first thing you do is go to the handy Microwaves101
attenuator calculator, and solve
the tee equations for 4.5 dB loss (R1=26 ohms, R2=36 ohms). Then
you play around with the capacitors and inductors within the ADS
model (be sure to keep the two capacitors equal). After five minutes,
you arrive at the following response:
Note that the loss
response is not perfectly linear with frequency, but it's close
enough for government work. When add this puppy between the two
TGA4830s, you will get a nice flat response. We'd show you the prediction,
but TriQuint hasn't posted an S-parameter file on this amplifier
yet.
Don't get too excited
at the success of this little project yet. You might not be able
to build this equalizer without all the parasitic elements messing
it up!
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