On May 3, 12:49*pm, msg wrote:
Greetings,

I would appreciate any citations to projects (etched boards, layouts,
anything that has been tested) for a small wideband amplifier with
hi-Z input (1mV - 100mV), AGC (over full gain range), 50 ohm output,
approx. 100kHz to 440MHz bandwidth with min. 15dB gain; the intent
is to provide a low impedance, leveled output from cheap old signal
generators (for setups to measure antenna impedance, circuit Q,
manual spectrum analysis, etc.) *Ideally, the AGC circuit would employ
an external level set control and three LEDs which indicate underrange,
locked and overrange respectively.

One would have thought that this would have been a popular item what
with the thousands of old Dynascan, EICO, Heathkit, etc. generators
still in the wild that have hi-Z unleveled outputs.

It could be built with discretes, but an AGC wideband IC makes sense,
such as the SL6140 (which is probably hard to find) -- recommendations
for any other device? Perhaps someone can point to some set-top box,
sat rcvr, cable tuner, etc. that may have something to use?

Thanks,

Michael

Over what range of output would you like the circuit to operate? I
suppose a 10dB range from, say, 0dBm to +10dBm would be appropriate.
You say "hi-Z input," but by the time you get to 440MHz, it doesn't
seem like the generators will have a high impedance output. Most of
the parts I can think of that would be helpful are already 50 ohm, or
similar, input. Maybe that's more because I deal mostly in 50 ohm
systems than that the parts aren't available. You didn't mention if
distortion (harmonic) is important, but I suppose so, and in fact, the
AGC loop should be so slow that you can pass AM modulation down to
pretty low frequencies.

It could be done with a small hand-full of parts, certainly: an MMIC
amplifier with an appropriate voltage divider at its input to get to a
higher input impedance and a reasonable level to drive a double-
balanced mixer (or similar; to control the level); another MMIC at the
mixer output to give the desired output level; an RF power detector IC
(suggest a Schottky-based one like the Linear Technology LT5507,
rather than a log detector, for this application) and a low frequency
amplifier/filter back to the baseband port of the DBM to control the
level. That's not a whole lot of parts, though there may well be
something closer to a single chip solution. However, a lot of RF ICs
don't cover down to 100kHz, and a lot of low frequency parts won't go
to 440MHz. I've played with a nice little 4-quadrant multiplier IC
from Analog Devices that would do the gain control part quite nicely:
ADL5391. Actually, its input impedance is probably high enough to use
directly with your sig-gen output, though the generator's output level
may be a bit too low for optimum performance. Also, beware that the
output impedance of the ADL5391 goes up with increasing frequency.

Hope this is useful food for thought...

Cheers,
Tom


Cheers,
Tom