The LMH6702 is one of the few current feedback amplifiers that is stable
with a relatively low value (237 ohm) feedback resistor.
Its estimated phase noise floor is around -171 dBc/Hz with +13dBm output
(and input) in the noninverting 2x configuration with a 50 ohm load and
a 50 ohm resistor in series with its output.
Thats about 3dBc/Hz less noisy than the measured 10MHz phase noise floor
of a OPA653 in the same configuration with the same input and output
signal levels.
For offsets below about 100Hz the phase noise of an OPA653 is
indistinguishable from the noise of measurement setup.
The effect of drafts and other air currents on the phase shift of RF
transformers is also significant for offsets of around 1Hz or less.
Extra thermal mass (encapsulation) and draft shields (even a piece of
paper) can work wonders.
Bruce
Joseph M Gwinn wrote:
I have seen National LMH6702 current-feedback video amplifier chips in
non-inverting amplifier configuration used to implement a wideband 10 MHz
distribution amplifier.
Joe
From: "Charles P. Steinmetz"<charles_steinm...@lavabit.com>
To: Discussion of precise time and frequency measurement
<time-nuts@febo.com>
Date: 03/27/2012 07:28 PM
Subject: Re: [time-nuts] Distribution amp - Use a video amp unit ?
Sent by: time-nuts-boun...@febo.com
Bruce wrote:
The above reverse isolation [~35 dB] is about 25dB lower than I would
expect.
D'oh! Bruce is right -- I calculated the reverse isolation
incorrectly. I had only been expecting 40 dB, so I didn't question
the result. The breadboard actually measured nearly 63 dB.
Stable operation at unity gain is necessary if a feedback capacitor is
used.
The Miller capacitance of the output transistors sees to that (with
an even greater phase margin when a faster transistor is used for Q1).
An LM329 has similar noise without the dissipation of the internal
heater in the LM399
I know. I just particularly like the 399, and have a pile of
them. I rationalize using it in this case by noting that the range
of frequencies where phase noise of the DA is important includes sub-
to low-Hz frequencies at which thermal effects could make the
unheated 329 significantly noisier (though if you keep drafts off
both of them, it might not be by a large amount).
Best regards,
Charles
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