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[time-freq] TADD-1 gain?

John Ackermann N8UR jra at febo.com
Fri Sep 8 01:27:12 UTC 2006

Hi Christopher --

I am just doing a quick email scan at the moment after being away for a
bit, so this isn't a researched answer, but you should be able to get
more output than that.

One limiting factor in the design is a voltage clamp in the MAX477 chip
that limits its output to 7 volts p-p unloaded, and about 5 volts when
loaded with 50 ohms.

You lose gain the TADD-1 in two places:  first, in the terminator at the
output of the first amp stage, which is a 270 ohm resistor and therefore
shouldn't load things down too badly but does have some effect, and
second in the series 50 ohm back-termination resistor in each of the
output amps, which effectively drops the output in half if the load is
50 ohms.

The overall gain/loss is a combination of the gain setting on the input
amp, the voltage loss in the 270 ohm terminator, and the loss in the
output terminators, with an overlay of the clamping voltage that may
limit the gain depending on the input level.

Finally, the R14/R15 voltage divider does somewhat lower the maximum
gain of the first stage from the theoretical value.  Reducing R14/R15 to
lower (but equal) values will increase the maximum gain a bit, but will
increase the current through those resistors.

Christopher Hoover said the following on 09/07/2006 06:52 PM:
> All power measurements below are 50 Z referenced (i.e. they were
> measured with a 50 Z power meter).
> If I put +10 dBm @ 10 MHz from a 3325A (or Z3801A or FTS 4050) into my
> TADD-1's with JP3 (50 Z term) installed and gain set for maximum, I get
> ~ +5 dBm on the outputs.
> With a single TADD-2, +5 dBm on the outputs works well enough.  I didn't
> even notice until I tried to chain one TADD-1 off of another: output
> levels drop down to 0 dBm.  And that's too low for some of my gear.
> If I pull the 50 Z termination, I can obtain unity gain plus a bit (~
> 0.5 dBm) more.  That's an unsurprising result -- getting rid of the 50 Z
> / 50Z voltage divider -- but now I've lost termination of the far end of
> the coax.
> This performance appears inconsistent with the manual.  Staring at
> schematic, I'm not sure sure where the gain has gone.   It looks like I
> should be able to get more than 10x gain (modulo clipping) from the
> input stage.  The output stages obviously run at unity gain.
> I'm relatively confident of the construction, but there's always a
> chance I made mistakes (or, one identical mistake?) on all three boards.
> Any help?
> -- ch
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