[time-nuts] 4 channel 1 Mhz - 25 Mhz distribution amp

Eric Garner garnere at gmail.com
Sun Jun 7 12:12:56 EDT 2009


I've been thinking about building something like this for some time
and I would be interested in seeing the schematics & layout. please do
post them!

thanks

-eric

On Sun, Jun 7, 2009 at 9:06 AM, wje<wje at quackers.net> wrote:
> I finally gave up trying to find a 50 ohm distribution amp I liked and
> wanted to pay for, so I built a simple 4 channel one that uses either a
> Linear LT1365 or an Analog Devices AD8044. It has an ac-coupled input and
> transformer-isolated outputs. I took care in the layout to keep trace
> lengths exactly the same for all channels, and they're also striplines. (not
> that using striplines makes much difference with 0.5" trace lengths).
>
> Warning - it doesn't have perfect unity gain, 0 chan-chan phase error,
> infinitely low noise, infiinte isolation, or any of those other
> characteristics Time-Nuts expect.
>
> What it does to (using the LT1365 @ 10Mhz) is accept a 10dBm input, provide
> 50dB worst-case channel-channel isolation, 68 dB worst-case output-input
> reverse isolation, and worst-case 0.2 degree channel-channel phase error.
> I'm not quite sure about the noise; it's down around the noise level of my
> HP 3588A, which reports -139dBm/rtHz for the amp. This is also the basic
> noise figure of the 3588, so the amp is probably lower. The amp specs say
> 9nV/rtHz, which works out to -147dBm referred to 50 ohms, assuming I did my
> quick calculations correctly.
>
> Interestingly, the phase error between 3 of the 4 channels is down around
> 0.02 degrees; just one channel has the 0.2 degree error relative to all the
> others. This was measured with my HP 5370 using 10k samples per reading. The
> LT1365 datasheet says 0.04 max. Hmm.. maybe I should check the coax for that
> channel.
>
> The gain is less than unity at all frequencies. Why? Because I wanted to be
> able to accept a 10dBm input without clipping and still get close to that
> out. Using the LT1365, this is possible. The AD8044 will start clipping at
> about 6dBm because it doesn't have enough current drive. However, it has a
> MUCH flatter bandwidth curve, managing 0.2 dB from <1Mhz to >50Mhz. The
> LT1365 is significantly worse, but for a single-frequency distribution amp,
> this isn't all that important. Using the LT1365, the gain is -1.5 dB @ 10
> Mhz, -0.9 @ 5Mhz. Of course, you can trim the gain for whatever you want
> within the voltage and current limits of the op amp. The opamp is set for a
> gain of 2 by default (to drive the series-terminated output transformer).
>
> If anyone is interested, I can put the schematic, pc board layout, and
> various plots from my 3588 for the amp on my FTP site.
>
> --
> Bill Ezell
> ----------
> They said 'Windows or better'
> so I used Linux.
>
>
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-- 
--Eric
_________________________________________
Eric Garner




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