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

Bruce Lane kyrrin at bluefeathertech.com
Sun Jun 7 12:14:48 EDT 2009


Hi, Bill,

	I would be at least curious about the beastie. Please do post what you have.

	Thanks.

*********** REPLY SEPARATOR  ***********

On 07-Jun-09 at 12:06 wje 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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-=-=-=-=-=-=-=-=-=-=-=-
Bruce Lane, Owner & Head Hardware Heavy,
Blue Feather Technologies -- http://www.bluefeathertech.com
kyrrin (at) bluefeathertech do/t c=o=m
"Quid Malmborg in Plano..."





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