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<[email protected]> 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. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > -- --Eric _________________________________________ Eric Garner _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
