I thought I would post my conversion of an old video distribution amp to a frequency standard distribution amp.
The unit in question is an Extron Versa Tools MDA 5V (The "5"" is for five outputs. The "V" is Video, not volts). It is powered by a 12V source. IF ANYONE KNOW WHAT THAT POWER CONNECTOR IS (see pics) PLEASE LET ME KNOW, AS I WANT TO GET ONE.
I paid around $17 delivered on ePay for this. Not a bad deal vs. building one from scratch. It has a pretty fast opamp Ti2691 (datasheet
http://www.ti.com/lit/ds/symlink/opa2691.pdf ). It already has the BNC connectors, and a case.
This unit has a switch to switch between 75 Ohm and High Z for the input. This is a nice feature. It is basically a termination switch.
I wanted to use it to distribute a 10MHz frequency reference. As most of you likely know, composite video uses 75 Ohm transmission lines and 75 Ohm terminators at either end. For test equipment you typically want a 50 Ohm termination at each end, using 50 Ohm transmission lines. So I needed to replace the 75 Ohm resistors with 50 Ohm resistors.
I cracked this guy open and located the 75 Ohm resistor on the input R1. I also located the output 75 Ohm resistors R3, R4, R5, R6, R7 (Nice low numbers
). Two output lines of the OpAmp are used-- one with three outputs from it (R3,R4,R5) , the other with two outputs from it (R6,R7). Each output port has its own output resistor, though.
All these resistors were
1206 0402 ? package SMD resistors, which are really fricking small. I did not have any of these resistors, and was too lazy to go to the store. So I wired it up with big through-hole resistors. I used pairs of 100 Ohms to make 50 Ohms, rather than using 47 Ohms (which would likely have been fine).
You can see from the pics that I soldered down a piece of resistor lead to the existing pads on each side. I then soldered my big through hole resistors to these pins (using a heat sink so they did not come off the board). For the pair replacing R1 (by the switch) I found an unpopulated through-hole in line with one of the pads and ended up using it. I then soldered the other ends directly to the switch. So now the switch switches between no termination on the input and 50 Ohm termination on the input. The astute viewer may see the pads for R8. These were unpopulated, and unconnected at the other end of the trace. So I let my wire touch these with wild abandon.
I am sure the RF engineers are screaming at their computers right now. given my rather hacky technique. Since I was really only worried about the 10 MHz, and I do not work with higher frequency stuff much (other than radio repair) I was not too worried. Also, all outputs have the same signal anyway. It looks like the original design did not match trace lengths for the various outputs, as it probably did not matter.
My question to RF engineers. How much of an impact does wiring it like this have? I am assuming it is better to have the wires down on the board, close to the ground plane backing? Short of using SMD resistors, what other ways might you do this to improvement RF performance? Thanks.