I recently started setting up to design the distribution amplifier system for my 10 MHz, so I've been rounding up the various parts I had set aside over the years. I had planned to use various transformers pulled from ethernet cards and routers, figuring they would be good at 10 MHz, but I didn't look at their specifications until now. I found that many of the common 10 Base-T interface modules contain Rx/Tx LPFs that cut off around 17 MHz, and provide about 20-30 dB attenuation at 30 MHz and up. I don't know if this has been discussed here before (is there a way to search these entire archives by subject?), but it's new to me, so I had to do some experimenting to see how well these might work. I air-wired a cascade of all sections of two Delta brand LANF3276 units, arranged so that their common-mode chokes would serve as baluns at the I/Os, to go from single-ended BNC pigtails into the differential environment of the transformers and filters. The impedance didn't match, but I was too lazy to make matching transformers for this quickie experiment. The rest of the wiring between sections was differential. Viewed with the TG/SA combo, it looked very promising - a sharp cutoff above 17 MHz, but lots of resonances at higher frequencies. I found that grounding all of the center taps and I/O cables was best, making quite a nice filter, but about as good as it gets with the open wiring connections. I am convinced that properly built with ground plane and shielding, and correct Z matching, this simple circuit would make an excellent brick-wall LPF for 10 MHz systems. Here's what I got as-is, with +10 dBm from the TG: 10 MHz insertion loss: 4 dB Attenuation: 20 MHz: 50 dB 30-70 MHz: 75-65 dB Resonances at 22 MHz (-45 dB) (maybe due to Z-mismatch or inherent to filters) and 75 MHz (-40 dB) (due to leakage around wiring and parts). Rising leakage above about 80 MHz, to about -45 dB at 100 MHz. Unknowns: Phase characteristics Phase noise Maximum power level capability I think the same filter properly implemented would provide over 100 dB attenuation from 30-100 MHz (and probably well-beyond), with about 6 dB insertion loss. It can also provide galvanic isolation, depending on the hookup. Not bad for a simple thing using readily available parts that were made in the millions of units. I plan to study these much more, and develop a final filter design to use when needed. I will likely use these or just the transformer-only type for the distribution system. These units are 100 ohm differential, and can easily be matched to 50 ohms with 5T:7T (or 10T:14T if center taps needed) transformers. For the analytically-minded, these types of units were made in such high volumes, and so thoroughly studied and specified, that a lot of info should be available to model and analyze the system behavior. I'm just going on experiments and the few data sheets I have found so far. BTW only the 10 Base-T types have the filters - the 10/100 types are wideband only. Ed