P57 feed-through terminator

26.08.2020 17:57

A quick note about this thing. It's a BNC feed-through terminator I've bought for cheap off AliExpress when I was on a kind of an RF accessory buying binge. After months of shipping delays it recently dropped into my mailbox. This one is marked "P57 load resistor 50 Ω". I've seen very similar looking devices being sold under various other names and brands. I've bought it because my TDS 2002B scope does not have a low-impedance input option.

P57 load resistor

The label says that the terminator is rated from DC to 1 GHz. The analog bandwidth of my scope is only 60 MHz, so that shouldn't be an issue. The DC resistance between signal pin and ground measures exactly 50.0 Ω on my Keysight U1241C. That's a good sign that it doesn't just have a standard carbon 51 Ω resistor inside.

The build quality looks fine at the first glance, although with the plastic body I wouldn't use this where any kind of significant power would be dissipated on the load.

Measured amplitude versus frequency with and without the feed-through terminator.

This is the result of a quick test I did. I connected the ERASynth Micro to the oscilloscope CH 1 over a coax cable. The red plot shows the signal amplitude measured at various frequencies without the terminator (so terminated with 1 MΩ at the scope's probe input). The blue plot shows the amplitude with the cable terminated with P57 on the scope end. The amplitudes were measured with the FFT function and hence only take into account the base frequency, without any harmonics.

The ERASynth Micro was always set to 0 dBm output level. If everything would be perfect, the blue plot would be at -13 dBV and the red plot would be 6 dB higher (twice the amplitude). Falling amplitude beyond 60 MHz is expected because of the limited analog bandwidth of the scope's front end.

I've measured between 8 and 5 dB difference between the terminated and unterminated amplitudes, which seems fine. Or at least not excessively wrong. There's a lot of unknown errors in this measurement. Cable and adapter loss, ERASynth Micro output matching and level accuracy and so on.

In conclusion, it does what it says in the description and seems good enough for my purpose.

Posted by Tomaž | Categories: Analog


Even Chinese cheat with 2 x 100 Ohms in parallel. DC goes to 150 MHz and the rest is RF. Fixing TV remote controller here. Similar class of problems :-)

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