A Pure Signal RF Sampler

To operate my station, an ANAN 100D and an SPE 1.3K Amplifier, with pure signal (Pre-Distortion), I need to sample the RF (RF-Sampler) from the amplifier output and feed a fraction of that signal back to the ANAN. Looking at the BiDirectional Coupler I made a while back (made a few) I decided to use the same principle. Using a current transformer to pickup a small
portion of the forward signal, attenuate it a bit more and then feed it into the ANAN as the processing sample for the Pre-Distortion algorithm.

My Requirements are:
Input power handling:  > 60dBm (future proofing)
Sampled power: 50dB down of through signal (-50dB)
Frequency response: 1.8 - 50 MHz

First I made a 30dB current transformer according to Building a Bidirectional coupler and then, to get the required 50dB, I'll add a 20dB attenuator (L-Pad), simple. So if we feed 1KW (1kW = 60dBm = 224V) through the sampler and, since the sample is attenuated by 50dB (30dB by the transformer plus 20dB by the resistiv pad) we get 10dBm (60dBm - 50dB = 10dBm) at the sampled port (10dBm = 10mW or 0.7V).



If you look closely, you'll find the mistake I made! Yep, the 10Ω R is positioned wrong! If you don't measure 50dB, you might have R1 soldered incorrectly. I measured 46dB with R1 soldered incorrectly.


Well, 50dB it is, and nice and flat up to 70MHz. A word about the Teflon tape, YOU DO NOT NEED IT ! I use it to make sure the the insulation of the wire does not scrape off on the edge of the toroide. So, it is an additional insulation for the protection of the wire. Wind the Teflon tape around the toroide and then add the 32 turns of the 30AWG wire. Checkout the BiDirectional Coupler page for more details on how to make the transformer. 

One more thing, this RF-Sensor (TAP) is a very useful addition to any shack. It will allow you to monitor your signal on a CRO or SpecAn (SDR-IQ, SDR-Dongle etc). As this unit is for my ANAN as a pure signal tap, I decided that 50dB is enough for me. If you like to use this as an RF-Sampler for monitoring/measuring purposes you might like to chose a different internal attenuation value. Instead of 20dB you might like to use 10 - or 30dB or any other value you fancy.

Parts List:
1x short piece of coax (RG213, RG402, RG316 or similar) to make the link between the input and output.
Teflon plumbers tape (there is no need for this YMMV)
85cm of 30AWG enameled wire (depends on the toroide size)
R1 = 10Ω ½ W Metalfilm (at 1500W the dissipation is about 300mW)
R2 = 39Ω ½ W Metalfilm

NOTE: It looks like the LO1230 (measured AL approx 600)  is hard to get in EU, a close relative would be the FT50(A)-43 (AL 440/480). So if you have issues getting the LO, use the FT.

You can play with the following spreadsheet to get a view of internal attenuation values.

Sampler Calc

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