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THE AM BULLETIN BOARD => Technical Forum => Topic started by: w1vtp on August 06, 2011, 04:17:30 PM



Title: Yet another sampler
Post by: w1vtp on August 06, 2011, 04:17:30 PM
We've had several good RF sampler topics on the forum.  Here's my version.  I've used this one off and on for decades.  Basically, I just tap off a tee with a resistor which is terminated with a 50 ohm feedthrough.

What I've done recently is crank out yet another Excel calculator that gives me the value of the divider resister I call R1.  The idea is simple, make a voltage divider that uses the 50 ohm termination of a scope.  There is a bunch of results that arn't necessary but the value of R1 and the power dissapated across it is useful for choosing the correct resistor.

I only use it on 75 meters but a similar project using carbon comp... resistors work up to 6 meters.

Input only in the yellow fields or an embedded formula will be wiped out.  This project could be put in a small Bud box.  I used a TEE and a Pomona  project box for the divider resistor that was fitted for the appropriate RF connector.

Al


Title: Re: Yet another sampler
Post by: AB2EZ on August 06, 2011, 05:05:19 PM
rAl

Just a few comments:

1. It is a little dangerous/risky  to depend only upon the termination impedance of a scope (nominally 50 Ohms) in making a single stage voltage divider. For example, if you plug the cable into a scope that (for whatever reason) doesn't have the 50 Ohm termination in place, you will have almost the full rf voltage across the scope (depending upon the input capacitance of the scope and the effect of the unterminated cable).

2. To obtain a 10000:1 voltage divider, the series resistor would have to be 500,000 Ohms. The parasitic capacitance, in parallel with a 500,000 Ohm resistor is likely to have an impedance that is much less than 500,000 Ohms. At 3.885 MHz, the capacitance whose corresponding impedance is 500,000 Ohms is less than 0.1 pF. Even a 1000:1 voltage divider will present challenges in this regard... if you want to get an accurate reading of the signal level on the scope.

For both of the above reasons, you might want to use a resistive network that consists of two or three stages of voltage division/attenuation. Even better, use a current transformer for the 1st N:1 stage... in order to keep the power flowing into the sampler as low as possible. The resistor across the secondary of the current transformer doesn't have to equal 50 Ohms (it could be 10 Ohms, for example), but 50 Ohms is often a convenient value to use.

Stu






Title: Re: Yet another sampler
Post by: KA3ZLR on August 06, 2011, 05:29:52 PM
Hi,

 In my Antenna Coupler the Feed line in I Have a Three Loop inductor
Placed around that line and there is no connection other than off to
my scope...werks great here...and it's inside and out of the way,

73
Jack
KA3ZLR


Title: Re: Yet another sampler
Post by: WD5JKO on August 07, 2011, 09:24:00 AM

Al,

   I have used a similar RF tap as you, but for reasons that Stu pointed out, I terminate to 50 ohms at the tap. Then if the coax is long it (as a guess > 10' for 80m), I terminate at the scope with 50 ohms. Years ago when I worked in a RF lab, I would build these taps, and then sweep them for XX DB loss from 160-6meters..Yes it does change. The goal was to take the maximum wattage to be sampled, and drop to 0 dbm. These can be very useful devices to feed frequency counters, scopes, spectrum analyzers, etc.

  It is also useful to add another divider tap to reduce the level from 0 dbm to maybe - 50 dbm. Here we can sample the RF with a receiver, and listen to yourself without any of the hum pick up common with other listening techniques. Could add a pot as well to vary level.

Jim
WD5JKO
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands