EMRFD Message Archive 10899

Message Date From Subject
10899 2015-03-20 12:59:02 ian_mccrum_mi5afl Measuring output return loss at low powers

Hi, new to the group, and homebrew really, about to start making modules for EI9GQ's HF transceiver, Avidly reading EMRFD and trying to learn the theory.


I can make a return loss bridge and can see how to measure the RL of the input of most of the circuits. With the higher output modules I can measure the output SWR directly as they feed a 50 Ohm load. At least I think I will be measuring the SWR/RL of the amplifiers output. Anyway, I can persevere with matching/feedback until I see good SWR into 50 Ohms at which point the actual RL doesn't matter.


How do people measure the RL of a module's output when it is outputting around or below a milliwatt?


I reckon I could drive a 50 Ohm load and use an oscilloscope for most measurements, perhaps a high value resistive divider on the output to give voltage and a series 1:n current transformer driving a resistor to measure current. (Bruene bridge I suppose). Phase might be observable by using XY mode...


I have read you can use an RL and a sig gen to drive into an amplifers output, with the amplifier input held low by a 50 Ohm resistor. I would worry that a parasitic oscillation might cause the amplifier output to damage the sig gen. In any case it is surely better to measure the output RL when the amplifier is running under more normal conditions.


Thanks for any advice

Ian


__Ian McCrum / MI5AFL

10900 2015-03-20 13:15:11 Ed Manuel Re: Measuring output return loss at low powers
Ian,

An AD8307 based Power Meter is sensitive enough to measure power and return loss at those levels.

73, Ed - N5EM



10901 2015-03-20 13:38:39 dave081360 Re: Measuring output return loss at low powers
10902 2015-03-20 16:44:48 ian_mccrum_mi5afl Re: Measuring output return loss at low powers
Hi Ed, yes but I am still clueless on how to measure the output RL of a an amplifier.

The input is straightforward ... Sig-gen ->---- RL bridge ------>---- input of amp
10903 2015-03-20 16:45:08 ian_mccrum_mi5afl Re: Measuring output return loss at low powers
Thanks Dave,   I will read it carefully, at first glance it only seems to cover testing an input - an antenna.

I can see how to do that...

 ... Sig-gen ->---- RL bridge ------>---- antenna
                               
10904 2015-03-20 16:57:11 vasilyivanenko Re: Measuring output return loss at low powers
Hi Ian

I've got a procedure in my old site archive:
http://qrp-popcorn.blogspot.ca/p/blog-page_9.html

Topics 2010-2011
10905 2015-03-20 20:18:01 dave081360 Re: Measuring output return loss at low powers
Why do you want to do that?  The SNA provides the RF for testing/analyzing the antenna.  You could use your QRP rig for the signal source by calibrating the SNA RF Power Meter for your rig's output.  The results should be the same as you would get using the SNA's DDS for the signal source.


73,
Dave
10908 2015-03-21 00:37:28 ian_mccrum_mi5afl Re: Measuring output return loss at low powers
Thanks Todd, a most clear answer, I will duplicate what you suggest, The anntenuator pads will protect the sig gen. After reading RF-Workbench-3 in the archive of Popcorn QRP: Old pops.net site downloads & info I have to say I am impressed, that is a very useful learning resource. - I shall now read all 945 pages!

 

All the best
73 de Ian/MI5AFL
10909 2015-03-21 00:38:11 ian_mccrum_mi5afl Re: Measuring output return loss at low powers
We are talking about testing the amplifier output, not the antenna. Cheers de IAN/MI5AFL
10914 2015-03-21 05:14:45 Thomas S. Knutsen Re: Measuring output return loss at low powers
Measuring output impedance is no different than measuring input impedance. You just rotate the amplifier 180 degrees, terminate the input and do the measurement using your choice of a coupler.

There is a alternative method, you can use the amplifier as a amplifier, and insert a directional coupler on the output. The coupler is then terminated in a resistive load, usualy 50 ohms.  This is the method used when measuring a oscillator output impedance. I have a sketch outlining the method here: http://imgur.com/1UPKA6o

A suitable coupler is shown in EMRFD fig.3.85. Its quite easy to build good directional couplers with the BN series binocular ferrite cores.

73 de Thomas LA3PNA.

2015-03-21 8:31 GMT+01:00 ij.mccrum@gmail.com [emrfd] <emrfd@yahoogroups.com>:
 

We are talking about testing the amplifier output, not the antenna. Cheers de IAN/MI5AFL

10917 2015-03-21 17:00:33 Kevin Morgan Re: Measuring output return loss at low powers
If anyone is interested, you can download the ARRL Test Procedures manual in pdf form here.

73, Kevin AB2ZI

--  Kevin J. Morgan AB2ZI  Blog: http://ab2zi.blogspot.com/ Photos
10918 2015-03-21 17:01:20 ian_mccrum_mi5afl Re: Measuring output return loss at low powers
Many thanks for the reply, I will try both methods (driving into the amplifier output with the input held at 50 Ohm and having the output drive a 50 Ohm load through a directional coupler.)
73 de Ian/MI5AFL