Wednesday, June 2, 2021

 Just an update on the QRM-180 and a bit of criticism of the X-phase:

I still get a lot of inquiries about the QRM device that I provided for a couple of years.  I thankfully have a large number of happy users out there and I am happy to try and answer questions as they come up.  I no longer list the QRM-180 on Ebay but a fellow Ham in Florida is now listing them and providing kits and assembled versions.  Contact Victor  if you are interested.

I unintentionally learned a great deal about noise cancelling in the process of creating the QRM-180 and in working with users for the past few years.  Some of what I learned follows:

First, the device relies upon getting a good sample of the noise signal separate from the primary signal.  That means that you must do some work to devise a suitable noise antenna.  A length of wire tossed on the shack floor is seldom satisfactory, unless your computer is the offender and the wire accidentally lands on it.

The noise signal must be from a nearby source and somehow different from the main signal.  If you can get a useful sample of a nearby man-made noise, you can proceed to try and cancel it.  But forget about trying to cancel general background noise or noise from the next city.  This is a QRM (Man-made noise) device not a QRN (Natural noise) device (one could only hope).  If you have a solar panel array nearby, noisy power lines, or some other motor/electronic noise one of these can do a terrific job of canceling that noise.  Otherwise this approach is probably not your answer.

As for the devices themselves, all of the ones I could dissect were very similar technically.  The ANC-4 seemed the best and also well made.  The MFJ 2026 has some decent circuit features but fell a bit short on being robust.  

The X-phase originated in Russia I believe, and is now available from multiple sources in China (there are also a few other copies of this device).  Sadly, it is just not useful as is, and I am afraid that it is creating a bad reputation for these types of devices.   

The big failing of the X-phase is that it is nearly impossible to get a large enough noise signal without having an antenna so large that is also picking up the desired signal.  So the good signal gets cancelled along with the noise.  The easy remedy is to add an amplifier on the noise input.  Ebay sells LNAs for that frequency range for under $10 and that solves the problem.  In fact it may be useful to lower the supply voltage on the LNA to get a little less gain.

The second X-phase problem is similar...  the output signal level is low.  To compensate you have to up the volume or change the front end setting on your receiver.  Workable, but a real pain in operation.  The MFJ unit had an additional stage of gain to alleviate this problem.  With a bit of perf-board you could add this stage to an X-Phase but it is a bit tedious.

The last complaints about the X-phase are power related.  If you run under 100W, and always with good SWRs the X-phase relay is barely large enough.  Anything else and that relay is destined to generate some unwanted smoke.  Making matters worse is the fact that there are no fuses of any sort to protect things in the event of some unexpected condition.  Even with added fuses I managed to burn the contacts on a couple of relays.

And while all of the QRM devices introduce some small insertion loss, the X-phase becomes a problem, especially above 20M.  Replacing some of the internal connector wiring might help, but I never tried.  Adding fuses would still be a good idea though.

If you are still reading, know that the QRM-180 has a good pre-amp with level adjustment, and the same buffer amplifier circuit as the MFJ design.   It also has fuses, a heavy relay good to 250W, and some circuitry to cut out the device if RF is detected.  If you have lots of money and know that this sort of device will help your problem consider the ANC-4.  But in most cases the QRM-180 will work as well.

Lastly, it is not a very complex circuit and the components are not overly critical or difficult to source.  If you are handy with a soldering iron, you could easily build one on some perf-board and put it in a nice cigar box.  Follow the schematic at the front of this blog.

73s and keep the noise down.


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