QRM-180
V3.41
Manual
07.4
Another
Noise Reduction System
BACKGROUND:
Unless
you live far out in the countryside you likely have your share of RF
interference from TVs, LED lamps, power lines, computers, solar
panels, etc. A common method to reduce QRM (popular since the days
of vacuum tube TVs) is to use a second antenna to collect a suitable
sample of the noise, invert that signal and add it back to the main
signal. It it is a fairly straightforward design that doesn’t rely
upon clever filters and traps specific to the type of noise.
And
the good news is that they can be reasonably effective. Don’t
expect miracles but
one of these can get rid of enough noise to make them well worth
having. A number of commercial versions available today are targeted
especially for amateur radio, and while they are designed with
amateur radio transceivers in mind, they will work equally well on
receiver only and SDR applications. Again, they can do a good job
canceling a local noise source, but not atmospheric noise. There are
a lot of reviews on line as well as a bunch of YouTube videos that
give some useful information on tuning them and noise antenna design.
The
QRM-180 came about because the commercial units actually worked but
seemed to be missing some needed features. Unfortunately they are
largely built with surface mount components and were difficult to
modify. The QRM-180 board (with ground-plane top and bottom)
managed to fix many
of the shortcomings (my subjective opinion) of the commercial units.
The circuitry is not all that original, it is a combination of what
was deemed to be the best of the existing designs. The board is all
through-hole components which makes it much, much easier than surface
mount to build by hand and also makes it serviceable in the future.
The circuit could have been laid out with surface mount and machine
assembled (much cheaper) but selling the kits is a way to encourage
more hands-on involvement in ham radio. Early hams had Heathkits,
Allied Radio, Eico kits, EMC, and others, but now there are just too
few useful kits on the market.
The
case is a compromise between being roomy enough for fumble-fingered
assembly and not so big as to take up valuable space next to the
transceiver. It sits comfortably on top of most external speakers,
or on top of the transceiver itself. It is thick aluminum so it is
well shielded and heavy enough not to slide about during use. The
electronics is easily removable from the enclosure for assembly and
service. The front and rear panels are made from PCB material with
copper ground planes on both sides to complete the shielding of the
enclosure.
FUNCTION:
A
separate noise antenna gathers what is hoped to be a good sample of
the offending noise without a lot of your coveted DX signal. That
sample is then run through a broad band amplifier before being
inverted by transformer T1 and fed to a phase shifter. Two J310 FETs
then act to buffer and combine the two basic signals. Several
schemes were tested for phase adjustment, and this one was not only
the simplest, it introduced less distortion.
The
combined signal is then buffered by Q4 and sent to the receiver
through a relay. In transmit mode, the relays serve to isolate the
device and allow the transceiver to be connected directly to its
antenna. The relay is large enough to comfortably handle 200w, but
make sure that you put the QRM-180 between your
transceiver and your
linear if you use one. It will
probably
be
toast if you try to install it following the linear.
The
PTT (really just the transmit signal
not necessarily
the
actual
PTT from
the mike)
input grounds the gate of Q2 which in turn turns on Q4 the power tab
transistor. This not only energizes the relays, it supplies power to
the rest of the circuitry. The relays energize on PTT low so that
the default state is safe
(circuitry bypassed) when the power is off.
There
are fuses and protection diodes on the signal
inputs.
In spite of some odd
suggestions
on the Internet, don’t remove them. They are useful and do not
degrade the performance at all (unless they are blown, duh). The
pi
filters are perhaps
debatable but for the bands where the
QRM-180
is useful they are a good thing.
The provided amplifier for the
noise antenna is a commercially available pre-amp (LNA) that
provides a nice flat, low distortion 30dB of gain that can be
adjusted down using RV1. In very rare cases where the noise is very
large and the noise antenna is also large, the LNA may not be
required and could be bypassed. You can also try plugging the LNA
into the 9V power connector. This reduces the gain and improves the
flatness a bit.
The
QRM-180
does
have transmit-detect
circuitry to disable the relays to try and protect things if
the PTT is not connected when
you transmit (C17
coupling some RF and eventually turning on Q1).
Like the other devices with this type of circuitry, it sorta
works. It is pretty good with AM but SSB
will be intermittent at best
and will
quickly blow a fuse. It is
useful in an emergency when you accidentally plug things in wrong,
but for normal use ... You
must connect the control wire from the
transceiver.
SETUP:
On
almost
any
transceiver built in the last 30
years there
should be a connection
on the back of
the transceiver designated
for operation of a linear amp. Look
for something labeled TX GND, PTT
or similar. If
you are lucky it is a separate dedicated connector. That
is probably what you want to connect
to the
PTT input of the QRM-180. But check it first: operate the
transceiver while monitoring that output with a voltmeter. It is
likely
just relay contacts to ground so if you read no voltage, try
measuring continuity. Whatever
the case, it must go to
ground
when in transmit mode. Depending
on your transceiver, you may have to obtain a suitable connector to
reach this signal. The
QRM-180 PTT input sets at 5V and needs less than 1 ma to ground to
operate.
If
your transceiver has a built in tuner, check the output when the
tuner operates.
The tuner should also pull this output to ground when it transmits.
In
nearly
all
cases, when you have a built-in tuner, the
output does not change during tuning and you
have to manually make
sure that the QRM-180 is turned off when actively tuning.
Failure
to turn it off will eventually blow the fuse. Note,
You may find that your transceiver’s autotuner has trouble on
the higher bands
with the QRM-180 installed, even when powered
off. C17 is probably the culprit. That cap is just enough to annoy
some autotuners, especially at the higher bands. Lifting one leg of
C17 and disabling the auto-detect is generally the best fix.
At
the worst, (on
a very, very
old transceiver
or
CB radio)
you
may have to
just tap into the microphone connector and find the push-to-talk
contacts.
Possibly
putting a male/female adapter between the mike and the radio. Check
the Internet for instructions on how to connect your transceiver to a
linear amplifier, the connection will
be
the same.
The
power
requirement is 12VDC
(11
to 13VDC).
The included wall wart will work (it
is a switching type, and
they all generate some sort of undesirable noise)
or any other good 12V source. If
you have a linear supply, use that. Most
transceivers have a 12V output that could be utilized, but you should
have at least 400ma available to power the QRM-180. Check
your
transceiver manual first.
For
automotive use, or
anything over 13V you
should drop the voltage by putting two diodes (1N4005 or similar) in
series with the positive lead.
OPERATION:
The trick to success is to get
just enough of the noise signal (and without the desired signal)
required to match the level of the noise component on the primary
signal path. The kind of noise will dictate the requirements, but
basically you will need a dedicated antenna for the noise and some
time to experiment with settings at different times. Don’t expect
to eliminate atmospheric noise-- you cant. But for a specific noise
source you should expect a significant if not complete reduction.
Read the reviews and manuals of the other devices (all available
on-line) to get the full picture. The antenna should NOT be
similar to the primary antenna you are operating on. You don’t
want to receive the primary signal, you want (as much as possible) to
capture only the noise.
If you somehow got this far
and have realized that you don’t really have a specific noise
problem, don't loose hope. There is generally a local noise
component of what we all assume is atmospheric noise. With the right
antenna, the QRM-180 might still find and eliminate enough noise to
drop an S-unit or two.
Step
one in
deciding upon the noise antenna is
to try
and know a bit about your enemy.
What time of day is it present, is it near or far, where in the band
is it strongest? If you have multiple antennas, is the noise the
same on each? Try to tune it in. Before
you insert the QRM-180,
use
your receiver
to find an antenna that picks up the noise without the desired
HF signal
(an old TV
antenna is great place
to start
if you have one).
Then install the QRM-180 and start
on an easy band, say 20 or 40M connecting
something to the noise antenna Some people swear by a long piece of
wire laying
on the floor. If
that works, the noise is probably in
the shack,
and should be fixed with some grounding and ferrite beads not a phase
inverter.
If you have an old 2m antenna,
it is often a good choice, especially if mounted horizontally.
CONTROLS:
-
The
GAIN
knob is just that, you are trying to find a level that just matches
the noise so that the result is zeroish. It is not
to find the best listening level. Use the radio knobs to do that.
-
The two PHASE knobs
jointly adjust the phase of the noise signal. You should adjust
each for a minimum. Unfortunately they also mess a little with the
level, so you have to go back and forth a bit between all three.
-
The NOISE Attenuator
control is rarely used and is on the rear panel. When the LNA is
installed it is used to get the noise signal in a range that can be
matched by the GAIN control. Without the LNA it will probably just
be set to max. It will depend on the size, type, and location of
the noise antennas, as well as the strength of the noise source. If
you find that it works best with the attenuator below half way, try
reducing the gain by plugging the LNA into the 9V power source, or
consider removing it completely.
-
The DELAY ADJ is
inside on the pcb. This screwdriver adjustable trimpot is used to
adjust the delay time of the relays after transmit. It is not
critical if you just use AM or SSB, the time can be fairly long
without being annoying. CW will require a shorter time and a little
more fine tuning.
Begin
routine
adjustment
by turning the
gain down a
little
on the transceiver, if it has a pre-amp turn it off (with
the gain of the QRM-180 it
is unlikely that you will need it).
Turn
on the QRM-180 and turn it’s gain to about half, then adjust each
of the both
phase controls for minimum noise. Keep adjusting things until the
noise seems to be minimized. You
should find a very sharp null point. If
the LNA is installed, start with the Noise Level control set to about
70%.
You
have to use your receiver controls as well as the controls on the
QRM-180 to get the best results. The
signal will likely be a
little different
than what you are used to. Try
with and without the transceiver’s
notch
filter,
NB,
and
different width settings. It
will take a little
practice to get things right quickly. With
a little practice, most noise can
be killed in 3-4
seconds of knob tweaking. Changing frequency within
a band should
require minimal
readjustment.
Try
with and without the QRM-180 frequently to be sure that you are
helping things. Try to ignore the level change and listen just for
the change in the noise component relative
to the signal.
Use
the transceiver volume control to match the levels for comparison.
Again,
you cant escape atmospheric noise, focus
on the specific noise source.
For
different bands you may get slightly
better
results with a different noise
antenna.
If
it seems to work better with a different antenna on another band, try
tying the two together. You may get away without an antenna switch.
In any case, you
should run
coax from the back of the QRM-180 to the noise antenna, but pretty
much anything
will work. RG-59
is
readily available and a good choice. It
doesn’t need to be 50 ohm, you
are not going to transmit on it. Try
a short dipole, an
old 2m antenna, a
rain gutter, or a fence,... you never know what will resonate with
your particular noise problem. Going
vertical is a good option if
your main antenna is horizontal, but
try both.
Remember
you are trying to receive just the offending noise, not the HF
signal. If
you can set up an antenna with some gain, great, point
it at the source.
When
you have established a decent reduction, note the knob settings. It
will be pretty repeatable for the same conditions on
the same band.
A word of
caution, whatever you
choose, don’t go with a big noise
antenna that is too
near to the one you
are transmitting on. The noise antenna could
easily pick up enough RF
to blow the fuse on the noise antenna line.
It
is a good idea to keep spare fuses in the case (the
small
6V bulbs). They do a good job of protecting the rest of the
circuitry, so if you do blow one, just
break
out the soldering iron and put in a new one.
And
while the QRM-180 works the 180-10M bands, the performance of the
phase adjustment drops off at higher frequencies. If the intended
use is 10-6M consider reducing the value of C6 & C7 to around
50pF, remove
C17, and
replace L4 with a jumper.
Other
Thoughts/ Uses:
The
QRM-180 is a collection of circuitry that provides a bit of
amplification and phase shift. If you have a couple of vertical
antennas you might try using
the QRM-180 to combine them and use phase shift to adjust direction.
At least two users have
modified the QRM-180 to work down around 475kHz with success. Thanks
to Tom WB4JWM for his efforts supporting the 630M band. You can
contact me for his recommended changes. It will however, disable the
higher bands.
Assembly Instructions
ASSEMBLY of the PCBs:
This kit may not be as challenging as building a HeathKit color TV, but it will still require a bit of time and skill. If you have not built an electronic kit for a while, you should probably practice with a simple DIY kit of some sort from eBay. Before you start the ‘180 assembly, thoroughly study the schematic and understand the function of each part of the circuit, it will be a help in construction and operation. Review the instructions and be certain that you can identify all of the components. Begin by sorting out the parts so as not to loose anything, get some good light, and a place to work comfortably for a couple of hours at a time. Complete assembly is probably a two evening job; there are a lot of parts.
You will need some basic tools: soldering iron (about 40W) with a smallish tip, your favorite solder, sharp flush cutters to trim leads, screwdrivers, needle nose pliers, and if you are not 13 years old, a good magnifying lens. A volt-ohmmeter, even a cheap one is pretty much a necessity (Harbor Freight has several for $10 or less, as does eBay).
You will notice that when soldering, that the ground plane is quick to dissipate the heat on grounded pads so you will have to stay on those pads a bit longer to get good flow to the top of the board. 3-4 seconds is usually about right.
It is generally best and easiest to install the smallest parts first, and gradually work up in size. Don’t stress about getting the components close to the board. It may look nice if they are all flat and neat, but keeping them a little above the board would make any rework easier and generally improve air flow. This is especially true of all the small transistors. Refer to the board layout drawing and install 6-7 parts at a time, then solder them. Then trim the leads and put in the next group. Checking them off of the BOM as you go will save time in the long run. Don’t worry if you have parts left over, there are often a few extra parts in the kit (I don’t count so good, but I try hard to never be short).
When the board is complete, you can clean the backside with a little rubbing alcohol and an old toothbrush to remove the flux from soldering. Check to see that all leads are trimmed enough so that they will not short to the case. Check especially those along the edge of the board.
MAIN PCB ASSEMBLY:
[_] 1) It’s a good idea to install the Zener Diode D7 first, this will keep it from being confused with the 1N4148 diodes which look almost identical. Then install the 1N4005 black rectifier diodes (D1 & D18). Now you can put in the three 1N4148 switching diodes (D2, D13, D19). As with all the diodes, watch the polarity and match the band with the silkscreen image.
[_] 2) Install the two full wave bridges, they are in the 4 pin dip packages. Note the + and – location on the board layout drawing.
[_] 3) Install the inductors, be careful, there are two different values and they can be difficult to tell apart. They look like resistors, but will measure nearly zero resistance on your ohmmeter.
[_] 4) The resistors are next. R9 and R7 will run warm in operation so mount them about 1/8” (3mm) or so above the board. Save the potentiometers until later since they are large and get in the way. Some resistors have 4 value bands (1%) and some have 3 bands. It didn’t matter but the 1% types were supplied when available.
[_] 5) Install the capacitors, just save C3 until later. Only C2 and C3 are polarized, the rest can go in either way. Each is marked with a very small 3 digit value code.
[_] 6) Now is actually a good time to install the board connectors. The pre-wired cables should be oriented correctly: The 4 pin cable should wind up with the RED wire in the square pad. The two-pin cables should also be inserted so that the RED wire goes to the square pad. The green connector and the SMA connector may require a little more heat.
[_] 7) Take a break from soldering and wind the transformer. Wind 10 turns of the enameled wire, and then three turns of the insulated wire. The wires can exit the ends or sides of the core, it doesn’t much matter. Just make sure that when it goes on the board the enameled wires terminate on the C7 side. Trim the wires to about the right length to fit the board (not too short). The insulated wire can be stripped with a good pair of strippers. The enameled wire must be prepped by scraping the ends with a razor blade or sandpaper, then tin the ends to make sure that all of the enamel has been removed before soldering. Double stick tape will hold the core in place on the board.
[_] 8) Install Q2,Q3, Q5, Q6. Leave out Q1 and U2 until after testing. Leave all the transistors well above the board, it will make removing them possible if that ever becomes necessary. On Q5, install the provided heat-sink before you solder it in.
[_] 9) Install Q4, the large PNP power tab transistor. The metal side of the transistor goes toward the inside of the board.
[_] 10) Add the bit of plastic shaft to the shaft of RV1. Use the adhesive lined heat shrink provided. This will allow this control to be adjusted from the back panel. Use a heat gun or very small flame to shrink the tubing.
[_] 11) Install the ‘delay adj’ trim-pot, RV5.
[_] 12) All of the larger parts can go in now. Don’t forget to install C3, and the 2 fuses (lamps). Solder in the controls and the relays. Note that RV1 is a 1K pot. The others are 5K. Be sure that the pots are firmly seated against the board so that the front panel will align with the enclosure correctly. Some of the pots have a metal anti-rotation mounting tab that will interfere with mounting in the front panel. Cut or break that tab off of the 5K pots before soldering.
[_] 13) You can now install the front panel. Do a test fit then carefully bend the LED to line up the the hole. Attach the panel with the washers and nuts on the three controls, just finger tight for now. Make sure that the LED is firmly snapped into its bezel and solder it in place.
[_] 14) This is not critical but it is a good idea to electrically connect the ground plane of the main PCB to the back of the front panel. Just solder a bit of wire (a piece of resistor lead) from one to the other. Doing this will allow operation of the unit outside of the case for testing. (the only other ground for the front panel is through the case and the paint can insulate connection)
[_] 15) Cut one of the two pin cables (red/black wires) to about 2 inches and solder it to the power switch. Polarity doesn’t matter. Install the switch into the front panel and tighten the nut. Plug the cable into the PWR SW connector on the board.
REAR PANEL ASSEMBLY:
[_] 1) The rear panel is really a PCB. It will provide a location for the fuse and protection diodes for the noise input.
[_] 2) You will need to surface-solder a full wave bridge pack to the back of the rear panel. Note the orientation and match the + & -. This is as close as we get to doing surface mount.
[_] 3) You should install the remaining 4 connectors as indicated. Tighten well, and if you have a dab of Locktite TM ,glue, or nail polish, this would be the place to use it.
[_] 4) Solder a small lamp (fuse) from the center pin of the BNC connector to the L-shaped pad next to the four diodes.
[_] 5) A 5.6uH inductor is also soldered between the ‘L’ shaped pad and ground. It is easiest to cut one leg to about 3/8“ (8mm) and prep it with a small bend. Then you can hold the other long leg to solder it in place. After the short end is soldered cut the long end and bend it into place and solder. Depending on your noise source you may get better results without this inductor. For now, a good plan is to leave the ground end floating so you can easily try it later.
[_] 6) On the four pin connector with wires, cut the white and red wires (don’t cut the yellow and black) to about 3.5 inches long. Then wind 3 turns of both the yellow and black wires through the green ferrite ring. This choke helps to block RF from getting in or out through the power wire. It also a good idea to solder a 0.1uf cap across the power connector on the rear panel. The yellow and black wires can now be soldered to the power connector on the rear panel. The yellow wire is connected to the center conductor, positive terminal. The ground lead should also be jumpered to the pad on the rear panel.
[_] 7) Solder the white wire to the center pin of the RCA jack. Then, solder the red wire to the L-shaped pad. You can also solder a 0.1uf cap across from the center pin of the RCA jack to ground if you like. Although not necessary, it can keep some RF out of the power.
[_] 8) Strip and prep the thin coax cable with the gold SMA connector, and using the pad next to the BNC connector, carefully solder the shield to ground and use the connectors ground lug as a strain relief for the cable. The center conductor should be soldered to the L-shaped pad. Use minimal heat to avoid melting the center conductor.
[_] 9) Using three pieces of about 2.5 inches of 18-14 awg wire, connect the SO-259 connectors to the three pin Phoenix connector. Note, the center pin is ground (GND on the PCB silkscreen)
[_] 10) You can solder the remaining 2pin red and black wire connector to the LNA. Run the wires through one of the mounting holes as a strain relief and connect the red wire to + and black to -. Don’t install the LNA until you do some basic testing.
TESTING:
[_] 1) It’s not a bad idea to check to make sure that the board slides nicely into the case and that the front panel holes line up. You may have to loosen the controls or re-solder them to get a perfect fit, but don’t install the case just yet. Double check the rest of your work and verify that all of the transistors are in correctly and that all of the diodes are in the correct direction. The LNA can be left out for now. Steps 2 thru 4 can be done with the boards out of the case.
[_] 2) Before the 4 pin power cable is plugged into the board, plug in the power supply to the rear panel and verify that 12V is present on the yellow and black wires and that yellow is positive.
[_] 3) Now plug in the 4 pin cable and the green 3pin connector. When pressing the power switch, you should hear the relays click and the the LED should light in a yellowish color. If it is green, the LED is reversed. If nothing happens check for 12V power at the power switch, the orientation of Q4, 5V across D7, and the location and orientation of Q2. If C3 gets hot and explodes violently, it was in backwards.
[_] 4) If everything works so far, and with power on, try grounding the control input (the RCA jack). The relays should click and the LED should change to red. If this fails, check D7, and the other diodes and components around Q2. When you are satisfied that the power and relay control is working, then install Q1 and U2.
[_] 5) You can put everything in the case at this point but leave the top off. You can tighten the controls and install the knobs. (It’s good time to put the rubber feet on the bottom as well)
CAUTION: When the transceiver is connected, there is a possibility of high voltage and RF exposure.
BE CAREFUL! Turn RF power down on your radio and take care not to transmit.
[_] 6) Continue testing with the transceiver, the power supply, your main antenna, and a noise antenna, connected. PTT is optional since you will not be transmitting. Hook up a noise antenna to the BNC connector on the rear panel. Maybe a 2m antenna or just 5-10 feet of wire in the shack for now.
[_] 7) The LNA board can be installed now, it simply sits above the main board held in place by a 90 degree SMA coupler to the LNA’s output. Power is provided by one of the two, two-pin connectors. Use the 12V connector for testing. The signal input to the LNA comes from the rear panel cable (the one with the SMA male connector).
[_] 8) When everything is in the case and connected, turn the QRM-180 power on and off. Listen for the relays to click and observe the power LED. When the power is on the LED should be yellow-ish. Set the gain, and the rear attenuator to max.
[_] 9) Turn on the transceiver. With the QRM-180 off, the receiver should work normally.
[_] 10) When you turn on the QRM-180, there should be a lot of noise in the receiver. Play with the controls to get a feel for them and try it with and without the noise antenna connected.
[_] 11) At this point you should be able to adjust the two phase controls to get a sharp null point. If not, recheck your work and use an ohmmeter to verify that the rear panel cables are connected and not shorted. You can lift one end of L1 on the rear panel to get a check of the resistance of the SMA cable.
[_] 12) If everything works the unit is now complete!! Go to the section on operation and begin testing your options for a good noise antenna. If you still have issues, review the “list of common problems” and continue testing.
You can operate with or without the LNA as needed, depending on the noise level and size of noise antenna. If you remove it just plug the SMA cable from the rear panel directly into the main PCB. For less gain, you can also try plugging the LNA into the 9v power source. While normal is 12v, the LNA is a bit cleaner on 9V. Try both.
TROUBLE-SHOOTING.
It is fairly robust circuit so you should not experience any problems if you managed to get all the parts in the right holes and in the proper orientation. But don’t panic if you do have an issue. Should something not function, check the schematic for that area and recheck the parts related to that function. It is surprisingly easy to miss-read a component’s value. Start with getting the relays to click when the PTT input is grounded. When that works, you know that the power circuitry is right.
If you still have signal problems, try connecting to the receiver but without the antennas. Use a small probe or bit of wire, and following the schematic, touch various points along the signal path. Just your body as a signal source will introduce plenty of static to locate a fault. If you can mess with the knobs and get a null in background noise, it is working properly. You just need to get the best noise antenna and get good at fussing with the knobs.
Common Problems: Power issue or relays don’t click:
• 12V red/black wires switched on the rear power connector. Check the power at the board.
• The rear panel gets its ground through the green connector. Be sure that it is plugged in when testing the PTT function through the RCA jack.
• A 1N4148 diode switched for the Zener? There should be approx 5V across the Zener.
• Is the power tab transistor backwards? Tab toward C3.
• Check pads that have top traces, sometimes solder doesn’t flow to the top and the plated-thru hole can break leaving the top trace unconnected.
• A few pads are very close to the edge of the board and can short to the case if the leads are not trimmed flush on the back of the board.
Signal issues:
• Check the resistance of the SMA coax cable to the rear panel, it is easy to overheat the shield when soldering and short it to the center conductor.
• Is the LNA in backwards? RF in goes to the rear panel, RF out goes to the board. Is the power correct to the LNA? Plus voltage to VCC on the LNA?
• Are the enameled wires of T1 sanded and soldered?
• Check all the fuses with an ohmmeter. It is difficult to tell when they are open. Spare fuses are included but if you need more, they can often be found in hobby shops or on eBay by looking for ‘model railroad lamps’. 6V 40-100ma lamps are best but small 12V lamps will work.
• Check the grounds between antennas and the radio. A loose connector will cause lots of grief.
All else fails, email me and I will try to help. robag6yj@yahoo.com
