Noise Phasing and MFJ-1025/1026 Data
The original concept of the 1025 is based on a bridge phasing
system. Many phasing boxes, such as the
QRN Squasher, do not rotate phase. They
are based on the concept that a 180-degree inversion allows rotation of phase
when mixed through a potentiometer, but that does not occur.
Mixing a 180-degree shifted phase signal with another
unshifted phase signal does vary the amplitude,
but it only produces an abrupt 180-degree flip at the zero-crossing
point. Some people have copied the flawed QRN-Squasher concept, and offer
I recently reviewed another phasing system design (on paper)
which is implied to be the “ultimate”. It uses a simple R/C phase
advance circuit, not a bridge. The maximum phase rotation of such a system is
limited to less than 90 degrees. With 180-degree or zero-degree inversion in a
later stage, that allows only a 150-160 degree slice of phase choice. Even
transposing inputs would only bring that to 320 degrees. Not only that, dynamic
range is limited by a very poor choice of operating parameters for the FET
driving the phasing system.
To have a perfect
system we need 360-degree
phase adjustment with no
level change. Even with perfect design,
such phasing systems are tedious to adjust.
Many systems rotate phase over a wide range, but have severe level change as
phase is rotated. If level changes
as phase is adjusted
we have to move
back and forth
between phase and level
This is because
ratios change with
any phase adjustment.
This makes phasing units
more unpleasant to adjust
than they normally
a true bridge system, amplitude
shift is minimal. The MFJ1025 series, despite being manufactured as
possible, outperforms any other unit I have tested (by a large margin) over its
intended frequency range of 1.8-20 MHz.
The exception is the
NCC-1, which is a
Cadillac of noise
The phasing system at
my station, used to pinpoint signal directions
of transmitters or jammers more than to actually cancel noise, is a more
sophisticated version of the 1025 with higher dynamic range. It uses tandem
bridges, and is calibrated in degrees shift. Phasing is spread over a wide,
linear, control range. Phase rotates a full 360-degrees with less than 1dB
amplitude shift. It is possible to resolve as little as 1- degree directional
difference in arriving signals.
A Word About Dynamic Range and Noise Floor
160-meter DX’ers have the most demanding noise floor and dynamic
range requirements of anyone using a phasing unit. 160 ops not only must hear
exceptionally weak DX stations using small inefficient antennas and low power,
they must contend with multiple 1500-watt signals (often with excellent
antennas) parked just hundreds of Hz away! CW operators often listen to CW
signals with 250Hz BW systems, pushing noise floors up to 50 times lower than
10kHz filters would produce at any location. Amplifiers and other components
must also handle the large powerful window of AM BC stations just below 160, as
well as the SW BC stations above. At my house the accumulation of signals from
stations *outside* Amateur bands is enough to light a small 12v light bulb when
using a 15dB amplifier!
Obviously what works well at MW for sorting 10kHz spaced wide
bandwidth signals apart is entirely different. Here we have the strong window of
stations, but we are looking in that strong window for other stations that often
are strong (compared to 100-watt transmitters with poor antennas). Noise floor
is also much higher, and so dynamic range and noise floor are of almost no
concern. The critical performance parameter is mostly adjustment range, and not
SW BC listeners need something a bit different yet.
Everyone should be cautious assuming a device optimized for one
application somehow makes it fit the other.
The only drawback of the higher noise floor of the MFJ-1025/1026
is you must place it after any amplifiers used with low-level low-noise
antennas. The MFJ-1025/1026 noise floor is typically around 14dB.
The dynamic range and noise figure of my personal unit is
excellent. Unfortunately such a system is impractical for amateurs and SWL’s
because of cost. Each unit has about $300 in raw material cost, plus the
assembly time. The FET’s alone are 28-volt devices capable of several watts RF
power while providing a 1.5dB noise figure. But for typical operation the 1025
is almost as good. If you want to spend several hundred dollars, I can sell you
a system that works almost perfect in almost any application!
Even with MFJ’s well-known QC issues and affinity for building
things cheap (not good) I still recommend the MFJ-1025 over all other mainstream
units. It has the potential to be better than anything else with only minor
changes, and is certainly betetr than all other mainstream units like the ANC-4
or QRN Squasher just as it is shipped. If you want a custom modified unit, and
can not make the changes yourself, I would be happy to help.
The following table are gain measurements for the MFJ-1025/1026.
Wiltron Network Analyzer (50 ohm detectors), transmission loss mode.
Stock Unit and Filters 50-ohm input and output
Aux Port Gain
|2.5MHz and higher
From the above losses, deduct -2dB (or add 2dB!) if the
input protection light bulb is bypassed or replaced with a jumper wire. I.e. 1.8
MHz gain becomes -3.4dB with the lamp removed.
Main Port Gain
|2.5MHz and higher
Bypass is essentially lossless.
Compression and IM
With gain controls wide open and pre-amp disabled, blocking
begins at about -3dBm. Third order intercept is about -21dBm. This is generally
well beyond what most receivers are capable of obtaining. (Click
here to see the receivers page on this site.) The drawback of
the 1025/1026 is the lack of any form of filtering, other than the high pass
filter. This means the MFJ-1015/1026 sees a very wide window of signals.
With that in mind, it is questionable if removing diodes
actually is worth the risk of RF or ESD damage. It would be much better to add a
simple bandpass filter system for the band you are operating. However,
there is some improvement offered by removing diodes. IM and blocking also can
be improved by increasing quiescent current in Q5 and 6, as well as Q4.
The bias modification is generally too complex for most people to do, and adding
a bandpass filter in from of the unit would be much more effective
Removing Diodes and Lamps
IM dynamic range will improve about 6dB with this
modification. The lamp will improve gain and noise figure of the AUX port
(reduce loss) by about 2dB.
If you never transmit through the unit:
1.) Remove C5
2.) Remove D1,D5 at the output, remove D12 and 13 (or D10 and 11
in 1025) and D6 and 9 at the inputs.
3.) Replace Lamp1 with a jumper wire.
NOTE: If you transmit through the unit do NOT remove C5
and D1 and D5.
Sensitivity to Load Impedance
Like any typical amplifier system, this unit is sensitive to
load impedance. Since it has an emitter follower at the output, it likes to see
a low load impedance. Gain will increase about 5dB if you add a 1:4 step up
transformer at the emitter of Q4.
The following are modifications or changes to
Gain Improvement Mod.
Increases gain and dynamic range by approximately 5dB (assumes
your receiver has a 25-ohm or higher input Z):
1.) Cut the trace from C6 to D1/D5 near the small dip relay.
2.) Add 1:4 step-up transformer, low impedance side towards C6
and high impedance side to load.
The MFJ-1025/1026 contains BCB filters that roll off at about
1.9MHz. To extend operation lower in frequency:
Remove L3, L4, L5, L6.
Short C8 and 16
Increase the value of C12 and 13 by the same factor as the
decrease in frequency. For half the frequency, double C12 and 13.
Caution: DO NOT remove filters if you intend to use this unit
for weak-signal reception on Amateur bands above 1.8 MHz, unless you are sure
you do not have strong BC signals.
C12 and C13 mods will reduce upper frequency limit
proportionally to the change.
Extending Phase Range
Phase range of the MFJ-1025/1026 is limited by the ratio of R16
to reactance of C12 or C13. Normally phase will rotate through at least
130-degrees. SW3B inverts phase 180-degrees, and this moves the phase shift to
the other side of the circle. Typically the MFJ-1025/1026 has about 280-degrees
of phase rotation.
The remaining 80-degrees can be covered a number of ways. You
can swap inputs (only when using it entirely passive), or modify the phasing
network. I also have successfully added a relay that swaps the outputs of the
low-pass input filters. A second method is to add an additional capacitor in
series with the ground lead of the bottom of R16 (the wiper remains grounded)
back to the junction of C14 and R17.
I’d recommend swapping the inputs, it is easier and does not
degrade amplitude response or bandwidth.