MFJ-1270B Electromagnetic Interference (and how to get rid of it!)


I have recently taken the opportunity to observe electromagnetic
compatibility of the MFJ-1270B as used by several of our NET/ROM nodes.
(some of this may apply to other brands also.) T4e initial esults were not
as good as they should have been. The question was 'Do the TNC's cause
interference to co-located sensitive VHF receivers ?' The answer is YES,
depending on the situation.

In two different sites measured, the noise coming from the TNC was
sufficient to cause 4-7 db worth of desense to a 0.2 uv (12 db SINAD)
receiver (145.01). There are two types of noise that are that radiate from
the MFJ-1270B. One type is broadband in nature (probably due to
data/address bus activity and DC-DC converter operation) the other kind is
harmonics from the crystal oscillator and sub-multiples thereof
((4.9152/2) * 59 = 144.998 MHz). The broadband noise can cause interference
on all frequencies if a sufficiently sensitive receiver in a quiet location
is involved. The harmonic energy can cause problem on specific

This interference is detrimental to the weak singals that serveral of our
nodes are required to operate with. In addition it may cause problem to
the communications of our hosts whenwe share a commercial site.

It is relatively easy to determine if you have a harmonic problem
because the receiver noise will quiet when the TNC is turned on. The
broadband noise usually can not be heard unless you observe the noise level
in the presence of a weak signal. (With the normal antenna connect, inject
a signal that provides 10 db of quieting then turn on the TNC and observe
the change in noise level. If you have a 0.5 uv receiver or are in an
otherwise noisy location you may not be affected. With a 0.2 uv receiver
you may lose 6 db of quieting.)

Cleaning up the MFJ-1270B

Sufficient reduction in the energy emitted by the 1270B can be had with
some relatively simple modifications to the 1270B. These modifications
should reduce the broadband noise level to a insignificant level for most
sites. Harmonics that cause problems to specific frequency can usually be
moved far enough off frequency by 'rubbering' the crystal with the trimmer
in the TNC.

  1. Scrape the paint from the inside of the cover and the outside of the
    chassis near the four screws that hold the cover on.

  2. Remove the circuit board from the chassis and scrape the paint from
    the top of the four posts that hold the circuit board.

  3. Add a low inductance strap from the mounting hole near the power
    switch to the ground trace at the edge of the circuit card. (The
    bottom side of the card works the best.)

  4. On the bottom side of the card add a 'small' 0.001 uf capacitor from
    each of the four active pins on the DIN connector to the ground foil
    immediately adjacent.

  5. Add a ground strap from the foil on the bottom side of the card under
    the DIN connector. The ground end of this strap can be left dangling
    until the card is reinstalled and then looped to the outside of the
    chassis and grounded with a small self tapping screw on the lip just
    below the DIN connector.

If you are at a shared site, you owe it to the other services to be a
good neighbor.

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