开云体育

Hello, I discovered this project last Fall and this reflector even more
recently. I have been reading the discussions about "Display Noise'
and"SPI issues" with much interest and since my experience is a bit
different I will share it here.

I have built a V12 radio with RF, Main, and Front panel boards in an
AI6YM chassis, using STDVer050.2 software.

I originally built the radio with a TFTM070A3-5 v4.0 (7") display and
immediately noticed significant amounts of noise not normally heard from
a radio with the antenna disconnected. This noise was below the level of
the noise coming from my antenna on bands up through 15M, but above that
the radio's internal noise was higher than antenna noise.

I made a simple probe from a piece of coax by removing 1 " of the shield
from the end, connected the other end to my spectrum analyzer, and
probed around the inside of my T41. I discovered what I believe to be a
smps in the upper right corner of the rear of the display running at
about 1.15 MHz. This smps exists in both the 5" and 7" displays. The 6th
harmonic of this can be heard at the bottom of 40M and the 12th harmonic
at the bottom of 20M; neither can be heard with my antenna connected but
are very noticeable with the antenna disconnected.

I discovered very high levels of EMI radiated from the middle area of
the circuit card on the back of the display. The spectrum of this EMI
starts in the HF band, peaks around 160 MHz and has very significant
strength in the 6M and 2M bands. I then noted that the layout of the
boards in my chassis places the receive input circuitry about 2" from
both the smps and the display EMI source. Not good!

I then clamped my coax to the chassis such that the 1" "probe" was right
next to the receive RF input circuitry. I then made a simple
shield/cover for the 7" display. In doing so, I discovered a few issues
with the displays. First, the pads around the four mounting holes are
not connected to the circuit ground, nor are they even connected to each
other. On the 7" display, the four oblong, exposed, pads across the top
are connected to the circuit ground, but the four across the bottom of
the board are not, nor are they connected to each other. When I then
measured the noise from the probe with and without the shield in place
on the 7" display there was some improvement, most noticeably on the
smps noise, but the noise on the upper HF bands was still higher than
the noise from the antenna.

I then looked at the 5" display. All four of its exposed, oblong, pads
are connected to ground. I soldered small clips (from a Harmon EMI kit)
to each of those four pads and made a shield to fit in those clips (see
attached picture). I had to run a short wire from each clip to the
closest mounting post to tie the shield to the chassis. This provided a
very significant reduction in the level of noise present at the receive
input circuitry.

To quantify the improvement, I measured the radio's sensitivity on each
band and converted that to a receive noise figure. That can be compared
to the level of noise that I measure from my antenna. This data is shown
and graphed in the attached Excel file. The T41 receiver is very quiet
now up through 10M. 6M is much improved, but further improvement is
desired. Perhaps the cable..., perhaps moving the RF board away....

73,

John W2TX