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G8MNY > TECH 31.08.20 10:00l 69 Lines 3276 Bytes #999 (0) @ WW
BID : 3730_GB7CIP
Read: DJ6UX DD3IA DF7EAV OE7FMI GUEST
Subj: Stopping HF Tx/Rx SMPSU QRM
Path: DB0FHN<DB0FFL<OE5XBL<F1OYP<IZ3LSV<IR1UAW<I0OJJ<GB7CIP
Sent: 200831/0856Z @:GB7CIP.#32.GBR.EURO #:3730 [Caterham Surrey GBR] $:3730_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jul 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Where a 13.8V SMPSU is used directly to power an HF Tx/Rx the normal mains & DC
filtering is often inadequate to remove all the switch mode spogies that a
sensitive Rx & nearby aerial can pick up.
10mVÄ´~~`Ä-..__ ³_S9+40dB
1mVÄ´ ~`-._ ³_S9+20dB
100uVÄ´ ~`-. ³_S9
10uVÄ´ `\ ³_S5
1uVÄ´ `\._ ³_S1
100nVÄ´ ~`Ä-._ ³-S0
ÅÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄ´
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 MHz
Typical QRM as measured on a commercial "RADIO HAM SMPSU" using a 50kHz wide
spectrum analyser. The 'S' meter readings depends on your Rig's QRM imunity to
power rail QRM as well as the SMPSU radiation & pickup to your aerial.
SMPSU often pulse on & off on no load, this is good indication that the
following Rx QRM elimination method will work 100%. But before you under take a
SMPSU modification, test the principle works OK with a variable bench PSU
first. If the test is OK & not too high a voltage is needed (watch those dial
lamps if over 15V!), theo for room inside the SMPSU or add a bolt on box
to the SMPSU unit if you like (case as heatsink).
The strategy is to stop the SMPSU from running while on Rx, by using a small
linear PSU for Rx instead. Stopping the SMPSU running is normally possible on
all SMPSU if the output goes slightly higher than the nominal 13.8V by only a
few 100mV. Note that too much overvoltage may trigger a crowbar overvoltage
circuit if that sort of protection is built in (I recommend U add one if not).
On a good insulated Heatsink
inÚÄÄÄÄÄÄÄ¿out
ÚÄÄÄÂÄÄÄÄÄÄÂÄÄ´ 7812 ÃÄÄÂÄÄ´>ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄ> +Tx/Rx
_³_ _³_ ³ ÀÄÄÄÂÄÄÄÙ ³ 1N5401 ³
MAINS /_\ /_\ ³ ³ ³ --------³---
ÄÄ¿ ÚÄÄ´ ³ ³ ÃÄ2k2ÄÄ´ | Ä´>ÃÄ´ | 14V Rx < 1A
)³³( ³ ³ 4700u³+ ³ ³ ÄÄ| TX ³+ | 13.8V Tx > 1A
)³³( ³ ³ 30V=== Pot ³ MAINS | SMPSU === |
)³³ ÀÄÄ)ÄÄÄ´ ³ ÚÄ>1k === ÄÄ| 25A ³ |
ÄÄÙ 18V _³_ _³_ ³ ÀÄÄ´ u1 ³ --------³---
1.5A /_\ /_\ ³ ³ ³ ³
ÀÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄ> -ve
Linear Rx PSU Noisy PSU
diodes 1N4001 or better.
The rating of the current limiting linear PSU needs to be just enough to run
the Rx at normal volume etc. Normally 1A will do, but 2 x 1A regs could be used
in parallel (with an 0.1ê Rs in each output) if 1A is not enough, or use some
other regulator.
Other than eliminating the SMPSU QRM there is 1A more current available on Tx.
The regulator(s) must be the current limiting type & well heatsinked, as it
will be working very hard all the time!
See my bul "Reducing Electronic RF QRM" for more tips.
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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