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G8MNY > TECH 01.07.06 11:08l 67 Lines 3182 Bytes #999 (0) @ WW
BID : 36594_GB7CIP
Read: DL1LCA OE5RCO GUEST
Subj: Stopping HF Tx/Rx SMPSU QRM
Path: DB0FHN<DB0FOR<DB0SIF<HB9EAS<DB0CZ<DB0GE<LX0PAC<LX0NET<DB0LJ<DB0RES<
ON0AR<GB7CIP
Sent: 060701/0743Z @:GB7CIP.#32.GBR.EU #:36594 [Caterham] $:36594_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Mar 06)
(8 Bit ASCII Graphics use code page 437 or 850)
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.
Typical QRM as measured on a commercial 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.
10mVÄ´~~`--..__ ³_S9+40dB
1mVÄ´ ~`-._ ³_S9+20dB
100uVÄ´ ~`-. ³_S9
10uVÄ´ `\ ³_S5
1uVÄ´ `\._ ³_S1
100nVÄ´ ~`-.._ ³-S0
ÅÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄ´
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 MHz
SMPSU often pulse on & off on no load, this is good indication that the
following Rx QRM elimination method will work 100%.
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 O/P 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 ÃÄÂÄ´>ÃÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄ> +Tx/Rx
_³_ _³_ ³ ÀÄÄÄÂÄÄÄÙ ³ ³ ³
MAINS /_\ /_\ ³ ³ ³ ³ --------³---
ÄÄ¿ ÚÄÄ´ ³ ³ ³ ³ ³ | Ä´>ÃÄ´ | 14V Rx < 1A
)³³( ³ ³ 4700u³+ ÃÄÄÄÄÄ)Ä2K2ÄÙ 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
ÀÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄ> -ve
Linear Rx PSU Noisy PSU
All 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 0.22ê Rs in each O/P) if 1A is not enough, or use some other
regulator.
Before under taking 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!), then look for room inside the SMPSU or
add a bolt on box to the SMPSU unit if you like (case as heatsink).
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!
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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