G8MNY  > TECH     21.05.25 09:00z 54 Lines 2296 Bytes #77 (0) @ WW
BID : 33117_GB7CIP
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Subj: DC/RF Sensing PA Switching
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From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                      (Graphics updated Dec 03)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

For PA with RF/DC sense switching (as some rigs put out DC on Tx), that used a
diode to detect any +ve signals (RF/DC) from the transceiver to operate the PA
PTT system....

The time constants are OK for SSB & FM. The 22uF can be omitted for FM only.
TR1 & 2 BC107 etc, TR3 a TIP type PNP, D1 RF diode 1N4148 etc. D2 1N4001.

Rig o)ÄÄÂÄÄÄÄÂÄÄ´ÃÄÄÄÄ> RF to Drive Relay
     ³ 100K _³_   1n
     ³  ³   \_/                                    ______+12-24V
     ³  ³    ³D1                                 ³/e
     ³  ³    ³      ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄ´ TR3
     ³  ³   22K     ³                       ³    ³\ PNP
     ³  ³    ³      ³      Ext oÄÄ¿        470     ³ +Tx
     ³  ³    ³    ³/       PTT    ³    LED  ³      ÃÄÄÄÄ> Drive, O/P,
     ³  ³    ÃÄÄÄÄ´ TR1           ÃÄÄÄÄ´<ÃÄÄÙ      ³      Bias, &
     ³  ³    ³    ³\e           ³/                 ³      Fan Relays
     ³  ³   ===     ÃÄÄ47KÄÂÄÄÄÄ´ TR2              ³
     ³  ³    ³10n   ³+     ³    ³\e                ³
     ³  ³    ³     ===     ÃÄÄÄÄÄÄ)ÄÄÄÄ150KÄÄÄÄÄÄÄÄ´
     ³  ³    ³      ³22u   ³      ³    +ve        _³_
     ³  ³    ³      ³     10K     ³  feedback     /_\
     ³  ³    ³      ³      ³      ³            D2  ³
    ÄÁÄÄÁÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄ

HOW IT WORKS
D1 detects RF & passes any rig DC to turn on TR1. TR1 turning on charges up the
22uF & makes TR3 initially conduct to give fast Tx attack. When 22uF is over
2V TR2 is turned on & passes higher base current into TR3 via a Tx LED ensuring
it is well turned on. The 150K +ve feedback ensure snap action (too low & it
may not switch off!)

External PTT earth can be put in/out on TR2 collector.

I have found it was heat sensitive, due to the sensitivity of the 1st NPN.


Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP



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