TX/RX sequencer V2


Proper changing from receive to transmit operation is vital for the receive pre-amplifiers as well as the used coax relays. Especially it becomes important with 1 KiloWatt power levels, mistakes will almost certain mean the immediate death of the pre-amplifier! To prevent including sequence control circuitry in every amplifier a standalone sequencer was built.



block diagram

The sequencer has 3 input sources: PTT, ERROR and RESET. PTT is the input which need to be asserted when transmission should start. When PTT is pulled low the sequencer starts the predefined timed sequence of activating the 3 outputs: RADIO PTT, PA ENABLE and TX/RX. Subsequently if PTT is released the sequence will be halted and reversed.

An ERROR input is provided to signal an ERROR situation to the sequencer, for instance this can be a high SWR situation or a separate signal from the TX/RX coaxswitch that this switch is for sure in the TX position. To resume normal operation when this ERROR has occurred the sequencer should be reset via pulling the RESET input low or via a full power cycle.

The 3 output signals are used to signal the transceiver between receive and transmit state (RADIO PTT), toggling the external power amplifier (PA ENABLE) and commanding the transmit/receive RF switch (TX/RX). Inputs are high impedance and active state (TX , RESET or ERROR active) is low/ground. Outputs are open drain active low signals, pulling the output low/ to ground when in TX position. In additions 3 LED's can be connected (anode to output, cathode to ground) for state indication.

Use and connecting

SSB operation

PTT input will be connected to a footswitch or microphone switch. Microphone element connected to the radio in the normal manor. VOX should be switched off. When PTT is asserted the sequence is started (see below for detailed information on the sequence).

CW operation

To be able to use this sequencer for CW operation the radio should be able to work in no "break-in" or vox mode, i.e. upon activating the CW KEY input of the radio it should NOT go into transmit (preferable only sidetone should be activated). A separate radio PTT input must be available for commanding the radio to go into transmit and only then RF power should be output following the CW KEY input.

In addition the CW keyer should provide an outgoing PTT signal (low active) signaling start of CW message with sufficient "tail/hang" time such it wouldn't go inactive between CW characters. For instance the Winkey device from k1el is perfectly suited.

CW output of the keyer needs to be connected to the KEY input of the radio and PTT keyer output to the PTT input of the sequencer. When PTT is asserted (start of CW) the sequence is started.

For both mode of operation a DC power supply should be connected and if applicable 3 LED's can be connected to provide an indication if the corresponding output is active. These LED's should be connected with the anode to the pin and cathode to ground.

"Go to RX" and "Go to TX" Sequences

During receive when PTT is brought LOW the "go to TX" sequence will start and will continue as long as PTT is LOW. In case PTT goes inactive during the sequence the "go to TX" sequence will be stopped immediately and reverted onwards.

Go to TX sequence:

Sequence descriptionAction description
1. TX/RX signal is brought active (Low) immediately The TX/RX coax relays will be switched to the TX position
2. Fixed delay of 100ms Wait till the TX/RX coax relays has been switched
3. Power amplifier is enabled for transmission
Enable the power amplifier, no power is output yet
4. Fixed delay of 100ms Wait till the power amplifier has switched internally to TX
5. Radio is set to TX mode (RADIO PTT = low) if no Error is seen (ERROR is not Low) The radio is put into TX, power is output


Go to RX sequence:

Sequence descriptionAction description
1. Radio is set immediately to RX mode (RADIO PTT = NOT low) The radio releases TX goes into RX mode
2. Fixed delay of 100ms Wait till the radio has released TX mode
3. Power amplifier is disabled for transmission (PA ENABLE = NOT Low) Disable the power amplifier
4. Fixed delay of 100ms Wait till the power amplifier is disabled
5. TX/RX signal is brought inactive (TX/RX = NOT Low) Switch the RF switch to RX position

State diagram

The sequencer is build around a state machine build in software. The four sequenced states can be seen together with the state of the output signals. Upon RESET (or a power cycle) the state machine will enter the RX_ACTIVE_STATE and remain in that state as long PTT is not asserted. When PTT is asserted the RELAISTX_STATE is entered. If PTT stays active PA_ENABLE_STATE will be entered after a fixed amount of delay time, however if PTT is released the state machine reverts to the RX_ACTIVE_STATE.

The other states and transitions are fairly self explanatory, special notice to the ERROR_STATE (this will be entered when the ERROR signal becomes active during the TX_ACTIVE_STATE). ERROR_STATE will disable the power amplifier and sets the radio to receive, however the RF switch remains in the (safe) TX/Transmit position. Only way to get out of the ERROR_STATE is to reset the sequencer via the reset signal of power cycle the unit.

state diagram sw



Output voltage VOUT TX/RX, PA ENABLE and RADIO PTT pins 100V
Continues output current IOUT PTT = GNDTX/RX, PA ENABLE and RADIO PTT pins 7A
Input voltage VIN DC power, PTT and Error pins 30V
VIN Reset and LED output pins  




Input voltage VIN(ON)

PTT input and ERROR pin

tbd 1 V
VIN DC input 8 12 30 V
VIN(ON) RESET pin -0.5 1 V
Power supply current ICC PTT= 0V 70 mA
PTT = Open 6 mA
Input current IIN PTT and ERROR pins Vin=0.2V - 50   µA
Vin=12V 100 nA
Output Low Voltage VOL

IOL= 200mA, RX/TX,

  0.1 V
Output current ILED

LED current each pin

10 30 mA



Timing delay Td Ta = 25 degree C 100 ms


In the picture below the timing is given as measured, note the awkward time axis scaling.



Length and width



Height Including components and PCB



Design implementation

CAD developed using Eagle 5.6. Board is a double clad FR4 with component silkscreen. For a good RF immunity the top plane is almost fully copper/ground. Inputs are well decoupled, but for optimal RF immunity it is advised to mount the PCB in a shielded box and use small valued (100pF) feedthrough capacitors.


For a .pdf version of the schematic select the diagram below or click here



Board layout


External connections

LSP1 Input DC power supply input, typical +12VDC
LSP3 Input RESET input
LSP4 Output LED TX/RX (Anode of LED)
LSP5 Output LED PA ENABLE (Anode of LED)
LSP6 Output LED PTT RADIO (Anode of LED)
LSP7 Output TX/RX
LSP10,11,12 Input PTT
LSP 13,14 Input ERROR


Software is developed in C using AVR Studio 6 and WinAVR GCC C compiler for AVR ATtiny13. Internal 9.6 MHz oscillator is used. Thanks to Mike G8TIC for a number of enhancements to the original source code.

CSEL1..0 10 Internal 9.6 MHz oscillator
SUT1..0 10 Slow rising power startup

Copyright © 2011 Carel Mobach, PC5M

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