[NLRS] Question about sequencing for an external RF amp

Fri Aug 9 21:29:32 EDT 2019

It is important that the RF transmit signal is quiet until the TR relay 
has moved to the transmit position. My FT857Ds have an option that when 
enabled and the rig is on USB to send CW only requires closing the key, 
no PTT operation. Unfortunately that puts out RF instantly and the RF 
keying in the DEMI control board on my 10 GHz transverter delays 
switching the relay but not the transverter so the relay switches with 
RF power present. The 3 watt DEMI amplifier has fried one relay for me 
using that mode. So for the transverter 857D (power turned down in the 
alignment menu transmitter 2m gain to 2 watts) I have disabled that 
option and require switching the mode to CW from SSB and require holding 
the microphone PTT button to enable transmit. That way I get sequencing 
of the transverter and the TR relay. There is a PTT output from the 857D 
that goes to the PTT input of the transverter in the cable between them. 
Use of VOX on SSB would cause the same problem so I stick without VOX 
and the PTT on the microphone for SSB also.

If you construct a sequencer circuit many IF rigs like FT817 and FT 857 
have a control input for ALC and applying an appropriate voltage to that 
control input will prevent any RF coming out until that control voltage 
is cut to zero. Something your sequencer can do.

Controlling output power of a FT857 by ALC generally leads to a RF power 
output spike on the first closing of the key, not good for the 
transverter and sequencer. Reducing the output with the alignment menu 
cutting down the transmitter gain prevents that spike in all modes.

73, Jerry, K0CQ

On 8/9/2019 1:05 PM, Larry Peterson wrote:
> The main purpose of a sequencer is to protect an LNA (low noise amplifier...or you may think of it as a pre-amplifier).  Any Tx power and you fry it.
>
> If you don’t have one and are just doing PTT-to-ground switching to turn on a linear, then it is really easy.  Just connect your PTT line between the rig&  linear.  When you transmit, it turns on the linear.
>
> PTT switching is superior to RF switching, but I think we all do the latter...I have with Mirage&  TE Systems amps.  But my Gemini’s and Beko’s call for the PTT-to-ground.
>
> Now if you do have an LNA, like I have on my 10 GHz station, then a sequencer is a must.  FYI, here is my sequencing order:
>
> 1. Turn off voltage to the LNA.
> 2. Switch T/R relay to transmit.
> 3. Turn on DC power for my 8W linear amplifier.
> 4. Turn the DEMI transverter to transmit by activating the PTT-to-ground from my Yaesu 817.
>
> Then when you are done transmitting, the sequencer goes back in reverse order to get back to receive mode.
>
> It’s pretty fast but you can hear the clicking, etc of mechanical relays in sequence.
>
> Good luck,
>
> Larry
>
>> On Aug 9, 2019, at 12:53 PM, David Palm<thepalmhq at gmail.com>  wrote:
>>
>> I am just about done wiring the switching circuitry and RF relays to add an
>> external 1296 MHz amplifier to my transverter (thanks, N4PZ!)  I know that
>> there should be some sort of sequencing so that the relays don't hot
>> switch, holding off Tx RF until they have had time to switch.
>>
>> But while I've been pondering how to do that in this specific case (any
>> suggestions are welcome), I thought about the TE Systems, Mirage, etc.
>> amplifiers we regularly deploy on 144/222/432 MHz.  We don't have to do any
>> additional sequencing for these amps, we just plug in PTT and RF from the
>> transceiver and go.  Is this because the PTT signal out of our modern
>> transceivers is already leading the RF output by xx milliseconds, so we
>> have that time built in for the relays to switch?  If so, do I need to do
>> anything else in this case, where I'm building my own relay transfer
>> circuit?
>>
>> Thanks and 73,
>>
>> David  W9HQ
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