[NLRS] Question about sequencing for an external RF amp

[Charlie Betz]

I agree with everything Larry stated, but I want to add that sequencing 
also helps save the contacts of the T/R relay.  I have been running high 
power (close to legal limit) on a number of VHF bands for awhile and 
even without preamps you do not want to be hot switching T/R relays with 
high power.  They will arc and eventually burn up or fuse - I've managed 
to destroy a number of them that way.  The bad part is I don't think I 
ever intentionally hot switches them - it was usually the result of a 
failure in wiring or in the sequencer itself.

BTW, for a good example, take a look in detail at W6PQL's SSPA 
amplifiers (you'll have to dig into his descriptions) - he uses a 
sequencer built into his control board to ensure the T/R relay switches 
before switching on the 50VDC supply to the RF deck - all on the name of 
protecting the internal T/R relay.

Most of the Mirage/TE Systems/ etc. are not running as high of power 
levels, but the reality is that over time, even those  T/R relays are 
susceptible to hot switching.  I've had a number of them fail as well 
although it was after a fair number of years of use.  You can probably 
get away with it for a long time before you start having problems, but 
over time they are also very likely to fail.

Bottom line is if you are running high power, a sequencer is a really 
good idea.  There are a number of good designs out there. W6PQL actually 
has one that's pretty reasonably prices and DEMI makes a pretty decent one.

Charlie, N0AKC



On 8/9/2019 01:05 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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