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I have recently purchased an amplifier for my repeater. This was actually my first amplifier purchase ever and should probably have asked more questions prior to buying it. The repeater is a Motorola XPR8400 UHF R1 running in dynamic mixed mode. The amp I purchased is TPL Communications PA6-1BE-RXRFPSM UHF 100W. I've heard that some amps have issues with digital signals and some don't depending on the amp class. I tried to do research on this but there is little to no information online. Was able to confirm though through an acquaintance who never heard of that is maintaining a repeater system running a 250W TPL amplifier in DMR without any issues. What is the theory behind digital signals vs analog and repeater operations? Will this specific amplifier transmitting DMR work or will I run into problems? Lastly, if this amplifier is not suited for DMR which make and models are?

Mike Waters
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Mike
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    This is a good question, I'm looking forward to some answers. Two things come to mind immediately: the "digital" modulation probably has an amplitude component to it, which may require a more linear amplifier than just phase modulation. And digital transmissions likely involve transmitting in timeslots, hundreds of times per second, and possibly also receiving between the transmitting slots, like a cellphone does. None of these will work well with a simple hard limiting "FM" amplifier. – tomnexus May 19 '23 at 23:59
  • ChatGPT gave me the best answer. Very detailed but it was unable to tell me anything about my specific amp. It’s a very common commercial amp and I’m hoping that someone familiar with it could provide some critical information on the specific amp. – Mike May 20 '23 at 17:08
  • Hah, yes, maybe it has swallowed a summary of the issues. But it also hallucinates and fabricates things. What is the amplifier, perhaps someone here will look it up. – tomnexus May 21 '23 at 01:39
  • The difference in the amplifiers depends on what modulation you are using. Some amplifiers take shortcuts that make them less linear. You would need a more linear amplifier for AM or FM vs. SSB and even less for CW. FM and AM need a fairly linear amplifier, so likely a digital signal would not have a problem with the same amp. If you've already hooked it up and you're not getting distortion that breaks things, likely it is fine. Duty cycle is a bigger concern -- especially for a repeater that might have 100% duty cycle. – user10489 May 21 '23 at 12:19

1 Answers1

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So... really what you want to shoot for with a digital system is relative signal consistency between among the 1's and the 0's. With that in mind, recall that:

  • Class A amps are pretty linear, reproducing the input signal with the least distortion. Good for high-fidelity audio applications or amplifying complex signals... like DMR. The tradeoff is in terms of power usage, typically only around 30-50% efficient.

  • Class B and Class AB amps are more power-efficient than Class A amps, BUT can produce more crossover/distortion if they're not designed properly for the application.

  • Class C amps are very efficient, but are not linear and produce a lot of distortion. They are usually only used for signals where the exact shape reproduction is not critical.

So you'll want a Class A or a well-designed Class AB amp for best signal integrity.

If this is the product, then it looks like each stage is a single common emitter transistor feeding the signal through an LC circuit... which I believe is a Class C design (if this is incorrect, please someone edit).

Hope this helps!!

webmarc
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  • What I havent been able to determine is what class my amplifier mentioned in the post is. – Mike Jul 28 '23 at 17:33
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    Just updated my answer w some info, confirm [that this is your product](https://www.repeater-builder.com/other-mfrs/pdfs/tpl-pa6-1ae-1be-1fe.pdf)? If it's a different one, share a link to the manual/schematic and we can figure it out. – webmarc Jul 28 '23 at 20:01
  • That appears to be an old manual and it appears newer versions of this amp was updated along the way. I believe this is the manual https://www.repeater-builder.com/other-mfrs/pdfs/tpl-uhf-rptr-amps.pdf – Mike Aug 16 '23 at 12:27