Repeater Amplifiers DMR vs Analog

Question

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?

Answer 1

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!!

Answer 2

Research update: The TPL PA6-1BE-RXRFPS-M amplifier, specified for FM mode with a frequency range of 400-512 MHz, input power of 8-15 W, and output power of 75-100 W, is likely Class AB. This class is typical for RF power amplifiers in communication systems, offering a balance of linearity and efficiency. The design likely uses solid-state components, such as LDMOS transistors, which are standard for Class AB amplifiers due to their cost/performance ratio in mobile and digital networks. The continuous duty operation and the need for handling modulated signals suggest Class AB, as it conducts for more than half but less than the full cycle of the input signal, providing good signal fidelity.

Investigation and Findings Initial searches for the TPL PA6-1BE-RXRFPS-M revealed detailed product descriptions but no direct mention of the amplifier class. For instance, the official TPL Communications website (Tplcom) describes the RXR Series as cost-effective, continuous duty power amplifiers, with specifications including frequency range, power output, and physical dimensions, but lacks explicit classification. Similarly, distributor pages like SYSCOM list input power (8-15 W, output 75-100 W) and note a consumption of 16 Amps, but do not specify the class.

Further exploration into FCC filings, using the FCC ID database (Fccid.io), showed applications for TPL Communications under grantee code BBD, but specific details for PA6-1BE-RXRFPS-M were not readily available. Searches for "TPL RXR SERIES amplifier schematic" and related terms, such as on Repeater-builder, did not yield direct schematics or class information, though user manuals for similar models (e.g., FCC ID BBD6-1AE-RXR) mentioned solid-state design options, such as a solid-state carrier operated relay (SSR), suggesting solid-state technology.

Comparison with Other Classes Class AB is a compromise between Class A (highly linear, low efficiency) and Class B (moderate efficiency, more distortion). Class C, while efficient, is non-linear and typically used for constant amplitude signals like FM, but may not suit digital communications requiring linearity. Given the user's interest in DMR, Class AB aligns better, though the FM specification might suggest optimization for analog signals.

Unexpected Detail Interestingly, while the amplifier is specified for FM, its likely Class AB design suggests potential use with digital signals like DMR, challenging the assumption that FM-specific amplifiers are unsuitable for digital applications.

Conclusion and Recommendation Research suggests that the TPL PA6-1BE-RXRFPS-M amplifier is likely Class AB, given its continuous duty operation, use in communication systems requiring linearity, and alignment with industry standards for similar RF power amplifiers. The design is probably solid-state, using LDMOS transistors, paralleling standard Class AB configurations. While direct confirmation from schematics or datasheets was not found, the technical characteristics and comparative analysis strongly suggest Class AB as the most probable classification. For precise verification, consulting TPL Communications directly or accessing detailed engineering documentation would be recommended. This conclusion is supported by the need for a balance between efficiency and linearity in communication applications, with Class AB offering a practical solution for the amplifier's specifications and usage. Interestingly, while specified for FM, the likely Class AB design suggests potential use with digital signals like DMR, challenging common assumptions about analog-only amplifiers.

When I put the amplifier in service this year I'll update my testing results to confirm linearity to handle DMR signals.

Answer 3

DMR and P25 use linear amplifiers. Using non-linear amps made for FM or CW will not work. All your digital downlink modes use a form of QAM.

Answer 4

So here’s the thing: DMR uses continuous-phase 4FSK. That means the signal’s envelope is as steady as a tax accountant—no drama, no spikes, no surprises. Which is to say, a Class-C amplifier will do just fine. Despite what some well-intentioned but mistaken folks have claimed, the amplifier class isn’t the problem here.

Addendum

There are largely two types of DMR repeaters. Depending on the country/region, one type is dominant but that is not common throughout the globe.

The types that use multiple frequencies for "uplink" and "downlink" like analog FM repeaters may not require super fast T/R switch.

However, the type called SFR shares one frequency for both up and downlink on timeslots. This type requires very fast T/R switch, making external amplification impractical in many cases.

The SFR mode is perhaps most commonly used in countries where repeater operation requires special licensing and frequency assignment but a single-frequency repeater does not trigger that requirement. That includes Japan.

Below, my answer assumed SFR mode.

But DMR’s not just steady—it’s twitchy. It’s a TDMA system, chopping time into neat little slices about 3.3 to 3.4 milliseconds long. Each frequency gets split between two alternating data streams, like two radio stations forced to share the same frequency—on alternating blinks of an eye.

Now here’s the kicker: the T/R switch must flip during a narrow “guard interval.” And by narrow, I mean a few microseconds—not milliseconds. You need something like a PIN diode switch. Fast. Solid-state. No drama.

Mechanical relays? Please. They switch like they’ve had three martinis and a long nap. By the time they’re done flipping, the conversation’s over and everyone’s gone home. Plus, they’ll wear out faster than a cheap non-YKK zipper because, remember, we’re doing this 300 times per second.

Anyway, I think that’s the issue. I see the original poster already did their homework and posted the findings—but what the hell, I just saw this. So it goes.