New SSTRAN AMT5000 High Efficiency Transmitter Preview

Dear PhilB

You are very soon to receive a flood of orders for your new product.

My pen is ready, the check will not be blank for long.

I'm even dusting in the space where the AMT5000 will be placed.

Can you comment on range improvements? And what kind of price range will this guy fall into?

Congratulations Phil! Looking forward to the roll out of the new product and more information.

Neil

The short answer is the range of the AMT5000 will be equal to or better than any other Part 15 compliant AM transmitter. That’s a bold statement, so here is the supporting technical information.

Transmitter range is a complicated issue that depends on the transmitter output power and the installation-specific antenna and ground losses. FCC rules limit the final RF stage power input to 100 mW. For any given antenna and ground system, the "best range" prize will go to the transmitter that is able to convert the largest portion of the 100 mW input power to output power. Percent Efficiency = 100 * output power / input power.

Choosing the point in the circuit where efficiency is measured is critical to evaluating the real efficiency of a transmitter.

The efficiency of the RF output transistor is the first part of the overall efficiency calculation. Output transistor efficiency is a measure of the amount of input power that is lost as heat in the output transistor. The AMT5000 transistor efficiency is 98%, so only 2% of the input power is lost as heat. The MOSFET transistor runs cool to the touch. No heat sink is required. The transistor is rated at 2 watts dissipation (not because it has to, but because it can), so it is not stressed at all.

Beyond the output transistor, on the way to the antenna, is the tuning coil. Whether or not the tuning coil losses should be considered in measuring the overall efficiency of a transmitter has historically been controversial on the various part 15 forums. Some transmitters don’t have a built-in tuning coil and some do. If the tuning coil is built-in, there is no option but to include its losses in the overall transmitter efficiency calculation. If there is no built-in tuning coil, then the losses in the external tuning coil should be included for an apples-to-apples comparison.

The AMT5000 has a built-in tuning coil in the form of an iron-powder core toroid coil. Toroid coils offer major advantages over other coil configurations occupying the same physical volume, but only if the core material is properly chosen for the operating frequency and power level. Some power is inevitably lost in the core. It is extremely important to carefully choose the core material that yields minimum loss. There are literally dozens of different core materials available, tailored to switching DC power supply frequencies way up to hundreds of MHz. Most people think “ferrite” when they think of toroids, but ferrite is a poor choice for transmitting at AM broadcast frequencies. Certain “iron powder” cores are best suited for the AM band. The AMT5000 uses the optimum iron powder core material to minimize core loss.

If the optimized built-in toroid tuning coil isn’t the Holy Grail for the die-hard experimenter, it can be bypassed in the AMT5000 with one jumper. You can use an external loading coil, or any form of externally tuned antenna. Large air-core coils, way to big to be practical inside a transmitter box, can be connected. You can experiment with base-loaded, center-loaded, top-loaded, helical or any other conceivable type of tuned antenna.

In summary, the overall efficiency of the AMT5000 will be equal to or better than any other Part 15 transmitter utilizing an internal tuning coil, and the overall efficiency will be essentially the best theoretically possible when using any conceivable super-efficient, externally tuned antenna system.

Phil,

It would be great if there is a measurement of efficiency which could be compared to other "internal loading coil" transmitters or just to establish a benchmark. I suggest attaching a series C (low loss mica or air variable) and R (2W composition works well) from the antenna output to ground with the circuit tuned to resonance. Reasonable values to simulate an antenna system would be R = 27 ohms and C = 27 pF. The DC input power is then measured with the load connected and the power delivered to the dummy R load will be Vrms^2/R with Vpeak read by means of an oscilloscope and converted to Vrms for the calculation.

Though not an exact predictor of efficiency with a real antenna system this would at least give some standard basis for comparison.

What do you think?

Neil

Neil,

You're right on the money with your suggestion. This scheme is exactly what I used in my evaluations of after-tuning-coil efficiency, both in simulation and in actual measurements with a scope.

My choice of capacitor and resistor values were only slightly different: 30 pf and 33 ohms, not significant enough to affect the efficiency results.

The dissipation factor, aka ESR, of the capacitor is an important consideration. I use a 30 pF Mica capacitor. Its DF is 0.3% resulting in an effective series resistance of 9.04 ohms at 1600 kHz (higher at lower frequencies). That's significant when compared to the my 33 ohm ground resistor, so it must be factored into the results. By factoring the capacitor ESR into the measurement results, you get higher true efficiency.

Phil

Ok - I'm dyin here. I really can't wait for this to be available.

Thanks, Phil, for your reply. Accounting for the ESR of the capacitor in the simulated antenna is appropriate since this would otherwise skew the data. The values you used are realistic.

I encountered the effects with ESR during my experiments with my transmitter design and found that using micas instead of ceramics in the output filter yielded a gain of several milliwatts in output power. This could be a matter of tolerance as well as ESR. Things are not always as they appear on paper are they?

You had suggested to me that an output filter was not needed and I assume your design omits this so the selection of the caps. is not a problem.

Neil

Phil's AMT5000, and Neil's design work, are both
exciting. I don't know what else to say to
capture the feeling!

Bruce, MICRO1690/1700

Neil,

You are correct. The AMT5000 does not have a filter other than the series loading coil. Spectral analysis shows a very small spike at the second harmonic, but it is 43 dB below the carrier. All other points in the spectrum are well below that. Since the FCC rules allow 20 dB spurious emissions, I chose not to add any additional filter components that would add some losses.

The circuit does use two capacitors in the RF output circuit: a 470 pF cap across the MOSFET for optimizing the Class E waveforms at the MOSFET drain and a .047 uF series coupling capacitor. Both are C0G (NP0) ceramic capacitors. C0G ceramics are the lowest loss capacitors. They have a dissipation factor of 0.1% compared to 0.3% for Mica, but they are not typically stocked by distributors in voltage ratings above 100 V. The ESRs at 1600 kHz for the two caps I mentioned are 0.212 ohm for the 470 pF and 0.002 ohm for the .047 uF cap.

The voltage before any series loading coil in a transmitter are low and 50 V C0G ceramics would be your best choice for lowest loss, but you also have to deal with inductor loss in the filter circuit, which likely will be the higher contributor to loss.

Another factor to consider for low-pass filters is that they are designed for certain input and output impedances. The resistive impedance at the input of the series loading coil can range from maybe 10 ohms up to maybe 100 ohms, depending mostly on the antenna ground resistance but also on the loss resistance of the loading coil. Designing the filter for a "standard" 50 ohm load value may cause problems at the high and low resistance extremes of the tuned antenna. I don't have much experience running the equations for low pass filters, so I don't really know how sensitive they are to the output impedance. I'm just pointing it out as a possible issue.

Spurious filtering performance of the series-tuned coil/antenna gets better as the coil and ground loss resistances drop (Q is higher) and gets a little worse at 100 ohms, but is still better than 40 dB. The resistance would have to go up to somewhere in the 500 to 1000 ohm range before the spurious filtering would hit 20 dB. That wouldn't even be a usable antenna.

Hi Phil,

Can you comment on using balanced lines to the unit. In one instance I may end up doing quite soon, I may have to run conduit and fairly long lines to get power and audio to the TX, therefore I want to run balanced audio., and very possibly AC to the DC PS in the same conduit.

The AMT5000 has a differential audio receiver IC with extremely high (95 dB) common-mode rejection. It accepts a standard Pro Audio balanced audio feed. A screw terminal block on the board allows connecting the audio +, -, and ground wires. The feed must be mono. Standard Pro Audio practice is to use a shielded twisted-pair cable. The two wires in the twisted pair connect to the + and - terminals and the shield connects to the ground terminal. The other end of the cable will typically terminate in an XLR or TRS plug to connect to your audio source.

Shielded twisted-pair audio cable is relatively expensive compared to unshielded CAT5, CAT5E, CAT6... Ethernet cable, but history shows that CAT5 cable works without problems. CAT5 has 4 twisted pairs. You can use one pair for the + and - audio connections, one pair for ground (both wires connected to ground on both ends), and one pair for the power. The remaining pair can be left unused or used for additional grounding.

Balanced feed has a couple of advantages for long runs. It will eliminate ground-loop problems and will reject noise and hum that may be induced over the long cable. The balanced audio input of the AMT5000 is protected against static discharge surges and induced RF.

There are no jumpers or switches on the AMT5000 for selecting the Pro Audio or Consumer Audio inputs. Just connect to one or the other.

Hope this helps. If not, ask away!

Phil

One distinguishing feature which marks gear as professional is its ability to handle balanced (3-wire)audio lines. As just explained, the AMT5000 handles not only balanced, but also unbalanced audio lines.

Another facet of balanced audio which can befuddle the first timer, is that the audio levels are typically much higher than with 2-wire unbalanced audio lines. But in the very first panel of this thread, PhilB links the specifications and we find that the AMT5000 will handle ALL normal levels: -10dB consumer; 0dB or +4dB professional.

Therefore the audio input section of the AMT5000 is ready for any and all audio situations.

Thanks Phil,

I figured you'd have it set up as you indicated. I haven't dug into the spec sheet yet, but I will. FWIW, I broadcast most of the time from my recording studio, regularly run sound eng'g for church, community theatre, live events, etc., so it's an everyday thing for me. Glad you spoke about the reasons for balanced lines, though.

My first choice would be wireless or optic cable and converters, but I haven't found either that I want for the price I want. I could once have gotten them from Being Surplus, but that option has mostly gone away, darn it! 🙁 ...so balanced line is the last choice in this case.