Abstract: This article discusses the design of a simple Class-E switching-type LowFER transmitter.
[Reproduced from Western Update #59, September 1988, by permission from the author]
My LowFER beacon, MPM, has been on the air for some time, utilizing a dual MOSFET push-pull transmitter design. Despite it's complexity I've been quite proud of the circuit, which I considered to be hard to beat for DC to RF efficiency. Because I have received several requests for a description of the circuit, and since efficiency has recently become a "hot" topic, I had planned on writing a short article on the transmitter.
However, no sooner had I began than I ran across Frank Cathell's article (WU #58) on Mike Mideke's "simple beacon". I'd previously glanced over Mike's circuit without being particularly impressed: "Maybe 50% efficiency on a good day" I thought to myself. But Frank's article started me thinking that maybe the efficiency could be pretty good if the active device could be operated as a switch. I decided to do a rigorous analysis of the circuit, optimizing component values, and then check the results with a circuit analysis computer program. The results were encouraging, so I went ahead and built a prototype and WOW! - was I ever impressed!!! Not only is it considerably simpler than my push-pull design, the efficiency is higher! As a result, I no longer recommend my push-pull design! However, my analysis uncovered a few surprising characteristics (as well as some minor errors in Frank's article) so, since I still have this ego-driven desire to write something, I thought I'd share my findings with everyone:
As Frank's article pointed out, operating the device as a switch can theoretically achieve 100% efficiency. What originally frightened me about this circuit was that the switch was directly in parallel with the tank circuit. Any voltage on the capacitor when the switch is turned on would be discharged by the switch, dissipating the capacitor energy and wasting power. Since the MOSFET drain voltage oscillates about the supply voltage (rather than ground) when the switch opens, it would not normally return to zero volts after one-half cycle. To remedy this situation, the tank circuit should be DETUNED so that it oscillates for more than one-half cycle while the switch is opened. It turns out that if the tank is made resonant at 1.2915 times the operating frequency (assuming a 50% switch duty cycle) the voltage on the drain will return to very nearly zero volts when the MOSFET turns on. (Contrary to Frank's article, a 50% duty cycle is not necessary - any duty cycle may be used, the only difference being that the detuning factor will change. However, since 50% is easily produced, it is used throughout this analysis.)
It is also desirable for high efficiency that the current in the
MOSFET
be zero immediately following turn-on: This tends to
minimize
average
MOSFET current for a given power output. This will occur if
the
inductor
is completely discharged during the MOSFET "off" period, i.e., all
energy
stored in the inductor is transferred to the load.
For a given supply voltage, frequency, and power output, it turns
out
that there is an ideal inductance value and load resistance which
satisfies
the above condition. When these values are combined with the
condition
for tank resonance above, we obtain component values and and load
resistance
for our "optimized" circuit:
Where: | |
L = tank inductance (Henries) | Z = load resistance (Ohms) |
C = tank capacitance (Farads) | P = output (or input) power (Watts) |
V = supply voltage | F = operating frequency (Hertz) |
Also: | |
Vmax = peak voltage on MOSFET = 3.6311 * V | |
Irms = rms MOSFET current = 1.1638 P / V |
The above equations (Figure 1) are the result of several hours of slaving over a table of Laplace Transforms, and numerical equation solving using a programmable calculator. Since I easily could have made an error in the calculations I decided to check my results using a circuit analysis computer program I have access to at work. I first calculated component values using the above equations, for P = 1 watt, V = 12 volts, and F = 179,000 Hz. These values worked out to:
For reference, the values for L, C, and Z referred to
in
the
past articles
(i.e. "Simple Beacon" schematic) are as follows: L = 75 uH C = 0.015 uF and Z = 72 ohms Again, note that these values are NOT derived from the equation in figure 1. |
L = 167.7 uH
C = 2826 pF
Z = 182 ohms
The analysis was run and the output voltage and switch current were plotted (see Figure 3.) Plots were made for both no load and full load. The output voltage closely approximates a half cycle sinewave at no load, but departs somewhat from sinusoidal at full load. The full load MOSFET current is seen to be zero when the switch first closes, as desired. Note also that the no- load MOSFET current is initially negative and ramps positive, with the result that the average current is essentially zero, which would be expected for no load.
Encouraged by these results, I decided to build a real working
output
stage. I picked up an IRF-511 MOSFET at Radio Shack, dug
through
my junk box and found a T-150-2 powdered iron toroid core, and
borrowed
several mica capacitors from work which totaled about 2800
pF. I
added turns to the toroid until it resonated with the caps at 231
kHz
(1.2915
times 179 kHz) - this required 105 turns of #30 wire. I
drove the
gate of the MOSFET with a 5 volt 50% duty cycle square wave from a
function
generator. For a dummy load, I used a 150 ohm resistor in
series
with a 3 mH inductor and 300 pF variable capacitor - the
resistance of
this combination was very nearly 182 ohms at resonance.
I first powered up the circuit with no load, and observed a
output
waveform
identical with the no load output voltage plot. The supply
current
was about 1.2 mA. I then connected the load, and by tuning
the
variable
cap obtained a waveform identical to the full load plot, with a
supply
current of 83 mA. Using the 'scope I then measured the load
current,
and calculated the output power and efficiency, which was an
amazing
98%!
The highest efficiency I ever measured with my push-pull design
was
95%.
The circuit was well behaved with changes in tuning, with maximum
RF
output
and maximum current drain occurring very nearly together.
A tune-up procedure which works well is as follows: With no
load,
the tank is adjusted for minimum supply current (this adjustment
is not
too critical and the circuit will work well over a +/- 10 kHz
range
using
fixed values). The antenna is then connected and the loading
coil
tuned for maximum RF. Since most LowFER antennas have a
resistance
of less than 182 ohms some means of impedance transformation is
required.
Probably the simplest way of doing this is to connect the antenna
and
loading
coil to a tap on the tank inductor. The position of
the tap
should be adjusted so the transmitter draws 1 watt DC (83 ma for a
12
volt
supply) when the loading coil is tuned for max. RF output.
If too
much current is drawn, tap the inductor farther from the MOSFET
end.
These numbers may look scary, especially the 2nd, but shouldn't
be
much
cause for alarm. A LowFER antenna with a Q of 100 (a rather
mediocre
value) will provide 37.5 dB attenuation of the second harmonic, so
the
radiated second harmonic will be over 43 dB below the fundamental.
For proper circuit operation, the load impedance at all
harmonic
frequencies
should be high compared to the impedance at the fundamental.
This is
easily
accomplished by placing a series tuned circuit in series with the
load.
A LowFER antenna / loading coil combination [will] provide this
automatically,
but if the transmitter is going to be driving a transmission line,
lowpass
filter, pi-network, dummy load, or a tap on a grounded loading
coil, a
series tuned circuit designed for an operating Q of 5 or greater
should
be connected in series with the load, and tuned for maximum
RF.
If
a low distortion sinewave is desired (e.g., for efficiency
measurements),
the Q can be selected based on the
above harmonic levels to provide the
desired harmonic distortion.
Finally, as Frank's article mentioned, it is important to ensure
that
the MOSFET gate be driven with a fast risetime squarewave to
provide
switching-mode
operation. Since MOSFETs have considerable input
capacitance, a
low
impedance driving source is needed. I have had excellent
results
driving MOSFETs directly from HC series CMOS. The HC parts
have a
driving source resistance of 30 ohms or less, as compared to
several k
ohms for the 4000 series. I also recommend AC coupling the
drive
to protect the MOSFET (what if the oscillator stops and the
divider
output
is stuck high??) (A sample design may be seen in Figure
4.)
In summary, I'm just about convinced that the 'simple beacon"
circuit
is the Ultimate Design! With the changes outlined here, it
provides
unmatched DC to RF efficiency, as well as being just about the
simplest
circuit one could hope for. What could be better?
Fundamental: 0 dB | 6th -30.24 dB |
2nd -5.54 dB | 7th -33.19 dB |
3rd -16.92 dB | 9th -35.20 dB |
4th -22.66 dB | 9th -37.62 dB |
5th -27.06 dB | 10th -38.93 dB |
Additional comments (not necessarily from the
article):
Go to
the "CT" MedFer PSK Beacon page
This page converted to HTML by Clint, KA7OEI
on
20000520.
Updated on 20110729