Friday, July 11, 2014
LT1910 AM transmitter
This circuit uses an LT1190, LT1007, and an LT1194 to form a AM radio transmitter. The carrier is generated by A1, set up as a quartz stabilized oscillator. A1's output feeds A2, functioning as a modulated RF power output stage. A2's input signal range is restricted by the bias applied to offset pins 1 and 8. A3, a microphone amplifier, supplies bias to these pins, resulting in an amplitude modulated RF carrier at A2's output. The DC term summed with the microphone biases A3's output to the appropriate level for good quality modulation characteristics. Calibration of this circuit involves trimming the 50 ohm potentiometer in the oscillator for a stable 1Vp-p 1MHz A1 output. The construction and operation of this apparatus may require Federal Communications Commission review and/or licensing.
Sunday, July 6, 2014
Protect your MOSFET final stage
by G3YCC
This idea was described in Sprat 52 in 1982 and basically shows how, by the addition of two cheap components, a resistor and zener diode, mosfet PA's can be protected from destruction by over driving. This is described in the article by Alan G3UZU who mentions seeing this recommended in Radio Spares data sheet 5342 on power mosfets. The resistor limits the dissipation in the zener diode in series with it from gate to earth and can be 10 ohms, 1/4 watt. The zener voltage is found from data sheets. For example the commonly used VN46AF needs a zener of less than the maximum drive volts of 15, say 13v, 400mW. The ubiquitous VN10KM would need a zener of 4.7v, 400mw. For other devices, look up the data in many books and catalogues and note the maximum gate voltage quoted and choose an appropriate zener. This simple modification will be found very useful in homebrew transmitters.
Steve Quest's Transmitter
by S.Quest
In various newsgroups Steve Quest has described an AM transmitter for mediumwave and shortwave that has produced great results and is quite inexpensive. Here is the verbal description he offered:
However, for now, just picture a standard Pierce oscillator, crystal controlled on your frequency of choice, with the output connected to a standard class C amplifier stage. The only modification to the standard class C amp stage is the insertion (series, so cut the trace and insert) of a modulation transformer between the power supply line and the isolation inductor on the collector. What works great is the $1.95 at my local audio shop, SPECO line matching transformer. Connect the transformer backwards! In other words, the input side goes to the class C amp, and the output side, normally the side connected to the speaker becomes input. Transformers work both ways you know.
Use the 500 ohm and common taps on the class C amp side, and the 8 ohm and common tap on the input side.
Now hook up a power audio amplifier to the (now) input of the transformer, and a mic or other audio source to the line input of the power audio amp. Adjusting the volume on the audio power amp will increase and decrease your MODULATION! Use a scope to set your modulation, just lay the probe near the antenna to see the AM waveform. Adjust to about 80% modulation, don't overmodulate, it sounds like crap AND is quite inefficient.
*IMPORTANT* Use a chebichev filter between the output of the class C and your antenna! If you don't your harmonics will be over a watt for sure!
This configuration, when powered by 12 volts will generally give you an input power of about 5 watts. That's based upon the class C amp giving you a times 10 gain, and the input power from the Pierce oscillator of half a watt. Sloppy design of the Pierce, and poor tuning on the class C will cause the power to go DOWN. However, if you're good, you can get 10 watts or more by tweaking the Pierce up to a 1 watt output into the x10 class C stage.
About a 4 to 5 watt input signal to a 150 foot random-wire antenna on the 41 meter band during the average sunny winter day resulted in a clear signal copied at greater than 200 miles and a weak signal copied at 1000 miles. This range example is reproduceable by several successful tests.
And here is the schematic:
Parts List:
R1 = 47k 1/4w
C1 = .001uF
C2 = 470pF
C3 = 470pF
C4 = 100pF
C5 = .01uF
C6 = 470pF
C7 = 820pF
Y1 = 7.3728 Mhz series type (off the shelf, CPU application)
Q1 = ECG or NTE282 or similar
Q2 = ECG or NTE235 or similar
T1 = Speco type T7010 70 volt line matching transformer
L1 = 28turns around 1/4" ferrite overwrapped with 9turn secondary
L2 = 28turns around 1/4" ferrite
L3 = 1.5uH choke
To make L1, tightly wrap 28 turns, then wrap 9 turns over that tomake the secondary. Space the windings out to evenly cover thefull length of the 28 turns. Use light gauge enamel wire for all.
To connect the modulation transformer, wire backwards using the normal secondary as the primary. Connect the red wire of theinput side to Vcc and the black wire to the class C amp choke. Connect the output side white wire to the amplifier speaker jack positive, and the black wire to the speaker jack negative. Use the amplifier volume control to adjust modulation level.
A few more details of this circuit:
... since the rig operates in the 41 meter band, I just used generic CB radio transistors. The Pierce oscillator uses a generic CB radio driver transistor, and the final is a CB radio final. Thesetransistors are cheap, readily available, and probably will never beoutlawed (like I suspect VHF RF components will eventually be). Since CB radio is 27 Mhz, running them at 41 meters (7 Mhz) is very easy on them.
Any type NPN, TO-220 package CB radio driver and final will workfor this application. There are probably _better_ transistors out there,but these are cheap, available at the local radio shop, and already instock for those of us who fix a few CB radios from time to time.And here's some additional detail:
I get my forms from a distributor, and my inductors as well. However, I know there are ferrites that are "close enough" in the Radio Shack assortments. You should be able to find one fairly close.
The question this time is could one substitute say 5/16" core for L1 & L2 and adjust the windings to come out with the same results. If the answer is yes, then could you recommend a good starting point to make the alterations?
Sure, use a 5/16" core, and the same number of turns. Remember, I didn't specify the wire gauge, and I should have said "about 1/4" ferrite rod" instead of being exact. Use a heavy enough enameled wire that you don't vaporize it with current flow, I used the middle gauge out of the Radio Shack 3 pack for my windings. This fact should probably be added at the website. :) You may have to adjust secondary turns between the oscillator/driver and the final to tune for maximum power. Don't glue the windings and leave plenty of extra (not looped but curved up) so you can do that. Once you find the right amount, glue them down. I use wax usually, but have also used shellac, varnish, silicone, you name it.
On the inductor in the final, an extra few turns will not hurt, but don't take away! Same goes for the primary on the osc/driver. However if you add turns, you'll have to adjust the secondary turns and capacitors. One thing to remember with HF verses VHF, the windings are NOT as critical. 3 turns in an air wound VHF oscillator tank coil will get you, let's say 88 Mhz, and one more turn and you're down to 64 Mhz, and another and you are in the 40's. It isn't like that with HF, it's not so touchy.
A small two stage transistor AM transmitter
This is two-stage transistor AM transmitter for the shortwave band, which gives about 1 watt in output. You can use a 2N2219 or a 2N3553 in the final stage. The output with a 2N3553 is aprox. 1.8 watt @ 12 Volt. The transmitter seems to give more output when you use 7 turns instead of 4 turns in the secondary section of T1 (the coil between oscillator and final stage). It will work fine between 6 and 8 MHz. Try with different (power) transformers to modulate the transmitter in Amplitude Modulation. You have to do experiments to find a suitable modulation transformer to produce a good Amplitude Modulation.
Also test with different transistors in the final stage, but be careful to not blow up the rig.
A small two stage transistor AM transmitter
This is two-stage transistor AM transmitter for the shortwave band, which gives about 1 watt in output. You can use a 2N2219 or a 2N3553 in the final stage. The output with a 2N3553 is aprox. 1.8 watt @ 12 Volt. The transmitter seems to give more output when you use 7 turns instead of 4 turns in the secondary section of T1 (the coil between oscillator and final stage). It will work fine between 6 and 8 MHz. Try with different (power) transformers to modulate the transmitter in Amplitude Modulation. You have to do experiments to find a suitable modulation transformer to produce a good Amplitude Modulation.
Also test with different transistors in the final stage, but be careful to not blow up the rig.
Simple Class E Transmitter
Class E amplifiers are very efficient amps and are generally built with MOSFET transistors. The principle is to drive the MOSFET's gate input with square waves to quickly put the device into it's low ohmic region and to do this when the voltage across the drain of the MOSFET is at or near zero volts. This greatly reduces the heat dissipated by the MOSFET and increases efficiency. A choke value for the drain is chosen so that it resonates at the operating frequency, in combination with the parasitic capacitance of the drain and the output filter. The "fly wheel" effect of the resonant tank causes the drain voltage to drop to zero before the MOSFET is switched back on. Efficiencies of 70% or more can be achieved this way.
The circuit show below is a simple Class E transmitter and is shown built for 40 meters. It uses a 74HC02 NOR gate as a crystal oscillator. The other three gates in the package are used to gate the clock signal to the output MOSFET. The crystal oscillator can also be used with a direct conversion receiver. An Rx offset curcuit is shown to shift the oscillator frequency when receiving.
A 2N7000 is used for the final power amp. Although only one is shown, three '7000's are used in parallel to reduce the "on" resistance, which inproves efficiency. 70-75% efficiency can be achived. The amplifier delivers about 2 watts output with a 9 volt supply and about 4 watts with a 12 volt supply. The plastic TO-92 2N7000's bearly get warm to the touch.
For best results the spacing on the output filter inductors with need to be "tweaked". Monitor the drain current and output power and calculate efficiency. Adjust the spacing of the windings on the output filter inductors until you get the best efficiency. L2 will have the greatist effect. L5 and C15 are used to suppress VHF spurs, which aren't effectively attenuated by the HF low pass filter.
If the square wave drive to the fet is simply gated on and off, the keying waveform is very steep and causes key clicks. Therefore, the supply voltage to the PA needs to be ramped up and down in order to provide for a least some wave shaping. Q1 is a PNP power transistor which is turned on and off by Q2. C19 slows the turn on and turn off time down and causes the voltage on the collector to rise and fall with about a 5 ms time constant. R5 and C14 form a delay to keep the drive active to the PA while the supply is ramping down.
Grenade Transmitter
The Parts List for the Shortwave Transmitter:
- R1 - 33K ohms, ¼ watt, carbon.
- R2 - 4.7K ohms
- R3, R6 - 100 ohms
- R4 - 22 ohms
- R5 - 100 ohms, 1 watt, or greater.
- C1 - 47 to 470 pF, or greater, silver mica or disc ceramic. Better yet, a trimmer capacitor, to slightly change frequency ±1 KHZ.
- C2 - 10 to 140 pF trimmer.
- C3 - 100 to 220 pF, silver mica or disc ceramic.
- C4 - .01 to .05 µF.
- C5 - 300 - 800 pF, or (? ? ?) variable capacitor. Use an Arco TC-4610 (260-900 pF) compression trimmer.
- C6 - .01 to .1 µF.
- C7 - 1000 µF or greater, electrolytic, 25 volts or greater.
- X1 - Fundamental mode crystal, 5 to 8 MHz.
- T1 - On an Amidon T50-2 toroid core, primary is 26 turns of #28 magnet wire closewound, secondary is 2 turns wound over the primary windings.
- T2 - On an Amidon T50-2 toroid core, bifilar wind 6 to 8 turns of #18 magnet wire. See note 1 for details.
- L1 - 2 mH or greater, .5 ohm or less resistance. Use the inductor from Radio Shack p/n 270-0030A HD Noise Filter. One could also use the 4 or 8 ohm secondary of a medium sized audio transformer, or the secondary side (6 to 24 volts) of an AC mains power transformer.
- L2 - .7 µH. Wind 7 turns of #16 magnet wire on a 5/8 inch diameter form, then remove form.
- L3 - 1.4 µH. Wind 11 turns of #16 magnet wire on a 5/8 inch diameter form, then remove form.
- Q1 - 2N2222
- Q2 - 2N2907
- Q3 - IRF510 or IRF511 (Radio Shack p/n 276-2072A)
... and this is another Audio section without compressor/limiter
A slight modification in the LF/RF section of the Grenade transmitter
How to winding and installing a Toroid Coil
The transmitter's low-pass filter uses a high-Q toroid inductor wound on a T37-2 form (T37 means the powdered-iron form is .37-inches in diameter). When winding the Toroid Coil, the numbers of turns are counted inside the form (not on the outside). That means, if the instructions call for a 12-turn coil, you must pass the wire through the centre of the core 12 times. When winding this coil, be sure to pull each turn up tight before starting the next. If the coil is wound loosely, its inductance increases - a condition that may reduce transmitter output power. Count turns on inside of form. Tin leads with solder before installing. Pull each turn tight before winding the next. Finally, before installing the Toroid Coil be sure to tin both leads with solder. The coil wire is coated with heat-strippable enamel insulation that breaks down at soldering-iron temperatures. If you touch the tip of an iron to the end of the wire for several seconds, the insulation should start to melt, allowing solder to adhere to the copper underneath. If your iron is not hot enough to start this process, carefully scrape the insulation off with a small hobby knife and tin. If necessary, refer back to these instructions at any time during assembly
The "Grenade" 10 watt AM transmitter for the 40 meter band
The Grenade Project RF-section
The "Grenade" is a 10 watt AM transmitter for the 40 meter band (6955 kHz).
It was designed, built and sold by "Radio Animal" of WKND.
The Grenade originally came built with one x-tal (6955/6950) and a dipole antenna at just under 100 dollars U.S. It is a great little radio, roughly the size of a brick, and weigh in at well under half a kilo. With a dipole cut to frequency you can cover the USA from the Rockies to the Atlantic in the evening under decent conditions. Output power roughly 15 watts when running from a 12 volt battery (That is constant carrier, it peaks in the 50 watt range) and will do close to 20 watts when powered from a regulated power supply. The limiter/compressor is really what makes the Grenade great. Makes it possible for someone with no knowledge of radio production to plug in a Walkman type tape deck and go on the air. The rig is cased is in a black steel box, with a finned aluminium heat sink covering the top. The transmitter can run continuously for 3 straight hours in hot temperatures (85+ degrees Fahrenheit) and the rig barely gets warm to the touch. Unlike ham transmitters, this one can run several hours per day with no problems and it is made so that it can handle running into mismatched or even no antenna.
Low-pass filters
"Station WRNY is equipped with the latest device known to the world of radio engineering, called a harmonic suppressor, the effect of which is the elimination of the signals of WRNY from every frequency except that to which it has been assigned by the Government, which means, in fan language, that the station may be tuned in on only one setting of the dial." Radio News magazine, August 1925
A low-pass filter attentuates the energy above a specified cut-off frequency. These filters are used to reduce the intensity of harmonics so that they don't interefere with other signals. Most of this filters on this page were designed for various ham radio gear operating in the bands from 1.8 to 14 MHz.
Unless otherwise noted these filters have 50 or 52 ohms input and output impedance. Capacitances are expressed in picofarads, inductances in microhenries. Most authors recommend using silver-mica capacitors.
for transmit frequencies
in the range of C1 C2 L1
1500-2000 kHz 1800 1800 30 turns #26 wire on T-50-2 toroid
3500-4500 kHz 680 680 21 turns #22 wire on T-50-2 toroid
5500-7300 kHz 470 470 14 turns #22 wire on T-50-2 toroid
5500-7300 kHz 820 820 2.2
approx.
cutoff freq. C1 C2 C3 L1, L2
1200 kHz .0039 uF .0056 uF .0039 uF 6.8
1800 kHz .0033 uF .0043 uF .0033 uF 5.6
2000 kHz 1592 pF 3184 pF 1592 pF 3.98
7300 kHz 470 pF 1000 pF 470 pF 42 turns
#26 wire
on T-50-2
cutoff frequency
(in MHz) 30 dB attenuation
point (in MHz) C1,/C4 C2,/C3 L1,/L3 L2
2.16 4.0 820 2200 4.44 5.61
4.12 7.3 470 1200 2.43 3.01
7.36 12.9 270 680 1.38 1.70
10.37 15.8 270 560 1.09 1.26
Based on an article by Ed Wetherhold published in UK Short Wave (Dec. 1983).
Friday, July 4, 2014
Simple AM Transmitter
There are not many AM transmitters that are easier to build than this one because the inductor is not tapped and has a single winding. There is no need to wind the inductor as it is a readily available RF choke (eg, Jaycar Cat LF-1536). To make the circuit as small as possible, the conventional tuning capacitor has been dispensed with and fixed 220pF capacitors used instead. To tune it to a particular frequency, reduce one or both of the 220pF capacitors to raise the frequency or add capacitance in parallel to lower the frequency. Q1 is biased with a 1MO resistor to give a high input impedance and this allows the use of a crystal ear piece as a low cost microphone.
Simple AM Transmitter Circuit Diagram
Thursday, July 3, 2014
AM Transmitter
There are two main approaches to generating AM:
1. Generate the AM signal at a low level (power/amplitude), and boost it to the required output power via linear RF amplification, or
2. Generate the AM at the final power level by directly modulating the gain/amplitude of the final (Class-C i.e. non-linear) RF power amplifier.
There are pros and cons of both approaches and which technique you choose will be influenced by the required power levels, frequency response, audio fidelity and the devices to hand. By way of illustration, here's a simple AM audio transmitter I cobbled together using an ancient 1 MHz TTL oscillator (which consumes the bulk of the operating current shown!) as the frequency determining element. (I've used the second approach to AM modulation in this circuit - also known as "plate modulation" from the old valve transmitter technique of modulating the anode [plate] voltage of the output amplifier).
AM Transmitter
There are two main approaches to generating AM:
1. Generate the AM signal at a low level (power/amplitude), and boost it to the required output power via linear RF amplification, or
2. Generate the AM at the final power level by directly modulating the gain/amplitude of the final (Class-C i.e. non-linear) RF power amplifier.
There are pros and cons of both approaches and which technique you choose will be influenced by the required power levels, frequency response, audio fidelity and the devices to hand. By way of illustration, here's a simple AM audio transmitter I cobbled together using an ancient 1 MHz TTL oscillator (which consumes the bulk of the operating current shown!) as the frequency determining element. (I've used the second approach to AM modulation in this circuit - also known as "plate modulation" from the old valve transmitter technique of modulating the anode [plate] voltage of the output amplifier).
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