Thursday, November 29, 2007
Other possible hookups
Micro Power AM Broadcast Transmitter
R = 80*[(pi)^2]*[(Length/wavelength)^2]*(a factor depending on the form of the current distribution) The factor depending on the current distribution turns out to be [(average current along the rod)/(feed current)]^2 for short rods, which is 1/4 for a linearly-tapered current distribution falling to zero at the ends. Even if the rods are capped with plates, this factor cannot be larger than 1. Substituting values for a 9.8 foot dipole at a frequency of 1.6 MHz we get R= 790*.000354*.25 = .07 Ohms. And the resistance will be only half as much for a monopole or 0.035 Ohms. Radiated power at 20 milliamps works out to about I^2 * R = 14 microwatts.
Wednesday, November 28, 2007
Parallel Port Relay Interface
Low Voltage, High Current Time Delay Circuit
Inductance = (radius^2 * turns^2) / ((9*radius)+(10*length))
R1,R3 = 10K Q1 = 2N3906, or equivalent
R2 = 680K (see text) IC1 = 4001, or equivalent
R4,R5 = 6K8 D1,D2,D3 = 1N4001, or equivalent
C1 = see text Ry = Relay, 12V
C2 = 0.1µF, ceramic
Tuesday, November 27, 2007
The Passive Aircraft Receiver is basically an amplified "crystal radio" designed to receive nearby AM aircraft transmissions. The "passive" design uses no oscillators or other RF circuitry capable of interfering with aircraft communications so it should be fine inside the cabin of the aircraft. Nevertheless, check the regulations before using this receiver on a commercial airliner. New security regulations probably prohibit this device on commercial flights. Do not expect to hear two-way aircraft transmissions with this receiver! It is a short-range receiver only.
The detector diode is a 1N5711, HP2835 or similar Schottky detector diode. The 10 megohm resistors provide a small diode bias current for better detector efficiency. The tuning capacitor may be any small variable with a range from about 5 pF to about 15 or 20 pF. The 0.15 uH inductor may be a molded choke or a few turns wound with a small diameter. Experiment with the coil to get the desired tuning range. The aircraft frequencies are directly above the FM band so a proper inductor will tune FM stations with the capacitor set near maximum capacity. (The FM stations will sound distorted since they are being slope detected.) Other capacitor and inductor combinations may be selected to tune other bands if desired. (Try the CB band at 27 MHz.) The LM358 dual op-amp draws under 1 ma so the battery life is quite long. A speaker amplifier may be added to drive a speaker or low-z earphone. The antenna can be a couple of inches if the receiver is near the transmitter or a couple of feet for maximum range. The selectivity is reduce as the antenna length is increased so best performance is achieved with the shortest acceptable antenna. Try increasing the 1.8 pF capacitor value when using very short antennas and decreasing it for long antennas. The receiver could be built into a small plastic box with a short antenna inside.
40 m Band Direct Conversion Receiver
Building a practical and usable direct conversion receiver for the 40 m CW band is not as simple as it might appear. Broadcast station signals from the adjacent 41 m band, will easily overload most direct conversion mixer designs with their unwanted (and quite nearby) S9 +40 dB signals. My solution incorporates a diode-ring mixer and a narrow band rf input filter. These choices result in observed overall receiver dynamic range than that experienced when using an IC mixer module (such as a NE612). Comfortable and undisturbed operation in the evening was possible when tested using my Windom antenna. Some of this project's objectives included:
- High dynamic range diode-ring mixer
- Stable oscillator with a 30 kHz tuning range
- Tuning range from 7005 kHz to 7035 kHz
- Narrow front end band pass input filter
- Broadband 50 Ohm termination
- Audio selectivity used in the AF amplifier
- Symmetrical (differential coupling) design
- Low battery / power supply current consumption
- 60 Ohm headphone output impedance
You may recognize many of the stage circuits - as they are similar to those used in some of my other projects. The VFO and the RF input band pass filter are each designed around ceramic resonators. These are becoming difficult to locate. [See the note below.] I have used a HPF505 type ring diode mixer - but other mixers such as the IE500 or SRA1 should be suitable. The diode ring mixer output drives a parallel arrangement consisting of R11 and the differential input impedances of IC1. This results in a more stable 50-Ohm broadband termination for the mixer output (by contrast to that possible on a typical RCL based diplexer).
The gain of the broadband amplifier following the mixer, IC1, is set by the choice of R10. A 40 dB gain is achieved by using 100-Ohm resistor at R10. Any IC1 rf output signals are shunted to ground - leaving only an audio signal to be applied to the following stage. IC2 is an operational amplifier that is used to amplify the remaining audio signals by yet an additional 46 dB. Passive audio filter components are used at the input of IC2 so that only signals at or near 750 Hz are amplified. The overall gain and output level is almost too great for comfortable headphone listening. An rf attenuator (P1 located near the antenna input terminal) is used as a means of controlling the receiver output volume
|P1||1 kOhm, linear|
|C1||20 ... 325 pF, variable capacitor|
|C10||2,2 uF, no electrolytice cap|
|C13||100 uF, 25 V electrolytic cap|
|L1,2||FT37-43, 5 turns|
tap at the second turn
|Q1,2||SFE 7.02 M2C, Murata|
|IC2||NE5532 DIP, dual opamp|
|M||Mixer HPF505, IE500, SRA1 e.g.|
|KH||Headphones Ri > 60 Ohm ( 32 + 32 Ohm)|
Description: 33 Volt DC To DC Converter
A lot can be learned when using strict design criteria to build a project. I set out to build an entire receiver using only 2N3904 transistors and at the end settled upon the design shown above. This design resembles that of the Ugly Direct receiver on this web site in a lot of ways and is also a low-cost popcorn project. A great deal of time was spent building and testing various VFO designs and investigating an interesting single-balanced mixer using two 2N3904 BJT's. The design process and reasons for abandoning my original criteria in the case of the mixer and VFO will be discussed.
If you do not have access to test equipment, tune the resonators at the center frequency while listening to the receiver in the headphones to obtain the greatest possible band noise. Confirm your adjustments by tweaking the trim caps while listening to a QSO as well.
The diode ring mixer ultimately used has 50 ohm ports and can be a homebrew or commercial unit such as the popular SBL-1 from MiniCircuits.
An (LC) VFO for 30 Meters
This design was by far the most stable design for both short and long term drift and is the most stable VFO that I have ever built.
The wound inductor should be cemented face down onto the PC board after removing a small portion of copper big enough to fit the inductor on so that it is not touching any of the PCB copper surface. I used a hobby tool and sanded off the copper in a circular shape about 3/4 inch in diameter. The inductor was glued on with epoxy. The Qu of these home spun audio inductors is very low and consequently have very low loss. The 0.56uF cap I used was a miniaturized metallized polyester film (DigiKey EF2564-ND) which is an expensive part at 95 cents Canadian currency.
AF Preamp Chain
Following the diplexer is the familiar grounded base amplifier popularized by Roy Lewellyn, W7EL. This stage presents a low noise, wideband ~50 ohm input impedance to the diode ring detector and diplexer. An active decoupler is used to help prevent any hum getting into this stage. The 22uF capacitor in the decoupler circuit is capacitively multiplied by the beta of Q1 and has an effective filtering value of 22000 uF. The second stage is an amp designed by Wes Hayward, W7ZOI. The DC negative feedback provides bias stabalization for this stage. It is interesting to note that W7ZOI made a break in the DC feedback loop with a 10uF cap to ground so that there is no negative AC feedback around the amplifier and it operates at maximum gain.
The source follower and two low pass stages were pulled from Solid State Design for The Radio Amateur published by the American Radio Relay League. The original article had the a ~1KHz cutoff frequency using 3K3 ohm resistors. The above schematic uses two 3K9 ohm resistors in each low pass stage for a cutoff frequency of 870 Hz. Other cutoff frequencies can be set by adjusting these resistor values as desired. The lowpass filter stages serve to improve QRM copy ability and attenuate a lot of the wideband noise generated and/or boosted in the preceeding stages.
AF Amp and Driver
Driving the final amp is a high gain common-emitter amp with its output connected to a 10K pot for volume control. The 0.0022 uF bypass cap is used as a highpass filter to help remove hiss.The final AF amp is a simple common-collector amp set for approximately 37 mA of emitter current. The 180 ohm resistor could be dropped to 150 ohm (~45 mA Ie) providing a heat sink is used on the BJT. A piece of PC board glued to the flat part of the transistor could be used to fashion a heat sink if you decide to stand more current than the original design.The 10 ohm resistor and the 22uF capacitor on the collector of Q8 form an RC filter to decouple the AF stage from the positive voltage supply. I have found this amp sufficient to drive a pair of Walkman style headphones with reasonable volume. Do not expect ear-shattering volumes levels however. Three sets of cheap headphones were tried and one pair gave very low volume when compared to the other sets. Keep this in mind if your not getting reasonable volume to your ears. The headphone jack used for this rig is a 1/8 inch (3.5 mm) stereo jack with both channels connected together for monoaural output.
Like all electronic projects, this receiver should be built and tested one section at a time. Ugly construction easily allows this to be done. I started with the final amp and then worked backwards through the schematic until the antenna input was reached. Build the 2 low pass filters and the source follower as one section as the source follower is needed to bias the lowpass filter stages. The AF amp stages can be tested with a home brew AF oscillator such as a free-running multi vibrator.