AM Radio Receiver Using the NE602 Balanced Mixer
Pictured above is a little AM superhetrodyne receiver that covers the broadcast band from 550 Khz to 1650 Khz. The circuit employs the 8 pin Signetics balanced mixer IC (NE602) which converts the incoming RF signal to the standard 455 Khz IF signal and provides about 13dB gain. The IF signal is amplified by a single transistor stage and audio is recovered using a biased shotkey diode (5082) and JFET buffer transistor. The LM386 audio amp is used to drive a small 2.5 inch speaker at about 200 milliwatts. The circuit contains four LC tuned circuits, all of which need to be fine tuned to obtain good results. An oscilloscope and RF signal generator are useful, but the circuit can also be setup using local radio stations and an additional portable AM radio. For test purposes, the circuit can be assembled on a solderless breadboard so changes can easily be made. Try to use short connections for the RF section with the antenna loop a couple inches away from the board. Assemble the NE602, ferrite antenna loopstick, tuning cap, oscillator coil and associated other parts on the breadboard. Next, connect the battery through a DMM and verify 3 or 4 milliamps of current flow. Next, adjust the tuning cap for maximum capacitance (550KHz) and connect a scope to pin 6 of the NE602 and verify a approx 1 volt p-p sine waveform. Adjust the red slug of the T1 oscillator coil for a frequency of about 1 MHz. Then adjust the capacitor to minimum capacitance and verify the frequency increases to around 2Mhz. To adjust the oscillator without a scope, use a portable AM radio to verify the oscillator is working and the frequency is right. To do this, place the portable radio close to the antenna coil and tune it to a AM station around 1000 on the band. Set the circuit tuning dial to the station frequency minus 455KHz. For example, if the portable radio is set to 1100, the circuit tuning dial should point to (1100-455 = 645KHz). This doesn't have to be exact, but set things as close as you can. Next, adjust the red oscillator coil slug until a beat frequency is heard on the portable radio. You should hear a squeal and then a null as the oscillator frequency gets near the station point on the band. This will verify the oscillator is running at about 455 Khz above the dial setting. If the oscillator doesn't run, the problem may be the connections to the (red) oscillator coil. The red oscillator coil will have 5 connections, 3 on one side for the tapped primary, and 2 on the other for the secondary. The circuit requires only the single, tapped primary side, but unfortunately the primary tap is too close to one end for the circuit to operate. To overcome this problem, the secondary is added in series with the primary which effectively moves the tap closer to the center and provides more feedback to sustain oscillations. The phasing of the secondary connections is also important. If the oscillator fails to work, try reversing the connections to the secondary of the coil. When the oscillator is setup, the remainder of the receiver can be assembled and calibrated. If you have a signal generator, set it for 600KHz, 30% audio modulation and 1 volt output. Connect the generator through a 1K resistor and a couple turns of wire around the antenna loopstick. Adjust the yellow and black coil slugs and also the position of the antenna coil on the loopstick for strongest response. Reduce the generator output as the signal improves so you can find a peak for all three coils. You can also make small adjustments to the oscillator coil to get the tuning dial to point directly at 600. This will calibrate the low end of the band. To calibrate the high end, set the generator for some frequency near the high end, maybe 1500 on the dial. Tune the receiver so it receives the signal and then adjust the two trimmer capacitors located on the back of the tuning capacitor for best response. You may find at this point the trimmer caps are all the way closed or open and a peak cannot be found. To fix this problem, note which way the caps are set and slightly adjust the main tuning dial to compensate. For example, if the caps are all the way closed, move the tuning dial to a lower setting (more capacitance) and then repeak the trimmers. Continue this until you can find a peak where the trimmers caps are not fully closed or open. At this point, repeat the adjustments at the low and high end until both are optimum. This same procedure can be done without the generator using a couple local radio stations at opposite ends of the band. The first thing to do is identify a station that can be heard somewhere on the band. Try to do this without adding an external antenna, but if no stations are heard, add a short 2 foot wire antenna to the antenna input at the gate of the JFET. Find the strongest station and slide the antenna coil along the loopstick for best response and also adjust the yellow and black IF coils. Here is a step by step procudure I found works reasonably well. My test station was located at 790 on the band. 1. If no stations are heard, connect a short 2 foot wire antenna to the junction of the antenna loopstick and gate of the JFET. 2. Adjust the tuning capacitor and slide the antenna coil for best response of some local station. 3. Adjust the yellow and black slugs of T2 and T3 for loudest response. 4. Reduce the length of the wire antenna and readjust the position of the antenna coil for best response. 5. Move the tuning dial slightly toward the correct point without losing the station. (i.e.) If the station is located at 700 and your tuning dial is pointing higher at 750, slightly move it down toward the correct 700 point (more capacitance). 6. Readjust T2,T3 and antenna coil for best response. 7. Repeat steps 4, 5 and 6 until the station is heard loud and clear and no further improvement can be made. 8. Remove the wire antenna and readjust the antenna coil, T2 and T3 for best response. Note the antenna coil should not end up at the center of the loopstick. This will indicate not enough inductance and a few more turns of wire may be needed on the antenna coil. The optimum position for the coil is near the center, slightly offset toward one end. If it ends up very near one end of the stick, you may want to remove a few turns which will allow the coil to be closer to the center. 9. At this point, several stations should be heard loud and clear but minor adjustments may be needed to optimize the entire band. Select a station near the bottom of the band (600KHz) and adjust the antenna coil and oscillator coil for best response. Note that only very small adjustments to the red oscillator coil may be needed. Then select a station near the top of the band (1500Khz) and adjust the 2 trimmer caps on the back of the main tuning capacitor for best response. Repeat this process until both ends are optimized. Be sure the 2 trimmer caps do not end up fully open or closed. If they do, note the position and slightly adjust the main capacitor to compensate. For example, if the trimmers are fully closed, adjust the main capacitor slightly lower (more capacitance) and then readjust the trimmers so the peak occurs somewhere between min and max.
AM Radio Receiver With Additional IF Stage
Pictured above is the same circuit with an additional IF stage added for greater sensitivity. Overall gain can be adjusted with the 1K resistors in the emitter leg of the 2N3904 transistors. The circuit board was assembled using multiturn 10K pots in place of the 1K resistors and then adjusted for best performance. The pots are the 2 little blue items just to the left of the tuning cap. I think I ended up with about 750 ohms. The emitter bypass caps are not needed since there is plenty of gain available without them. The caps (two yellow items near the pots) are still in the board but not connected. I didn't know if they were needed or not, so I put them in there anyway and later disconnected them. Removing the bypass caps also increases the input impedance so that both IF stages can use the black IF coils which have higher secondary impedances (and thus more voltage) than the yellow or white coils. You might be able to replace the yellow coil with a black one for greater signal transfer since the input to the first transistor is much higher without the bypass cap, but I didn't try it. You may notice one of the black coils is actually white in the picture but it was rewound for a higher secondary impedance. Actually, it was removed from a junk radio purchased for a dollar and didn't have any secondary, so I added a 27 turn secondary which is close to what the black coils use. Overall, the performance is very good except for the AGC circuit, which has limited range and may not be able to compensate for very strong stations which may overload the circuit. The AGC voltage is derived from the IF amplitude at the cathode of the detector diode (output of T4). As the IF amplitude increases, the DC voltage at the gate of the JFET will move negative, below ground. The audio signal is present on both the gate and source terminals of the JFET, but the audio DC offset voltage will change as the IF amplitude changes. This DC voltage (about 2 volts) is fed back through a 15K resistor and the two IF coil secondaries to control the transistor bias points. The audio signal is filtered out by the 47uF cap leaving a stable DC voltage at the base of the transistors. As the base voltage drops, the emitter voltages also drop resulting in less operating current and lower gain for two IF stages. But the range is limited to maybe only 6-12dB which isn't enough to compensate for very strong signals. One solution to the problem is a manual gain control consisting of a switch and a few turns of wire around the antenna coil which can be seen in the picture (3 turns of solid insulated white wire on left side of loopstick). Closing the switch loads the antenna coil and reduces the signal level. Notes: IF transformer data and Mouser part numbers can be found at: http://www.electronics-tutorials.com/filters/if-amplifier-transformers.htm There are a couple different versions of the yellow and black transformers. The total turns used and position of the tap varies with the version. I'm not sure which is better, or which ones I used since they were recovered from old radios. However, either version should be driven using the shortest section of the primary. This means connecting the transistor collector to either the tap or the end closest to the tap and the power connection to the other point. Leave the transformer end farthest from the tap unconnected. You can use a DMM to measure the resistance from the tap to each end of the primary to determine which end is closest to the tap. Resistance will probably be a couple ohms or less. A couple sources for ferrite rods for antenna loopsticks can be found at the links below but the longer rods are expensive ($25 for a 5.6 inch rod). The second source has 7.5 inch rods for $20. You may also find them on ebay. http://www.stormwise.com/page26.htm http://www.universal-radio.com/catalog/misc/amidon.html A source for a miniture variable tuning capacitor was found at "Ocean State Electronics" http://www.oselectronics.com/ose_p98.htm Near the bottom of the page is the listing: Miniature 2 Gang Poly-film Variable Tuning Capacitor For Broadcast Band " Tunes AM band from 540Khz to 1600Khz. Ideal variable tuning capacitor for miniature circuitry and use as exact-duplicate replacement in current transistor receivers. Rotates through a full 180 degrees Maximum capacity: Antenna section. 15-140PF, Oscillator section, 10-60PF. Trimmer capacity: variable to over 12PF. Trimmer adjustment on rear of case. Completely enclosed to clear polyethylene plastic case to protect plates. Includes calibrated dial, screw, and knob. Small size, 3/4" Square x 1/2" Deep. BC-540...........$3.95 " If you use this cap or similar from an old miniture radio, the antenna loopstick inductance will need to be about 600uH to tune 550KHz with the capacitance at maximum (140pF). This amounts to about 80 turns on a 4 inch, 3/8 diameter ferrite rod. Shorter rods will need more turns.
Loop Antenna for AM Radio
A loop antenna can greatly improve medium wave reception. Loop antennas are directional and receive signals along the plane of the windings. The directional quality improves signal to noise ratio of the desired signal while rejecting signals perpendular to the plane of the windings. Larger loops are better than smaller ones but good results can be obtained from moderate sizes of one or two feet on a side. The shape doesn't make much difference so the loop can be circular, rectangular or a triangle shape. The main idea is to cover as much area as possible, so I would imagine a circular loop would be the best. The loop pictured here measures 15 inches on a side and is about 1.5 inches wide. It was wound with 16 turns of #35 copper wire, and has a Q of about 100 at 600 KHz. Larger guage wire might have been better (less resistance) and therefore higher Q and selectivity, but the arrangment here works pretty well with a bandwidth of about 6Khz at 600Khz. The loop is tuned with a 30-365 pF capacitor and covers the standard broadcast band of 550-1700 KHz. The antenna signal is inductively coupled to the radio's internal ferrite rod antenna so no wire connections are needed. Simply place the radio near the loop antenna and adjust the position(s) for best results. You may have to adjust the tuning of both the radio and antenna several times for optimum results.
More detailed information on loop antennas can be found at: AM Loop Antennas A calculator for figuring the number of turns needed for various rectangular loops can be found here: Loop Loop Antenna Calculator - By Bruce Carter