Documentation Available:

• Schematic and article are available from the EDN web site: EDN (Search for "Hageman")
• A PCB is available from FAR circuits: FAR Circuits

Designers Update: Since I first published the article I have built several of these receivers. I have found a few ways to improve the performance.

1) The EDN article left off a few things. The zener diode is a type 1N748A. The NE605 pins that are not connected were left off of the EDN article. The pinout was shown from pin 1 around the IC to pin 20 (just like looking down on the actual IC). EDN left off the last three pins on the bottom right of the 605 (Pins, 8, 9 and 10). Following around to the top right side of the 605 they left off pin 11. The rest of the pins are shown in the correct order.

2) Add a simple high-pass filter to the input. I found that at some locations I was getting severe FM and AM interference. The HPF prevents this. The filter also needs to be added to FAR's PCB.

' ........................22pF..........1nF

'Ant Input > -----||----+------||-------> To Pin 1 of the NE605

'..................................|
'..................................L =  120nH
' .................................|
' ..............................___
'.................................-     PCB Ground

These parts are available from Digi-Key (See my links page). Using surface mounted parts, these are easily mounted on the bottom side of the PCB from FAR.

3) With the input filter added, performance is improved by substituting the original designs 12 kHz IF filters with the same type Toko 6 or 8 kHz parts. This lowers the in-band noise level quite a bit. If Digi-Key is out of the exact filter you want, any of the Toko AM filters will work. (jump to Toko web site).

4) Fred's PCB has some grounding problems, leading to instability. To cure this add four, 0.1 uF capacitors from pins 2, 6, 13 and 17 to the ground trace under the 605 on the bottom of the PCB. Again 1206 size surface mount caps will work best.

5) Use a three terminal regulator instead of the zener diode originally specified for improved tuning stability. I found that with low impedance headphones (real communications types, not the walkman variety) some tuning instability was found when crossing a strong station with a weak battery. This is because the strong station pulls more current from the battery. A weak battery has more resistance and it's voltage would drop causing the VCO voltage to drop (hence changing it's frequency somewhat). The three terminal regulator may be added in place of the zener on the top side of the board by placing the "input" pin where the zener cathode was, then placing the regulators "output" pin where the zeners anode was. The "ground" pin from the regulator may be soldered to the negative end of C9 on the top side of the board. C9 is the 2.2 uF tantalum. I used a TO-92, LM78L05A type regulator.

6) Be sure to build the receiver in a shielded box, otherwise you will get too much "Hand" effect on the tuning. A shielded box also helps broadcast band and FM rejection.

7) Last time I checked, Radio Shack had NE605's (or SA605 / SA615's as they are known now).