Simple
Aircraft Band Radio
By: Steve Hageman
This simple, single IC Aircraft band radio is just the thing for easy
building. The design is based on the Philips, NE605 down converter /
IF chip and followed by a simple JFET audio amplifier.
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).
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Modified -
7Jan02