Document text
MATH’S NOTES
BY IRWIN MATH, WA2NDM
WHAT’S NEW AND HOW TO USE IT
A Miniature HF to VHF AM/FM Receiver Using the NE605
circuit for a miniature receiver we get
lots of mail, most of which is comipli-
mentary and indicates that this type of pro-
ject is what “tickles a lot of fancies.” | am
not certain exactly why that is, but if that’s
what's necessary to spark homebrewing,
we will be glad to present such circuitry
whenever we become aware of suitable
offerings that may be applicable to ama-
teur radio. A resurrection of the home-
brewing portion of the hobby is, after all,
one of the objectives of this column. As a
result, this month we are pleased to pre-
sent another one!
The Signetics (now Phillips) NE605 is
alow- power mixer/IF amplifier integrated
circuit normally designed for single-con-
version FM receiver applications which
can easily be used as the heart of a sim-
ple receiver. The circuit to be described,
using this chip, will enable you to construct
a complete AM and FM receiver that can
be used for local monitoring or for casual
HF and VHF listening. With the addition
of a simple BFO oscillator, it can even
form the basis of a simple entry-level com-
munications receiver. Before proceeding,
however, be aware of the fact that this is
a medium-performance circuit, not the
end-all and be-all. Although operation
from HF to 500 MHz for the front end is
certainly possible, the noise figure is only
around 5 dB, so sub-microvolt signals
probably will not be received as well (if at
all) as with your HT or standard commu-
nications receiver, both of which usually
offer noise figures of less than a dB at
VHF. A low-noise tuned pre-amp ahead
of the NE605 will obviously increase per-
formance, but that will not be covered at
this time. In addition, the on-chip local
oscillator will only go to about 50 MHz
using the internal circuitry, limiting opera-
tion to 6 meters using the chip alone. An
external local oscillator will be required for
operation up to 500 MHz. This also will
not be covered at this time. However,
most sirnple pre-packed microprocessor-
oriented crystal oscillators (with a stage
or two of multiplication as required) will
work for higher frequency single-channel
applications. Nevertheless, keeping the
limitations mentioned in mind, a very sim-
ple receiver that may suit many needs will
be the result, so here goes!
Fig. 1 is a schematic of the simple, sin-
gle- conversion FM and AM receiver using
the NE605. Only iwo tuned circuits are
[: seems that every time we present a
c/o CQ magazine
Pre-set
10pF
*= .001uF ceramic
Levels
100K
100K
L1,C1 resonate at desired
operating frequency mi
FL, FL2 = 455KHz a
ceramic filters
+5.7V
L2,C2 resonate at Vec
455 KHz below desired
operating frequency
Fig. 1- Simple HF to VHF AM/FM receiver.
necessary: the input, which will provide
some degree of selectivity, and the local
oscillator, which should operate at 455
kHz below the desired input frequency.
Choice for the absolute values for these
circuits will be left to the experimenter and
probably will be dedicated by what is in
the junk box. The two items labeled FL1
and FL2 are common, low-cost 455 kHz
ceramic filters designed for AM radio use
and are readily available.
In operation, signals received by the
antenna are applied to the input of the
NE605. This input goes directly into an
internal mixer stage, which accounts for
the higher noise figure. Also applied to the
mixer is the local oscillator output, the fre-
quency of which is determined by the set-
ting of L2-C2. The difference between the
two is the IF frequency, 455 kHz, and the
two ceramic filters are used to provide a
narrow pass-band IF amplifier at that fre-
quency. You will note that the local oscil-
lator is tunable. If fixed frequency opera-
tion is desired, C2 and its parallel padding
capacitor can be replaced by a crysial of
the appropriate frequency. L2 and the
.001 uF capacitor, however, must be left
in. If you use the variable-oscillator ap-
proach, be sure that C2 and its padding
SL Se TRE a TP I A EN a OR TT cc
52 + CQ «+ August 1998
Say You Saw It In CQ
capacitor are chosen so that you have a
reasonable tuning range. Attempting to
make the tuning range too wide will result
in instability. Keeping the range to 1-1.5%
of a given frequency is a good rule of
thumb. Also be sure to use high- quality
capacitors and inductors for all circuits.
Their overall stability will be directly
reflected in the stability of the entire
receiver.
The 270 WH coil and 390 pF capacitor
connected to pins 10 and 11 form a quad-
rature detector that detects an incoming
FM signal and provides recovered audio
at pin 8. Pin 7 is usually used for a received
signal strength meter. As a result, its out-
put level is a voltage that is directly pro-
portional to the incoming signal amplitude.
Since AM is a variation in signal ampli-
tude, this outout makes a neat AM detec-
tor. Two pre-set potentiometers are
included at the AM and FM audio outputs
of the NE605. These are used to equalize
the outputs so that switching from one
mode to the other provides a roughly
equivalent signal. An LM386 audio ampli-
fier completes the circuit, providing ade-
quate drive for a small speaker or head-
phones. The entire receiver operates from
5 to 6 volts, which is derived from a 9 volt
transistor-radio battery and a low-power
three-terminal regulator. Current drain is
a function of the audio output level.
When building the circuit, take care to
use high-frequency layout techniques.
This means shortleads, keeping input and
output components away from each other,
and building everything over a good
ground plane. So-called “dead-bug or
ugly” construction methods are fine as
long as you are careful. Overall results are
dependent to a great degree on layou,t so
be careful.
The completed receiver can be mount-
ed in the enclosure (meta! preferred) of
your choice. Since it is a single-conver-
sion circuit, strong signals at 455 kHz
below as well as above the local oscilla-
tor frequency may leak through. This is the
price you have to pay for single conver-
sion. A more elaborate bandpass-type
input tuned circuit with higher Q that
“tracks” the local oscillator can reduce this
effect, but that’s something you will have
to “play” with. Regardless of what your
final design is, the NE605 is a chip worth
experimenting with.
As a final note, we have not purposely
described exact layouts or bills of mater-
ial for this project, as it is experimental.
The whole point of building such a circuit
is to “cut and try” until you acheive results
with which you are happy. If you need
more details, request the data sheet for
the NE605 from Phillips Signetics Com-
pany, 3811 East Arques Avenue, Sunny-
vale, CA 94088-3409. Good luck and let
me know of your results.
73, Irwin, WA2NDM
Say You Saw It In CQ
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August 1998 «