Direct Conversion Receiver using NE612 for HF Bands
When I first began to disturb
the ionosphere by generating radio waves, owning a integrated transreceiver was
still a dream.
For some time I was thinking on a
simple-to-build receiver to seriously launch my home-brewing career and give me
pleasure which can be had from building station equipment. The direct conversion
seemed ideal for such a project because it holds up the chance of acceptable
performance from simple circuitry. Further more, coupled with a simple amateur
band transmitter (QRP of course), to be alongside with the receiver one could be
in business at a modest budget.
However, adding a QRP transmitter to
an existing station receiver can be done with a minimal list of requirement. It
really is easy !
Most radio amateurs will be
familiar with the technique of radio reception by Direct Conversion. The signal
from the antenna are fed via input tuned circuits to a mixer or product
detector. Here the rf signals are tuned into audio signals, by mixing them
with a local oscillator. If that difference is within the audio frequency range,
say 50hz to 5kHz, it will appear as an audio signal out of the mixer.
If a received signal at 14.1000
Mhz is mixed with a local oscillator tuned to 14.0008 Mhz, an 800 Hz audio tone
appears. Incidentally, it will also be possible to produce a 800 Hz tone from
the same signal by tuning the local oscillator to 14.0092 Mhz (800 Hz away from
the signal on the other side). This is the principle of the product detector for
receiving c.w. and s.s.b. signals on the superhetrodyne receivers.
The NE612 is an integrated
circuit which contains a balanced mixer with its own on-board local oscillator
and voltage regulator. The mixer can provide up to 18dB of gain at 45 Mhz, and
the local oscillator will operate up to 200 Mhz.
It was my sincere attempt to
experiment and experience the Phillips NE612 , the chip of choice in the Amateur
Radio QRP (Low Power) circles
The receiver is roughly an idea when
NE612 , LM358 and LM386 are thrown in the air and fall down on a circuit board.
The circuit of the Direct Conversion (DC) receiver is as shown below
The signal from the antenna meet a
pair of two crossed diode, which is an easy way to protect the input circuit of
the receiver. They act as limiter, only allowing about 250mV (quarter of a volt)
of signal to reach the receiver input winding on L1. L1 and L2 with their
associated capacitors (C1, C2 & C3), form a band-pass filter for the
required band. The values are to be experimented to allow 'flat' tuning across
the required band, without the need for a variable capacitor in the input
circuit. Once set up by adjusting the cores of L1 and L2, the input circuit
requires no further adjustment.
The Oscillator section of the NE612 is
located at pin 6 and 7. Our receiver is varicap tuned (or varactor diode to
clever people) using tuning diode.
With the value shown the receiver tunes a little more than the 20M band.
Frequency variation is accomplished by varying the voltage across the varicap
diode.VR2 is the main tuning (MT) control, while VR3 is the fine tuning (FT)
control which varies the tuning voltage by a smaller amount. Preset Pot PR1 is
used to set the frequency range while R2 is used to make the tuning more linear
over its range. Try experimenting with the values of these resistors until you
get just the required band spread on the required band (In the prototype I have
omitted this resistor R2, C11 and the unmarked Capacitor).
The following LM358, dual
operational amplifier provides audio amplification and processing. The first
section is a high gain low pass amplifier, which also incorporates a clipper
with two diodes. This prevents strong signals from overloading the audio
amplifier. The second amplifier serves as a low pass filter with unity gain.
Finally LM386 IC provides more
audio gain into the loudspeaker
All stages except the audio
amplifier work on regulated 6 volts. Regulation is provided by a 7806 3 pin
regulator. As the audio amplifier works well with 12V DC do not drive the set at
more than 12V.
Though the board is compact, the
layout is not cramped and the project is suitable for a beginner to build.
Small, modern components are used to minimize board size. Use IC sockets for the
three IC's. Before you start, please try to understand the given PCB, the
Overlay pattern and the circuit and visualize the parts placement on the PCB
without which going ahead would be futile.
I have used the following Coil formers in my project. These formers offer excellent shielding and ease on winding. All Coil winding Data pertain to these type of formers.
Start out with, by building the
Oscillator first and test it. This is the only job which requires a little skill
and effort, in the whole project. For this you will have to
assemble / solder IC socket for the NE612 , C5, C6, C7, C9, L3 (wind coil
according to details given in the parts list), VCD, C10, R1, Cy (Solder this
component directly across the terminals on the Pot), PR1, VR2, VR3, REG1, C12,
and C26. Before applying power to the PCB check the PCB for any
visual shorts due to soldering. Apply (not more than) 12 VDC at the point close to C26
and check the voltage at pin 8 of the NE612. It should be 6V. After confirming
6V at this point you are now set to place NE612 in the socket. Remove the
applied voltage, plug in the IC and again apply the voltage. Check the IC and
the REG1 if they are getting heated up. If yes, then quickly turn off the supply
and check for possible shorts. Shorts could occur at Pots or shorted capacitor.
Turn the wiper of PR1 in the mid-way position and check the frequency of the
oscillator. To do this you may require a digital readout receiver or a frequency
counter or a ordinary SW Rx covering the 14Mhz. frequency. I use my ICF-2001D
receiver for convenient tuning. Keep the assembled unit close to a good Rx and
try to observe the beat. Move the core of T3 in/out so as to get the beat on the
Rx or check it out on a counter. You could also try the MT control and observe
the beat. You should get the beat. After this your job is just to calibrate the
swing of the Oscillator i.e. adjust PR1 and experiment with R1 / R2 till you get
just enough to cover the 20M band. After having adjusted the swing (Oscillator)
of the DC-Rx (remember to put a little wax on the core of T3 so that it does not
move or shake till you assemble the other components) you are now ready to
assemble the remaining components on the PCB. Now start out with assembling
L1/L2 and so on. Remember , to wind more number of turns first and over them the
link winding (2 turns) in the same groove. There is nothing that can go wrong in the Operational Amplifier
and the Audio Amp stages. The final part incorporates tuning of the front end
Band Pass Filter (BPF). For this, tune in a weak station and peak the coils L1 /
L2 till you get a flat response throughout the band.
Take care with the LM386, some may manage to destroy them with supply over-voltage.
To decrease the low pass frequency
Increase the value of C17, and vice versa. Changing the value of C14 will also help. Replace C16 with 0.1uf for CW use. To get better results, the values of R7,8 and C16,17 should be recalculated from an active filter design book and calculated values substituted
for CW use : R8 = 27K, C16 = 0.1, C17 = 1000pF
for SSB use : R8 = 27K, C16 = 0.04, C17 = 470pF
Points to remember
Happy listening on 14 Mhz. !
Parts List (for 14 Mhz Band)
|D1 - D4||1N4148||C15,C21||10uf|
|L1||2T Primary / 9T Secondary||C14||150pf|
|L2||12T Primary / 2T Secondary||C17||3n3|
|L3||10T (5T per section x 2)||VR1, VR2||10K|
|C2 - C3||56pf||VR3||470E (Vertical Mounting)|
|C4||2.7 pf||R2||Not required|
|C5, C16, C11,C27||103 (.01)||R9||1K|
|R5 , R6||10K||R10||22E|
|R7 , R8||15K||C23,C24,C28||100uf / 25V|
|R4||470K||C22, C10,C25||102 (.001)|
|C12,C18,C26,C26,C27,Cx - 104 (.1)|
|VCD - MV2109 ; I have used varicap found in TV Tuners , SMD type. They also work fine.|
All coils are wound
using enameled 41 SWG Cu wire.