Direct Conversion Receiver using NE612 for HF Bands

Introduction

 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 !

 Direct Conversion

 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 Circuit

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, C27, C28, 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.

Modifications

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

1. All resistors are mounted vertically.
2. Solder the components with shortest possible leads on the PCB.
3. Use shielded wire for Speaker connection and two core shielded wire for all Pots.
4. Use a thin 50 ohms cable from the PCB to the Antenna socket.
5. Final assembly must be in a metal box, this offers proper shielding.
6. Remember to ground (solder) the case of all the coils. Check to see that the ferrite cap are installed before you solder the case to ground.

Happy listening on 14 Mhz. !

Parts List (for 14 Mhz Band)