Sunday, February 17, 2013

Building The 40M Direct Conversion Rcvr







I've decided to start the build on Steve's (formerly N5JBB) direct conversion receiver.  My goal is to build a replica to the original in his article.  (The first post of this blog)  The procuring of parts has been a very time consuming task.  I managed to find everything through ebay and various other sources, including the project box and vernier dial and so it will at least look the same as the original.  As of now I'm waiting for all the parts to arrive so that I can start on the printed circuit board layout and component location.

Here is the original description of the Receiver that is more legible than the photo copied one posted before on Box.net:


An Experimental Receiver Using the NE602 Oscillator Mixer

Steven Morris, N5JBB
4913 Rustic Rd
Sand Springs, OK 74102
719-245-0582

          The NE602 is an 8 pin oscillator/mixer chip made by Signetics which was meant for use in battery powered receiver circuits.  I was intrigued by the possibility of a very simple direct conversion portable receiver based on this chip and was able to get a fairly good 40 meter receiver onto only 3 square inches of board space.

          The NE602 contains an active mixer circuit optimized for best performance at 6 volts.  Biasing of the mixer is taken care of on the chip, making it simple to use.  An oscillator/buffer stage is also provided on the chip.  The base and emitter of the oscillator transistor are brought out on pins 6 & 7 respectively, where the frequency determining components can be attached.  The signal input is into pins 1 & 2 while the outputs are from pins 4 & 5.  Fig. 1 shows a block diagram of the NE602.

          This chip has several advantages over some of the active and passive mixers currently in use.  Since this chip provides about 20 dB of signal gain from the input to the output, there is not as much gain needed in the audio strip as that needed for a receiver using a passive mixer.  Its small size and on-chip oscillator and biasing eliminate much board space and many of the components that would be needed if another active mixer such as the LM1496 were used.  The chip has very good sensitivity but mediocre dynamic range, potentially a problem when working on the 40 meter band.  Nevertheless a good receiver can be built based on this chip, giving a very small, low power set good for portable operation.

           Looking at Fig. 2, the receiver is split into two sections, the oscillator/mixer and the audio strip.  In the first section T1 is used to match the 50 Ω antenna impedance to the 3 kΩ input impedance of the mixer.  It is tuned to reject out of band signals.  R1 is used to drop the 9 V supply down to 6 V.  The oscillator circuit is of the Colpitts type with C2 used as the bandset and C1 used for main tuning.  Output from the circuit is single ended, being taken off pin 4.  C8 R2 and C16 are used to filter RF before the audio signal is sent to preamp Q1.

          The audio strip contains a 2ND5210 preamp and an LM386 power amplifier.  The preamp is designed to have a gain of 10 although more gain can be had by bypassing R6 with a 10 µf capacitor.  R7 is the receiver volume control.  The LM386 is designed to have a gain of 20 but that can be increased to 200 by adding a 10 µF capacitor between pins 1 & 8.  The LM386 tended to oscillate when using 4 Ω headphones so I rewired the headphones for 16 Ω operation which cured the problem.

          I laid out a circuit board just to see how small I could make it.  The circuit board is 1.5 X 2 in., occupying 3 sq. in. of space.  The layout is shown in Fig. 3 and the component location is shown in Fig. 4.  The completed receiver was built in a metal cabinet purchased from Radio Shack and is shown in Fig. 5.  The cabinet was modified to make the cover more easily removable when replacing the battery.  This was done by using #6-32 knurled thumbscrews instead of the sheet metal screws supplied and cutting slots in the cover so it can slide under the thumbscrews.  Fig. 6 shows the internal layout of the box.

          I believe this receiver would make a good portable set because of its small size and low power drain.  There is much room for experimentation too.  The antenna circuit could be reqorked to eliminate the input attenuator and raise the Q of the input tank.  The oscillator circuite could be reworked to obtain better frequency stability.  I think that some improvement in dynamic range could be gotten by using an audio transformer across pins 4 & 5 of the NE602 for balanced output from the chip.  Some experimentation with gain in the audio strip is called for.  The headphones I use are very sensitive, other headphones might require more gain.  By increasing the gain in the LM386 enough ti might be possible to eliminate the preamp.

          You may have to go through a Signetics distributor to get the NE602.  I have not seen them for sale in any of the common mail order outfits.  I bought mine from Schweber Electronics, a nationwide distributor which handles Signetics.\

References:

Signetics 1987 Linear Data Manual, Vol. 1:  Communication, pp 4-60 to 4-74

Signetics application note AN198, “Designing With the SA/NE602”

Signetics application note AN1982, “Applying the Oscillator of the NE602 to Low Power Mixer Pllications:



Here is the parts list:


PARTS LIST


J1                BNC Connector
J2                1/8 in. phone jack
T1                T50-2 toroid
 L1               T68-6 toriod
C1                15 pF variable cap
C2, C6            75 pF trimmer
C3                 100 pF
C4, C5           680 pF
C7                .1 µF
C8, C9, C10     .047 µF
C11               .068 µF
C12               .01 µF
C13, C14        10 µF
C15               .05 µF
C16               .22 µF
C17               1000 µF
R1                120 Ω
R2                1.5 kΩ
R3                100 kΩ
R4                2.2 kΩ
R5                12 kΩ
R6                220 Ω
R7                10 kΩ Pot
R8                10 Ω
U1                NE602
U2                LM386
SW1             SPST Miniature Switch
Vernier dial
Head phones
Circuit board
Stand offs
Machine screws
Project box
9V Battery
Battery connector
Enamel wire
Hook up wire
Thumb screws
Labels
Foot pads

I'm working on the schematic.  The original can be viewed from the link below:


15 Mar 13:  Today I'm starting the Regen project.  I've gathered all the major components and onto the proto board they will go.






















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