How do I...?The December, 1998, meeting of the North Georgia QRP Club was dedicated to construction of a 1 watt 80 meter transmitter. I was only able to stay at the meeting for a short time, so I built mine at home, using "butt ugly" construction. Silly man that I am, I decided that "wouldn't it be nice if I had a simple receiver to go with this transmitter?" Discovering that Tech America stocked the SA612AN, which is the same device used in the Columbus QRP Club's MRX-40 (they used an NE612). The MRX-40 is a nifty little XTAL controlled receiver that was created as a companion to the Micronaut transmitter, featured by Dave Ingram, K4TWJ in a series of articles in 1997 in CQ Magazine. Just to show how limited my design skills were, I made the following post, one
Saturday night around 9:00PM (EST) to the QRP-L list:
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Inside view of NOGA-80 (first attempt at T/R switch is at lower center of PCB). |
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QRPers Never Sleep!By 9:00AM (EST), Sunday morning, I had 4 responses in my e-mail! To net out the responses, I learned that the way to scale a resonant circuit from one frequency to the next is to calculate the values of XL (inductive reactance) and and XC (capacitive reactance) in the resonant circuit. Where: Glen Leinweber, VE3DNL was the first to write, and really helped me understand this. With the above equations in mind, Glen said, |
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Let's Get That Sucker Tuned Up!After a couple of days fooling around with scaling the input tuned circuit, I made my first trip (of many!) across town to Tech America. Cool! The SA612AN goes for $2.52 (less than the SA602AN), so I got a bunch of them, plus some toroids, perfboards, trimmer capacitors, etc. For my first stab at the "MRX-80" on the breadboard, I chose a 220pF capacitor for C1 and a 910pF for C2. L1 became 10.4uH, which was worked out using the ARRL handbook formulas for ferrite toroids (I chose ferrite for its high permeability and given that heating/saturation should not be a problem--I sound smart, but these recommendations came out of W1FB's QRP Notebook). L1 is 23 turns of #30 enamel wire on an FT-37-63. The first tuned circuit for the NE612 Local Oscillator, used a 3.686MHz crystal, 39uH choke (from the junk box), and a 1N4001 diode (forward biased with +6V though a 10K variable resistor and a 100K resistor) all in series from pin 6 on the NE612 to ground I'm not showing the diagram for this circuit, because basically, I was not at all pleased with how well the crystal would warp. I tried several incantations and instantiations of the VXO, without results that in any way pleased me. It may be the Q of the circuit. I'm not sure. All I know was that the best I could get the crystal to warp was about 500Hz--not very good. I finally decided to use a BFO--more about this later. Anyway, I put the scaled MRX-40 circuit (with the VXO mod) together on the breadboard, with both the NE612 and LM386 (AF amplifier). I fired the whole thing up, and got a wonderful howling sound. Lesson learned: Always use the shortest leads possible, and bypass capacitors, especially on the power supply and output leads. I configured the LM386 with a 10uF capacitor across pins 1 and 8, which yields a gain of 200. This is sufficient to lift the signal coming out of the NE612 to a comfortable listening level, but with that much gain in a single stage, it's really easy to get feedback and self-oscillation. |
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RCI BCIOnce the sun was down, and the howling dispensed with, it was time to try out the receiver with a real antenna attached. I hooked it up (using clip wires) to my HF6V (80-10). My, how nice Radio Canada International sounded on 7.3MHz (in my 80M receiver!). I remembered that Sam Billingsley, AE4GX (who also helped me with the scaling exercise), had had severe problems with 39 meter BCI in his PIXIE kit, which he fixed with a specially designed filter. So, I grabbed my W1FB QRP Notebook and wired up a quick 3-pole low-pass RF filter for 80 meters (page 103), to see if that would help. Knocked the RCI BCI down quite a bit, but still a S9 or better signal. At this point, however, I started to hear 80 meter CW signals! Wow, I was impressed! This theory stuff really does work! So the RF 3-Pole Low-Pass Filter shown here became a permanent part of my new receiver design. |
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Hey, this is a pretty good receiver!At this point, I had a functioning receiver, and the 39 meter BCI from Radio Canada International was nearly all gone. I was lazily enjoying listening to the music of the Novice band CW (about all I'm really any good at copying!). At this point, I began moving the entire circuit off the breadboard and using a more permanent perfboard construction method (my preferred technique). In this photo, I've moved both audio stages off of the breadboard. The variable resistor on the perfboard is the AF Gain control. The two chips are the LM386 and a TL082 used as a low-pass filter, as described in the next section. For what it's worth, the purchase of a breadboard similar to this one is a must for anyone serious about construction. I can no longer imagine life without it. Also, heavy investment in solder wick is highly recommended! |
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Finishing TouchesI like rigs in interesting containers. I've had these coffee tins for quite some time and decided that this was the project for them. The audio/RF board fits quite nicely in the bottom of the the tin. The top of the BFO box, which is made out of dual-sided PC board, becomes the top of the rig. This inserts over the top of the perfboard and screws in snuggly. There is room next to the perfboard for the 9V battery, and a small 1/8" jack is used to allow for an external 12V source. The speaker fits in neatly to the side just over the headphone jack, and is held in place with small bolts (the nuts press against the backside of the speaker). |
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The NOGA TwinsAnd here are the NOGA Twins, the NOGA-80 transmitter (also called by some, the "Georgia Cracker") and the PGR-80 (Pretty Good Receiver). My version of the NOGA-80 has two crystals, 3.686MHz and 3.579MHz, switch, and the VXO tunes about 800-1000Hz around the center frequency. I experimented with a diode Transmit/Receive switch, but only managed to cause very severe chirp in the transmitter (I suspect I was seriously loading the final). So I settled on a manual T/R switch for this version of the project. Always room for improvement! |
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The Pretty Good Receiver (Summary)In summary, this project did not yield any new innovations in receiver design. Every technique that I used was borrowed, and I relied heavily on the advice of my Elmers. But it was a whole lot of fun, and I got a whole lot more than I had bargained for when I put out my first question on the "L". I learned that every design project is one of balancing conflicting objectives, such as price vs. performance, bandwidth vs. selectivity, etc. Everything is a compromise, but I found that a pretty good receiver can be built with a very low parts count (about 50). One of the coolest aspects of this project is that every part used can be purchased at Tech America. When I get a hankering to build something, I hate waiting for parts to arrive (I'm seriously into instant gratification). The parts list will be forthcoming. Thanks again to all my Elmers, particularly, Sam, AE4GX, and Wes, W7ZOI. 72! |