|Updated April 9, 2014|
QRP-Tech Group Projects:
(4 digit # refers to QRP-Tech group message)(Click on a project in this list to go to my page(s) for that project.)(This is the page for Project-000.)
Project-000 = Simple EMRFD DC Receiver 4960
Test Equip used here: Tek DSO scope TDS2012C, Hatachi scope V-422 (very old but still works), HP Sig Generator 8640B, B&K Freq counter, B&K DVM, HP3400A RMS volt meter (good past 10MHz, 3 each HP lab power supplies, Simpson 260 VOM, AADE LC meter, Veleman DVM, Step Attenuator ARRL design.Building a couple of 50ohm loads, a return-loss bridge, a couple of AD8307 power meters, and a couple of scalar network analyzers.
W5DOR - Project 000
(Please be aware that it will take me twice as long because I'm not getting around very well in this wheelchair. . and, taking pain meds makes my mind kinda fuzzy.)
Another problem us old folks have is being able to see. I have several things I use. I have my normal pair of reading glasses (1.25 diopter), another pair (3.00 diopter)that I use for most construction work and for real closeup stuff a pair of 6.00 diopter reading glasses. The 6.00 diopter pair I could only find online. Then I have that little USB microscope that displays on the laptop but the very best is my stereo microscope. I always put finished pcb's under the microscope to check for solder shorts. I also use the microscope for soldering SMD parts. I will describe my SMD soldering proces elsewhere on this web site.
1st day - Gathering parts and making plans:
Since the schematic was relatively simple I didn't make a parts list as such but just printed out the schematic and checked off the parts as I was able to find them. There were a few variations but my goal was to get as close to the parts specified as possible. I added the 470 ohm resistor in the power supply lead of the NE602 and the rectifier diode in the positive supply lead to protect against accidentally hooking up the battery backwards. This is just a .jpg picture that you can just copy/paste if you need an updated schematic.
Perhaps the hardest to find would be the tuning capacitors. I have a large supply of air variables in my "junk box" so I was able to get close. The main tuning cap read 82.56pf on my AADE LC meter. BTW, that's a good bargain in an accurate LC meter for the lab. I had purchased a few years ago a cheap LC meter and it was very inaccurate on the low scales. I guess I shouldn't "bad mouth" them here but it was a very well known company producing inexpensive test equipment. For the 50pf tuning cap I was able to fine one that read 39.45pf. That's as close as I could get from my junk box. However, it will probably be a great "bandspread" capacitor because I was pleasantly surprised to find that it had a 4:1 gear drive. The other variable cap is the 180pf cap used for tuning the front end. I was able to find a 200pf that will probably work just fine.
The other front panel control is the 1K potentiometer (pot) at the antenna input. A 1K, 2W pot was found without any difficulty.
Here's the rest of the parts:
This is a standard 8-pin IC socket soldered onto a 8-pin IC pad. I made these IC pads myself but really nice looking one's can be purchased from QRPme.com. Go to http://www.qrpme.com/?p=product&id=MEP
I made my pads using ExpressPCB layout software (free) and their mini-board service which gets you 3 boards without silk-screen or solder mask for $51.00. I was able to fit 20 8Pin pads on each of 3 pcbs resulting in $0.85/pad and a total of 60 pads.. However, I really like the pads that you can purchase from QRPme.com. The QRPme.com pads just break apart so you don't have to figure a way to cut the pads apart when you make them yourself.
Well, that's my parts gathering exercise. I suspect the variable caps will be the hardest for folks to find at a reasonable price. So, when I get it finished I plan to make a little YouTube video showing how well it works. then I plan to remove the 2 air variable caps and replace them with a couple of caps from QRPme. I believe their polyvaricon 265pf variables to be the least expensive of all the variable caps I've looked at. It will be an exercise in re-calculating the values of the other caps and maybe even the toroid L2. If you're having difficulty finding the air variable caps or maybe you found some but they are terribly expensive then the little polyvaricons from QRPme might be the least expensive and easiest to obtain. Make sure you purchase the 2 x 265pf polyvaricon cap "KIT" so you'll have the shaft extension for mounting a knob. These variable caps are also smaller and easier to fit into a small enclosure.
Other modifications to consider after completing the standard unit - - -
2nd day - Sketching a layout of parts on pcb
Today I'm sketching a layout so the manhattan breadboard construction will have some sort of logical organization and work good. I'll use a 8 1/2 x 11 quadrille paper and a plastic template(s). I'll outline the actual pcb size and place the components so that I can be sure of having room for all the parts and they're well organized.
Now that the parts are placed I'll draw the layout with a pencel and plastic template.
Started to assemble the rcvr and realized the audio amp layout wasn't well thought out. Re-layed out the parts like this.
I've soldered it together but had to undergo major back surgery and have not had a chance to test it. (May 17)
May 29 - Plugged in headphones and powered up the audio amp and got the expected noise. I'll now hook up my signal generator for an actual test. . . .and, here goes - turning on the power and NOTHING. Even the noise I heard the first time I turned it on is gone. Did I zap the chip? Do I have a solder short?
Just had an idea. I have a 4SQRP Hi-Per-Mite filter that includes a LM386 audio amp. When I get the receiver working I might substitute the Hi-Per-Mite that will give me great selectivity as well as the audio amp. Oh well, Just a thought.
FIXED IT ! Was only that I hadn't plugged in the DC power plug all the way. Of course if I'd gone so far as to trouble shoot I would have noticed right away that the voltage to the amp wasn't present.
Now, to wire up the RF section. Oops, Haven't really tested the audio amp yet. I'll rig up a voltage divider to the output of my boombox to provide a low-level signal to the input of the LM386. I don't have a speaker L-pad so I'll just make a voltage divider out of some resistors.
What's an L-pad? http://en.wikipedia.org/wiki/L_pad
Mounted some parts for RF section:
OOPS again. Forgot the protection diode in the power supply lead. Have found a 1N5822 schottky rectifier - perfiect for this kind of thing. It'll handle the supply current and because it's a schottky it'll have a very low voltage drop. Notice in the pix above the red supply leads go diredtly to the DC power input jack. Below it goes thru the protection diode.
Next I'll finish adding the RF componenets.
Just a side note: I'm 72 years old and mostly blind in my left eye. . so, what do I use to help build circuits with small components? Several things. I normally use reading glasses of 1.50 diopter but for bench work I use 3.00 diopter or if I really need to see close up I use 6.00 diopter reading glasses (normally not found in stores but online). I also have a stereo microscope that I use to solder some SMD ICs but could do it with the 6.00 glasses as well. I purchased one of those microscopes that plug into your PC USB port but not that fond of it.
Winding toroids. OOPS! again! Need to start with enough wire. I started with a 10" and only got 18 turns so make sure you start with 12" of wire. These toroids are so small that I'm just gonna let them be mounted by the wire itself and not try to come up with a screw and nylon washer mounting scheme.
Finished soldering in all the components. BTW, I didn't have NPO caps of some values so I had to use a few silver mica caps in place of those ceramic NPOs.
Checking it out:
1. tracing circuit and marking it with yellow highlighter on schematic. Found one capacitor left off and two bad commections. One was a connection I forgot to make and the other a broken wire. Then there was one bad solder joint. All fixed now.
2.Hooking up the power supply. Wow! Actually hearing some signals as I tune and I haven't even attached an antenna yet. Now I'm really excited.
3. Checking some important points in the circuit to see if the DC voltage is what I expected. I'll add these voltages to a schematic later. I checked the power pins of the 2 ICs and they read about 4.5VDC. A little lower than expected.I touched pins 4 and 5 of the NE602 and could hear 60Hz hum - a good sign.
4. Checked pins 6 and 7 of NE602 and have an AC signal so I'm assuming it's correct. I didn't bother to take time to set the scope trigger so I could actually see the waveform. I'll do that later and add to this web page.
5. Now the big test. Hook up the antenna. Ok. Working but I'm out of the 40m band. I hooked up the high-impedance input of my freq counter and I'm just below the 40m band. I could only get up to 6.9xxx MHz. So I'll have to lower some capacitance in the osc circuit to raise the freq. BTW, I turned up the 1K RF gain pot and there's plenty of sensitivity or at least for such a simple rcvr.
The easiest way to increase the freq is to increase the freq of the LO so I just lifted the 33pf cap and I'm now receiving a CW signal at 7.050. I'm using my freq counter as a digital dial. A little later this evening I'll video the rcvr and plug in a powered speaker so we can all hear it well in the video. I plugged the antenna back into my FT-450D to see what 40m CW band sounded like this afternoon. It was quite dead so the fact that I could copy a CW signal on 7.050 was very good. Early evening should bring more 40m activity and I'll make a video with a powered speaker plugged into it so we can hear easily the 40m signals.
I'm now trying to decide what to do next. Here's some of my ideas:
1. Add a voltage regulator so I can hook it up directly to a 12VDC battery.
2. Build a more selective front end to attenuate any broadcast or other unwanted strong signals. Probably just a simple DTC (double-tuned circuit)
3. Disconnet the LM386 and replace with the Hi-Per-Mite kit from 4SQRP group. http://www.4sqrp.com/HiPerMite.php It has a 200Hz active filter that has NO RINGING and its own LM386 audio amp. Might also add a volume control.
4. Change out the LO circuit below the two 680pf caps. I might try a varactor tuned osc or a variable crystal osc(VXO) and then finally a DDS with display. If the DDS LO works well I might use this as the receiver section of a 40m portable QRP transceiver..
. . . and, here's a video of the DC rcvr working.