I am, I suspect, the KING of unfinished homebrew projects. My latest efforts have been focused on the tx/rx IF unit, and I've built a couple of units in that direction. The first one, I didn't like, and the second one hasn't seen the soldering iron for a few years.
But the March 2006 QST article on the HBR2000 home brew HF transceiver got me fired up to resume construction on my own homebrew SSB transceiver. The upside of the lull in construction is that DDS has exploded onto the scene since then. At the time I started, DDS was so hard to come by that I was thinking of ordering it as a 'part' for a Yeasu or Kenwood.
What I have actually built so far is a 9mz tx/rx IF. The top board is the tx modulator and the bottom board is the ssb demod/AGC/audio amplifier. The board on the right is the beginning of the filter unit/first tx/rx IF, and whatever else will fit on the board (vfo mixer? First conversion mixer?). As shown in the photo, it's all wired up in test mode.
For now, things are going pretty slow as I am still getting around on crutches, and that makes it nearly impossible to go dig for stuff in the radio room. Two more weeks (first week of April) and the Doc will set me free.
03/19/06 45Mhz I.F./Up/Down Converter
The big news is I found a FREE schematic capture program, and it works great! I did this diagram using it, and the first time out, it takes a while as you get used to the way it works.
I did a basic first draft of the 45Mhz I.F./Mixer, and it's pretty daring. I won't know if it actually works until I try it on the breadboard and do some measurements.
Some reservations I have about it are that the loads from the switching circuits might be a bit heavy for the application. It may be heavy enough to wipe out my gain factor; but on the other hand, the heavy loads will keep the circuit stable against input/output coupling when it is switched between tx and rx. I expect that I will have to tune the switching bias resistors, coupling capacitors and the input/output impedances. The other thing to worry about is that 36Mhz is a multiple of my last I.F. and I'm hoping that shielding and bypassing will be the answer. The crystal was 'off the shelf' and cheap, so it's worth a try. Hey; what do you expect, it's just a concept drawing for now.
Update: the 1N4148 diodes don't give enough isolation. I'll have to redesign this with PIN diodes (I have no idea where to find some of those).
Jan 20, 2009 - Ok, I've actually been working on my ssb transceiver. I have a new digital oscilloscope, and I completed the DVFO-II; mounted in a box and calibrated it with the Instek 'scope (which has a high-resolution freqency counter in it).
The doggone receiver IF wasn't working that great, and I was wondering why. Took a good look at the NE602 wiring, and discovered that I had accidentally wired the 9mhz carrier into pin 7 instead of pin 6! Well, doggone it, I built that dumb thing so 'dense' that it was real hard to get in there and swap pin 6 and pin 7. But I got it done, and now, I have so much AF into the LM380 audio amp that it overloads. No bigee, I'll add a little resistance at the top of the volume pot.
Today I built the input filter for 80 meters (with the help of the free filter software at AADE) and the rx rf amp. Built it with some 'mystery' toriods I had around; wrapped them with enameled wire (whatever I had around), and measured them with the LCMeter, and removed windings (I purposely over-wound them) to come up with the required uH value. I then took the scope and the DVFO-II and wound it through the frequencies I expected to see at the output of the filter, and it was so close to PERFECT, that I couldn't beleive it!
Above you can see all the components laid out on the bench, all aligator clipped together for a test! On the left is the DDS/Attenenuator. The upside-down pc-board is the input/output filter and rf amp (shown on the right). The proto-board in the center is the rx mixer, with a 5.5 to 5.0mhz vfo above it (rescued from some other old radio and modified to the freqs I needed), with an FCC-1 indicating frequency. The two assembled boards are the SSB gen/tx IF (lower board) and the Rx IF/AF board above it. Pretty standard 9mh IF/VFO design, but I figure, that first I'll make it work, THEN get fancy with the DDS and all. Looks like a huge mess all laid out like this, but it all works.
BTW, I had a HP 355D attenuator (had it around for years, and knew I was saving it for something) with 10db steps that I added to the DVFO_II, and with some calibration with help of the 'scope, I know the millivolts for each step of the attenuator. That'll be handy as I tweek sensitivity and AGC.
Jan 23, 2009 - I've been working on the diode switching for TX/RX switching, and have a pretty good design for the 9mhz crystal filter switching. Trouble is, it's designed with PIN diodes. Hmmm... don't seem to have any of those.
So I decided to test RF switching with some more standard diodes and see what works and what doesn't. My signal source was the DVFO-II with 0db attenuation set, which gives me about 400mv at 10mhz. I tried various diodes from the junk bin, and they work, but not that well.
Well, just for laughs, I thought I might as well try some of the green LED's I had laying around! To my surprise, they actually worked fairly well! Only thing was, with 400mv into the input port, I still had about 50mv on the unselected output. To fix this, I added an LED to the output of each port that would be forward biased to ground when the other output is selected. To my further surprise, this worked quite well, and now the selected port has about 350mv output, while the unselected port has virtually none (can't seem to measure that low). But that's just some stupid LED's!
I was also surprised that I was not getting any clipping from the LED that is on the output; but it seems that a forward biased LED will have a DC offset of about 1.8 volts. So as long as my signal stays below .6v p-p, that isn't a problem. If the signal is above .6v p-p, then I might have to use two or more LED's in series.
What's really funny is that it's easy to see which diodes are forward biased, since the LED is ON if it's forward biased.
But of course, the dumb thing is rediculously complex, but the results are truely interesting. Perhaps, with some simplification, it would be usable. Might be interesting to try some other LED's too.
Here's a good article about Diode Switched Band-pass Filters by Doug DeMaw, W1FB. The problem with his design is that it's hard to come up with some chokes with that high a value, and I think I came up with a better way to do that with generic diodes and just resistors. I would like to exchange emails with him, but I think I saw something that he is now a Silent Key. Darn! The guy was a GENIUS.
|Out with the OLD
|I finally have a DDS VFO assembled for use as a signal generator on the workbench. I haven't mounted it in a chassis box yet as it's still a bit hard to do some basic navigation while I'm still on crutches. I expect this DDS VFO to take the place of the old military surplus URM-25D that I've been using for years. Anyone out there remember the URM-25D? I used one of these regularly over 30 years ago while I was in the US Navy. Would you believe mine still works? Of course, these days, you really need the digital accuracy and DDS stability. The only drawback with this DDS is that it doesn't recall the last frequency you dialed into it. It always defaults to 10mhz.||In with the NEW
Progress is still pretty unimpressive so far. But to get back into it, I did some surfing around the Internet to see what others are doing and what stuff is available.
Here's some of the stuff I found on the Internet:
|HBR2000 home brew HF transceiver by
This site provides a detailed description of the 9 band HF High Performance Transceiver built by VE7CA. Since low oscillator phase noise is one of the pre-requisites to obtaining a high overall receiver dynamic range, he decided to use an analog VFO and mix the output of the VFO to the required injection frequency with separate crystal oscillators for each of the 9 lowest amateur radio bands. The VFO tuning range is 1 MHz. he can thus tune from the low end of 20 meters up to WWV at 15MHz. As well, a 1 MHz tuning range allows him to cover the lowest 1 MHz of ten meters without having to build another crystal oscillator.
Transceiver by I6WJB & I6YPK
I like the design, but couldn't seem to find any schematics. But it does have block diagrams and design info aplenty.
His site has lots of good schematics and photos, as well as tips on construction techniques.
Primary frequency control is provided by a PLL (MC145163) under processor control. Lots of good photos and schematics.
|Homebrew General Coverage Receiver
This is a general coverage receiver
capable of tuning from below 300 kilohertz
(kHz) to 30.259 Megahertz (MHz). It will
receive amplitude modulated (AM) and single
side band (SSB) transmissions. Continuous wave (CW)
transmissions may also be received in SSB mode.
Here's a cute little SSB single bander built by WB4QXE. The one shown is a 75 meter rig, but he's also done a 20 meter rig with the same basic design. Schematics, as he states, are somewhat incomplete, but I see lots of good valuable stuff here. The SSB is just the tip of the iceberg here, as he also has done some linears, transverters, and ATV as well. He also has bandpass calculator software (downloadable), a handy tool for the rf homebrewer.
|EI9GQ multi band SSB transceiver
Like mine, is still under construction. This site will describe the construction of a multi band SSB transceiver. Because he does 99% of his operating on his four favorite bands, he decided to build a four band rig with provision for adding a fifth band later. The design can be used on any band from 160M to 10M. This one covers 80M, 40M, 17M, 10M.
Shown here is the frequency display and vfo stabilizer unit.
|Some Interesting Sub-Assemblies to consider|
A 0-60 MHz coverage DDS VFO with built-in amplifier and variable output level from 0 to 4V p-p, manually adjusted with a trimpot or software controlled with a digipot. Once your controller-of-choice serially loads the 40-bit control word into the DDS, the raw waveform is presented to an elliptic filter that removes unwanted high-end frequency components, resulting in a signal of sufficient quality to serve as a local oscillator for a transceiver. This kit comes without the AD9851, but a free sample is available from www.analog.com.
The NorCal FCC-1 is a highly flexible frequency counter that incorporates features not found in any comparable unit. Its small size allows it to be incorporated into your favorite rig and your battery will hardly notice its modest current requirement. A high impedance input buffer provides high gain and isolates the counter to minimize loading of the signal source. The 4-bit band select input enables the counter to directly display your operating band and supports programmable parameters on a per-band basis. These parameters are stored in nonvolatile EEPROM. The 16 character by 2 line display is a high contrast, super twisted nematic LCD that's easily viewable over a wide angle in modest lighting conditions. Three menu-driven pushbuttons are provided to support user programming. Operation is as simple as connecting power and a signal source.
The FCC-2 is the second part of a DDS VFO, frequency counter and keyer project being designed by Bob Okas W3CD and kitted by the Northern California QRP Club. I had thought of building my own controller, but the compilers for the pics are typically $100 or more; so if I can get one 'off the shelf' at a reasonable price, I'll go for that instead. This unit is currently in the BETA testing stage and is expected to be available in the later part of March 2006.
MODE1 HF FAST: max input frequency
40MHz, resolution 10Hz., gate time .1sec
IF offset is programmable from +/- 0 to 2GHz in 1KHz steps.
|DDS VFO with
PIC16F84 and AD9850
Here's a wonderfully-simple project first done by Curtis Preuss, WB2V in the July '97 issue of QEX. The article was entitled "Building a Direct Digital Synthesis VFO" and was geared as a straightforward starting point for building and evaluating a DDS VFO based on the Analog Devices AD9850 chip.
His site also includes assembly and hex code programming for the PIC 16F84.
This project is a fun and easy one ... you should give it a try. It's a pretty simple next-step of plugging this VFO into your Sierra, OHR, or whatever QRP rig, thus giving you a stable and precisely controlled VFO down to 1 Hertz resolution! It also can serve as a precise RF signal source for the bench.
qsl.net can sometimes be excruciatingly slow, but this one is worth waiting for.
Seems I'm always out of the common resistor values. Same with the capacitors. These folks seem to have pretty good prices on resistor and capacitor kits.
Ocean State Electronics
Components, Kits, Test Equipment, Tools, Hardware, Gadgets, & More. A good resource for Torriods as well. NEW - Maybe not such a good place to order from... I ordered my Toriods from them TWICE and still don't have them... Does anyone know where to order these? Email me at: wizard at kl7gq dot com.