DGM Rig Reincarnated with new Features & Fun

Starting off 2019 with a new rig!

[A famous French Painter whose name totally escapes me was asked about marriage and he said it was best described as a long dull boring meal with the dessert served first. So this project to some may seem like a dull boring meal, afterall it is not a Bitx40, uBitx, QCX or QSX but rather an N6QW creation. So to the dessert part. The rig performs flawlessy and is now fully operational. The clunky, quirky problems like clock noise, sub-marginal audio amplification and an enclosure that was too small are all things of the past. The rig has received solid audio and signal reports from some very picky operators. There are no complaints of audio that has no lows and it is on frequency. It doesn't get any better than that! In fact many unsolicited reports start by saying you have a great signal. Imagine what can be done with two Dual Gate MOSFETs?]

About 10 years ago I built a SSB transceiver that employed Dual Gate MOSFETs in a bilateral configuration wrapped around a 9 MHz Crystal Filter. Truth be told it worked but suffered from problems such as not only the power output seemed low but so was the audio output sub par.

True it did have a K5BCQ VFO kit and much like the KWM-4 could be operated from a keyboard or keypad, but it was not what I call my finest work

Needless to say I was not totally satisfied with the performance that I knew was possible and after a short troubleshooting stint I just put it away for another day. Subsequently I robbed it of many parts and up until about two weeks ago it was just a hulk of a circuit board. For those who still subscribe to QRP Quarterly there was an article by me on the original DGM transceiver.

That hulk calling from the bottom of the junk box kept saying to me: Why don't you fix me? Well between Christmas and New Year I devised a plan to do just that. It is now a working rig. But I should also mention that the hulk had many parts missing. BTW for those with an eye for detail the case including front panel was repurposed for use with the Heathkit Filter Rig built in 2018.

 

The New Rig!

 

 

By and large this is probably one of the simplest transceivers based not only on part count but in the number of active devices. Read low cost. Here for your dining pleassure is a recap:

 

  • 2N2219 Receiver RF Amplifier
  • 3N209 1st Bilateral Amplifier
  • 3N209 2nd Bilateral Amplifier
  • NE5534 Microphone Amplifier
  • 2N3904 Audio Pre-Amplifier
  • LM386 Audio Amplifier
  • 2N3904 & 2N2219 Transmit Driver
  • IRF510 Final Linear Amplifier
  • 2N3904 Power Conditioner
  • TUF-1 Rx/Tx Mixer Stage
  • ADE-1 Product Detector / Balanced Modulator
  • Arduino Nano & Si5351 LO / BFO
  • 128 X 128 Color TFT

Yet this rig stands up and talks. Hidden in the design is the ability to replace the manual IF gain control with AGC and ALC to handle large received signals and to prevent over driving downstream stages during transmit.

 

A little history is in order. In SPRAT #128 Ron Taylor G4GXO presented a design for a bilateral amplifier stage using a Dual Gate MOSFET (or pseudo MOSFET by cascading two JFETs). The circuit featured 19 dB of gain and was capable of a 35 dB AGC gain change. Worked for me. This circuit found its way inot my KWM-4, my one and only 17M rig and of course the DGM transceiver. The tri-band rig built in 2009 also has ths circuit. Thanks Ron! It is a superb design.

Here is a page from SPRAT #128 showing G4GXO's design and some of my handwritten notes about modifications.

I have done some additional "diddling" with this circuit by making R7 now 30 Ohms. [W7ZOI on his old website did some experiments with a BF998 and made R7 = 30 Ohms. I didn't just guess that!]

So why did I have such less than stellar experience with the DGM Rig? For starters it had some strange parts values in the audio pre-amp stage. By substituting the pre-amp stage with the circuit values I derived for the microphone amp used in the Sudden--the audio is now rock crushing. The low power output problem was a problem in my RF power amplifier chain which has been totally replaced. Now using an Arduino Nano with Si5351 we have USB/LSB select, two VFO's and a Tune Tone generator. Did I mention it also has a neat Color TFT display.

Now for a block diagram of the reincarnated DGM SSB Transceiver.

Below we see the main board which follows the block diagram. Just to the right of the crystal filter is the TUF-1 DBM which is dwarfed by the Crystal Filter.

The IRF510 Final Amplifier Assembly

   

Issues that had to be resolved!

  • Trust me when I say that often when I build something it does not work properly just as the last solder connection is made. Keep in mind how this current project started off
  • The first issue was I could hear the Arduino Clock in both receive and on the transmitted signal. It is not unusual to hear a bit of a faint clock on receive with the antenna disconnected; but this was loud with the antenna connected! It is unusal to hear the clock as the Arduino goes through the loop on the transmitted signal. Well I found that a few bypass caps resolved those problems. Mind you my hulk of a board had the Microphone Amp and Audio Amplifier already in place. Now there is no clock signal on transmit and just the faint hint of the clock on receive when the antenna is disconnected. In a recent project there is no hint of the clock signal on receive with the antenna disconnected. The solution there -- the Arduino is in a shielded enclosure.
  • Initially I had just the stock EMRFD driver stage (2N3904/2N3866) connected to the LPF and with two external amps got a whopping 120 watts out. On the air reports (three) gave this high marks. Normally with the two external amps in line this is a serious 600+ watts to the antenna. But there was no hint of feedback and it was a clean signal with just the driver stage.
  • Wishing to get the full QRO capability I added my stock design for the IRF510 final amp. That is when I ran into trouble. Since many of the circuit components were in a physical location, a result of using an existing board, the issue of feedback paths reared it ugly head! As luck would have it the LPF was about a quarter of an inch from the BPF. Well you can easily guess what happens. So one solution was to move the back panel away from the chassis baseplate (by about an inch) and place a shield over the LPF. Those measures resolved the problem. A little mechanical work will resolve the 1 inch gap from the base plate to the back panel.
  • Noise Clean Up: I took another measure with the clock noise in the Microphone Amp circuit and that is to decouple the source rail from the source. Three parts gets you there and I have N2CQR to thank for this circuit. Upon application of power form the Transmit rail the 2N3904 is put in an ON State and 12 VDC appears at the Emitter of the 2N3904 which powers on the microphone amp. Any AC noise is passed to ground thorugh the 100 uF Filter Cap and pure DC applied to the amp circuit.
  • THE CASE SIZE! I started with a piece of aluminum that I had that seemed the perfect size for the base plate. It was (TRGHS) exactly six inches wide and 8 inches long. For the front and back panels I simply took a 4X6 inch sheet of single sided copper PC Board and on my mill split it lengthwise so the front and back panels would be 2" X 6". This also gave me an opportunity to turn the back panel into a PC Board for the IRF510. Now I was on a roll. The back panel was affixed to the baseplate with a peice of angle stock taken from a defunct former transceiver project. It was perfect. So I assembled the transceiver and as luck would have it the Low Pass Filter was adjacent to the Band Pass Filter -- so close that RF energy from the output was being coupled back into the BPF. A really bad case of RF Feedback! Thus I had to provide a greater separation and to add some shielding over the LPF. A quick fix was to add some 1 inch long threaded spaces so that the back panel was now affixed to the baseplate using three spacers that aligned with the holes in the back panel to the piece of angle stock affixed to the baseplate. The small circuit board that had the LPF was now affixed to the back panel PC Board and threaded spacers were once again used to affix the LPF to the back panel PCB. These spacers were the kind with a threaded stud on one end. Thus the threaded portion connected the two boards and the open end used to connect the shield over the LPF. Mechanically the three spacers connecting the back panel were fairly rigid but that did mean I had a gaping one inch hole between the baseplate and back panel. This is where a bit of mechanical engineering solved the problem. The next time you pass through Home Depot find the roofing section and look for the small pieces of galvanized sheet metal. I keep a stock of these and I think the nominal size is about 7 X 5 inches and the cost is about 50 cents. So a bit of work with a pair of tin snips and I had a piece of metal 6 inches long and 1.75 inches wide. After careful measurement I concluded that I could make an "L" bracket that was 1.375 inches on one edge by .375 inches on the other edge. Having a metal brake it ws a simple bend. Then more careful measurement and I marked four drill holes for the bottom that aligned with the anchor holes for the origianl angle stock to the baseplate and then three holes that aligned with the back panel & spacers. Boom. Now this has bit or work made the chassis one inch deeper and the added metal bracket has made the assembly solid. I offer this to those aside from myself suddenly find themselves with six pounds in a five pound bag. Oh lest I forget --- No RF Feedback!