Thursday, June 22, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.24

Back to the Dual Conversion DifX Build

6/25/2017 ~ Working Field Day (at QRP level) with the 20M Shirt Pocket SSB Transceiver. At 16 cubic inches --smaller than a Bitx ! I actually made QSO's with this transceiver and it is a testament to good things come in small packages. Definitely a DifX.
Pete N6QW


6/23/2017 ~ See video at the bottom for more MMIC Applications 

Now that I was able to resolve the Crystal Filter that will be used in the second IF (it is not a Dishal nor any other form of homebrew filter) I have turned my attention to the issue of various amplifier stages that are used elsewhere in the rig. One such circuit is the bidirectional amplifier board I developed where there are two amplifier circuits comprised of a single 2N3904 in each leg. One leg is the receiver RF amplifier stage and the second leg is the Transmit Pre-driver stage. Relay switching is used to direct the signals in/out of the board -- a total of three relays.
While the 2N3904's are cheap, the relays are not! While the stage was broad band there were issues of making the gain constant over a range of 30 MHz (160 to 10 Meters). Since most of my rigs are single band units or perhaps two bands like 20/40 Meters this was not so much of a problem.
So in noodling the problem to have broad band amplifiers stages that are constant gain, are 50 Ohms, involve minimum switching (no more than one relay) and are termination insensitive. While you could use the Hayward/Kopski Termination Insensitive Amplifiers -- I wanted this to be a Pete design and not use the work of others.
My noodling took me back to 2010 and my very first article in QRP Quarterly where I used MMIC (Microwave Monolithic Integrated Circuit) amplifier blocks. The project was a 20 Meter MMIC based QRP SSB transceiver. A pair of MMIC amplifiers are diode steered so that they are bilateral (operating in two directions depending on which amp is powered on and steered for the proper mode). Yes Alice (or Virginia) it has a Digital VFO,

(Interesting note in that same issue we have G3UUR Analyzing Crystal Filters)

The MMIC device selected was the TriQuint (Watkins Johnson) AG-303-86G, This device is good to 6 GHz and has a fixed gain of 20 dB with a Z in/out of 50 Ohms and operates from 5 VDC. It also is termination insensitive, Did I also mention they work quite well.
Thus it was an easy decision to once again employ the MMIC amp and we have a board operating with a test transceiver. The alternative for those who don't want to enjoy using MMIC's in your next rig then you can always use the Hayward/Kopski TIA amps. Shown below is a board that was just installed in the transceiver that was recently used to evaluate the 128X32 OLED noise issues.

BTW I did have several conversations with TriQuint and they provided information about the use of the 1N3070 diodes as the "best choice" over the 1N914 or 1N4148. The actual building of this bilateral stage will present construction problems in that the MMIC's are surface mount and really small. It will be difficult to build this amp using Manhattan techniques. So while the benefits are superb, the construction may well beyond the skill/capability of those who have never done this type of homebrew construction. In the last 7 years the price of the MMIC's used has increased significantly so that may be a factor in your decision but my having a stock of the devices makes it a non-issue for me.
As you are perhaps gathering,  the Dual Conversion DifX will have features and functionality not found in other currently popular designs.
For those sitting on the edge of your chairs regarding the detail of second IF filter --patience grasshopper (for those who remember the Kung FU TV series). Oh I will tell you it is not 9.0 MHz.

Below is a video of MMIC's (four of them) used in a 40M CW transceiver built about the same time as the 20M MMIC SSB Transceiver. I do not know of many other transceivers that employed MMIC's in various transceiver stages. Perhaps another tip of the spear from N6QW.

A video from 2017
A video from 2010
Pete N6QW