A Bi-Lateral Amplifier Stage using a single MMIC Device. Watch and Wonder.
Good friend, Todd, K7TFC in his most recent Mostly DIY RF. com newsletter released a new product which is the MAR-6SM+ MMIC device mounted on a circuit board.
For those homebrewer's who will only use W7ZOI TIA amplifier stages or only use circuits from the original Bitx20 in their projects, it is OK to color outside the lines. By this I mean that the MMIC amplifier devices are inherently termination insensitive, they are 50 Ohms in/out and can supply 20dB of Gain from DC to several GHz. They are SMD parts mounted on boards from Todd, so get over it.
My first published article in QRP Quarterly involved the use of three bilateral MMIC stages employing the AG-303-86G. These devices made by Tri-Quint are obsolete but a sub in the form of the Mini-Circuits MAR-6SM+ is a drop in after changing one bias resistor.
So, how does this amp work? With +12 VDC applied to the two "R" points Diodes D1 and D3 are forward biased, and a signal applied at the left side passes through the amp Left-To-Right. Diodes D2 and D4 are back biased, and signals can only go Left to Right.
Now if we remove the 12VDC applied to the "R" points and now apply that voltage to the "T" points, signals on the right side and because D4 is forward biased then that signal goes to the input of the MMIC. From the output side of the MMIC and because D2 is forward biased the output appears at the left-hand side of the circuit module. Thus, on Transmit the signals move from Right to Left. D1 and D3 are reversed biased and isolate any input signals from the output signals.
The 51 Ohm resistor with a source supply of 6VDC provides the proper Bias Level for the AG-303-86G. I know, I know, for most wannabe homebrew hams (many who are amateur extra class licensee's) it is too hard to look up the data for the MAR-6SM+ so I will tell you that the value for a 6 VDC supply should be a 1%, 154 Ohm resistor.
Thus, to build a bilateral IF Module, you would need two of the MAR-6 SM+ MMIC boards and get one of Todd's Crystal Filters. You will need to match the 50 Ohms of the MMIC amps to the Zin/out of Todd's filter.
In a filter I bought from Todd, the Z value was about 140 ohms or a 2.8 to 1 match which is easily matched using a FT-37-43 ferrite core with a 10-turn winding tapped at 6 turns.
Let us do the math ~ 6^2 = 36 and 10^2 = 100. Thusly 100/36 = 2.777777778. My friend that is close enough to get her pregnant! For the wannabe homebrewer's, the 6-turn tap connects to the MMIC devices and the 10 Turn to the Filter. If your filter is other than 140 Ohms, then use this process to find a close match.
Todd offers several filter frequencies and mine happens to be a 4.9152 MHz which is more suitable for use on 17M versus a 9 MHz Center Frequency which is problematic when used on 17M.
The two MMIC amps would give a gain block of 40dB and allowing for filter loss would yield a final gain block of around 35dB. You can also get mounted ADE-1's from K7TFC so add two of those to the order. This now gives you a Rx/Tx mixer on one end and on the other end a Balanced Modulator/Product Detector. This has another advantage in that the LO is always connected to the Rx/Tx ADE-1 and the BFO is always connected to the PD/BM ADE-1. Thus, you can eliminate having to hot switch RF signals during receive and transmit.
With this small number of boards, you are 50% of the way to a SSB transceiver. Just think you didn't use any W7ZOI TIA amps or any circuits from the Bitx20 -- you colored outside the lines!
In line with my recent post regarding looking at old projects, the parts count with what I discussed in this post is far less than using the TIA design or pieces of old Bitx20 circuits. That affords a smaller footprint and less opportunities to make wiring errors while holding down the project cost with a bonus of delivering a strong performance. That is the essence of the engineering process.
Them that know can make it go.
73's
Pete N6QW