The IC Transceiver Design (Tnx to K1BQT)
1/22/2018 ~ Not so Fast Batman --The Batmobile may have a problem!
For many years now it has been obvious that while asleep my brain keeps churning away thinking about problems/issues/concerns with designs I am working on at that time. It is both a blessing and a curse. A blessing because I may find a problem before it raises its head as a problem. The curse part is that it is usually around 3:00 AM and I am awakened --never to get back to sleep. It happened last night.
Sometimes when we chunk a project into modules --we miss something at the interface. This might be the case with the K1BQT original design. Below are two parts of the original schematics. Take a minute to look these over. Read on.
T4 on the output side of U7 is a trifilar wound transformer consisting of 9 Turns X 3. Thus two of the windings are connected bifilar fashion which essentially puts two of the windings in SERIES for a total of 18 Turns. The third winding, the output is stand alone with one end grounded and the other end that ends up on Gate #1 of the 40673 which has a 10K resistor to ground.
So we have an impedance transformation from the output of the MC1496 to the input of the 40673. So lets look at that transformation in light of turns ratio squared. Thus 18^2 = 324 and 9^2 = 81. We have a classic 4:1 transformation (For the OBTE 324/81 = 4). So the output impedance looks like 1/4 the value in terms of the input to the 40673.
You might want to Google Maximum Power Transfer Theory in relation to impedance matching. You get max juice when the impedances are matched. So now the Question would be what is the Output Impedance of the output of the MC1496 and what does the input impedance look like for the 40673. Not withstanding the actual values --there is a 4:1 transformation taking place. So we need to go down that bunny slope to assure there was matching taking place as that impacts what we do in the redesign.
In the Simplecever Plus Project I addressed this issue by forcing the input of the J310s' DGM to be 2.2 K Ohms and then did a match from 50 Ohms to 2.2K Ohms with a 3 Turn to 20 Turn Broad Band Transformer. Thus 3^2 = 9 and 20^2 = 400 resulting in 400/9 = 44.4. Now if you take 2200/50 = 44 --close enough. Thus 50 Ohms at any output will look like 2200 Ohms at the Input of the DGM.
Below is an example of how this matching was done on the Simpleceiver IF stage. The 1st match was 3 to 20 turns and the 2nd out of the crystal filter was 170 Ohms to 2200 Ohms thus a slightly different turns ratio but always presenting 2200 Ohms to the input
I have now modified the two driver amps to include input matching from 50 Ohms to 2200 Ohms
Some tribal knowledge here -- if you simulate these in LT Spice and add the inductor mutual inductance notes (K9 L8 L9 1) be certain that when you make that note to check the dot that says invoke Spice Command --otherwise you will get no output.
These changes added about 10 DB to the output. Now lets see if we can track down the output Z of the MC1496. Maybe K1BQT accounted for this in the Out/In impedances but given I am changing his design I want to make sure I have accounted for the matching.
You can scroll down this posting and look at the original plots. Matching is important!
When I get the output impedance of the MC1496 we can modify Transformer T4 on the schematic (if necessary) so that its output winding is 50 Ohms. Now we could be really clever and once we know the output impedance we could use a single transformer so that the primary Zout matches the Secondary Zin to 2200 Ohms. I prefer two transformers but it could be done with one.
So let us now check the MC1496 Data Sheet that can be found here. The Output Resistance for the Parallel Outputs is 40K Ohms. So if we look at K1BQT's transform it is 4:1 and thus 40K looks like 10K into the base of the 40673. Thus the design as presented is absolutely correct. K1BQT was spot on!
Now how do we match that into my 2.2K Ohms?
Well lets us start by looking at T4 so we would have a 40K to 50 Ohm match which is 800:1 and with only 18 turns on T4's Primary -- you would end up with less than a half turn on the secondary --that won't work. So now lets look at the transformer on the input of the J310s and what can be done with the primary side. A 10K to 2.2K transformation is a 4.55:1. So now if we modify my input transformer to have a 15 turn Primary and 7 turn secondary we have the following: 15^2 = 225 and 7^2 =49. 225/49 = 4.59:1 is close enough. So I will run some plots with this transformer.
Now alternatively we could change the input to 10K and see how those plots look with a source of 10K pumping RF into Gate #1 with a 10K resistor to ground.
This exercise has served the purpose of looking at impedance matching and how to adjust input / output impedance matching.
Stay tuned tomorrow for the plot data with the new input transformers.
1/20/2018 ~ Prototype board layout for the Driver.
I will start the build of the first prototype Driver stage. My process begins with using G Simple to lay out island squares and the cut it on my CNC. The layout facilitates parts changing, circuit changes and lots of measurements. Once I get a working circuit I will convert this to the single sided copper board which will be another prototype. Finally I will build two "production units" for 20 and 40 Meters. I have left space on the board to add shields should that be necessary. If one desires this could be done over a sheet of copper PCB using Rex's W1REX MePads. For reference purposes the CNC will cut this board using a single piece of 4X6 board and pretty much fills the board.
That is the beauty of having a CNC Mill. Five minutes after I finished the design I had a board in my hand.
CNC Engraving/Router Mills can be had for about $200 to $300 on eBay. So within reach of many homebrewer enthusiasts. This board is way larger than needed but since it is a prototype we will be doing lots of "peaking" "tweaking" and adjusting and thus the extra room
1/19/2018 ~ Cleaned up Schematics for the 20 & 40 Meter Driver Assemblies.
I took some time to clean things up on the schematics so that the components were more readable and to include any coil winding data. The plots previously presented are essentially the same.
The 40 Meter version has slightly more gain but the minimum gain is 5 DB in both cases. Note that "R" on the schematic is a combination of a 10K trim Pot (Drive Control) where the top end is connected to the bottom end of the 22K resistor, the Center Wiper connected to gate #2 and the other end is connected to a 2.2K resistor which has its other end grounded.
K1BQT has a ferrite bead connected to the lead on Gate #2. Since my trim pots are board mounted I have not found this necessary. That said should you panel mount a pot then more care in lead routing and a ferrite bead may be required.
Caveat Emptor -- These circuits were simulated in LT Spice and not actually built which will be the next step. However, The J310's stage and the Band Pass Filters have been built and used in other rigs so these parts are known quantities. The substitution of the BS170 for the 2N7002 and the simple Low Pass Filters are new and thus will be evaluated. Should these two circuits work well then they will become a standard module that could be used in any rig design. I am hopeful this is the case.
I would encourage those with more sophisticated test gear than I have to build one of the circuits and run a Spectrum Analysis and share with us what you find.
One more thing: I used T-68-2 Red Cores for the 14 MHz networks. I invite your attention to the information in the chart below provided courtesy of Amidon. Red Cores are OK at 14 MHz. The Q's are good enough with the T-68-2 cores (Al = 57) . But if your shorts are all wadded up, then with the inductance values provided you can calculate the turns required for the type 6 (yellow) cores for the 14MHz networks. Al value for yellow T-68-6 cores = 47.
Stay tuned -- this is the year of SSB Transceivers!!!
We have earlier presented the reference document for this project and in looking over what will be kept pretty much intact versus design anew, the first piece to be designed is the driver stage following the IC lower level stages.
The basic LM1496/MC1350 pieces will literally go untouched. That said the driver, final RF amp and Audio Amp stages will likely get a going over. My initial thoughts were for a two band rig that will be switched by a simple DPDT toggle switch. This starting point affected what I did with the driver stage.
From K1BQT's design the driver stage looked like this:
In 1985 this was state of the art and we will build on the idea of a Dual Gate MOSFET driving a N Channel Enhancement Mode FET. Since 40673's are not like a stock item I will build one using two J310's. This is a proven approach and works perfectly! The IRFD1Z3 is like un-obtanium and so we'll substitute a BS170. Touring Rick's design the 40673 can have an adjustable "Drive" level. We will do likewise with the J310s. The 40673 was followed by a Band Pass Filter and then to the IRFD1Z3 with a Low Pass Filter on the output. K1BQT was very careful to state that if you use bands other than 75 Meters you will need additional filtering.
So if we use just the J310's and the BS170 as a single amp we would have to be able to switch the Band Pass and Low Pass Filters which adds a lot of diode and/or relay switching.
So what if we built two complete separate driver assemblies and then it would only involve a SPDT relay at each end of an Assembly to switch between the Assemblies and then a third SDPT relay to provide power to the Assembly being used. This approach does several things for us including making the switching a bit less complicated but more importantly each driver assembly can be optimized for the band being used. The added cost would be for a couple of devices (2X J310 and 1 X BS170) and a few resistors, caps and ferrite cores.
Today I took a crack at designing or should say simulating two separate Driver Assemblies. Should mention that the Dual Gate MOSFET Stage is a stock design that I have used in several rigs as is the Band Pass Filters. What has not been previously tested is the BS170. So after presenting this design I will build a prototype and run some tests. the simulated plot as a starting point shows great promise.
Take a hard look at the schematic and note that the Band Pass Filter is terminated at each end with 50 Ohms. If you want a rude awakening remove the input side 50 Ohm resistor and run the plot. Do not remove this resistor.
Again these are the first pass and I will build a prototype to insure they works as deigned.
Stay tuned and start ordering the parts.
Stay tuned and start ordering the parts.