Wednesday, May 10, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.14


Back to the LM373 SSB Transceiver.


While I take the time to re-measure the 9 crystals for the Dishal Filter, I thought I would update the LM373 SSB Transceiver project. This rig qualifies as a DifX as it is not a Bitx and the subject matter that will be covered has direct applicability to the DifX Dual Conversion project.
 
W5BAA's LM373 SSB Transceiver (Ham Radio November 1973) used 11.0 VDC as the supply voltage for his rig. If you look a the photo of his rig in the article you will see it connected to a 12 VDC Gel Cell. So connecting the dots, this gave him some lee way as the Gel Cell discharged. My evaluations of his approach actually demonstrated that his design did indeed "love" 11 VDC and so I followed his direction.
 
But now you have to have a source for 11 VDC (at least for the lower level stages (which is most of the rig) and you also needed a means of efficiently switching 11 VDC to the steering diodes to change the application of the circuit elements.
 
So task #1  was to come up with a 11 VDC regulator circuit. For this I went to an LM317 TO-220 style that is good for 1.5 amps. My circuit has the element of being adjustable which you will shortly see the why. The Internet abounds with regulator calculators and favorite circuits. I simply lifted one of those circuits and used values from one of the voltage calculators.
 
Essentially the LM317 samples the output voltage and using a voltage divider feeds that back into the V adjust pin. The ratio of the feedback resistor and the resistor from V adjust to ground sets the final output level. The LM317 can handle input voltage up to 40 VDC in and will regulate voltages up to 37 VDC out. The delta difference is needed for the regulator to work properly.
 
In my design the two resistors were 220 Ohms and 1715 Ohms. The 220 Ohm is a stock value but 1715 is not! So I homebrewed a 1715 resistor by placing in series a 1K connected to a 200 Ohm pot wired as a variable  resistor and that in turn is connected to a 680 Ohm resistor to ground. Thus with the pot a "0" we have 1680 Ohms and with the pot at 200 Ohms we have 1880 ohms. The pot also compensates for the tolerances of the 1K and 680 Ohm. The circuit works very well and this is shown below.
 
 
 
So now we have a 11 VDC source and as was mentioned when I first covered the LM373 Transceiver, W5BAA used steering diodes to route signals through the single direction IF block and it is through the application of the steering voltages that the LM373 rig goes from Receive to Transmit. Long ago I designed a circuit to do this. It was out of necessity as I was using an analog VFO (Yes Bill one of those old technology devices) and a power relay to switch voltages. Well the back EMF was so large (even with a snubber) that the VFO would jump frequency. This resulted in my solid  state switch design shown below to replace that pesky relay and has been updated for the 11 VDC. This same switch will be used in the DifX Dual Conversion rig although at +12 VDC.
 
Stop! I can see all of those who are into power FETs that will quickly make a posting about doing this with power FETs --you know will supply the arguments  that no current is drawn and how when you take this to the field battery life is precious and how your design is so much better. Well good for you. I am not taking this to the field, I have a stock of parts and it works just fine.
 
So how does this work. First I thought it would be cool to use an optoisolator like the 4N35. That device is also important as the next element in the circuit is quad NAND Gate and a bouncing input signal will give a lot of false triggering --the 4N35 acts as a buffer between your mechanical PTT switch and the 7400 IC. Two of the NAND Gates are wired as inverters with one Gate triggering the second Gate. With the PTT in an OFF state the upper 2N3904 has a signal applied to the base which powers on the TIP32C and we have juice for the receive condition (R). When the PTT is engaged the 7400 is toggled so that the HIGH output is not on Pin 6 but Pins 3,4,5 and thus the upper 2N3904 is now OFF and the lower 2N3904 is ON and we have juice for the Transmit stages.
 
You can build your self a truth table to verify what I said. Since there is a pull up resistor on Pins 1 and 2 that means it is HIGH (with the PTT disengaged) which results in Pin 3 being Low. But Pin 3 is bussed with Pins 4 &5 which make them low and now Pin 6 is High --driving the upper 2N3904 to ON. Now when you trigger the 4N35 with the PTT the output goes low which means the 10K is to ground and that drives Pins 1 & 2 to a Low state. Now Pins.3.4.and 5 are high and Pin 6 is Low. But since we are tapping off of Pins 3,4,and 5 and in a  HIGH condition we now see a signal on the lower 2N3904 and we have a transition from R voltage to T voltage. No relays are involved for this part of the circuit. Below is the build.
 
 
Yes there is a combo of leaded and SMD parts and the layout is close to the schematic. This is a keeper and no power FETs were involved in the build! When I connected the regulator and solid state switch to my LM373 I measured the voltage at the R and T ports and then adjusted the 200 Ohm pot so that they read 11 VDC at the R and T pins (collectors).
 
After some additional tweaking I now find that with my single 2N3904 pre-driver stage it will produce 10 Millwatts into a 50 Ohm load - (2 Volts Peak to Peak) . Now we will proceed with the driver and final stages. This is really moving along.