Tuesday, January 26, 2016

Simpleceiver ~ Part 27

Some light at the end of the Tunnel!

My XYL has been steadily improving and hopefully we expect her release from the nursing facility in about a week or two. That means I can get back to heating up the iron and continuing with this project. I have not had any comments regarding those who forged ahead and started work on the transmitter stage and so that either means there is a complete loss of interest or the project is dead. I sure hope not as I think given what I have observed from the receiver performance this project could rival some of the currently popular homebrew radios.
The basic transmitter board had 4 elements including the microphone amplifier, the carrier oscillator, the SBL-1 and the IF Amp/Filter block. The output is of course at 12.096 MHz which is the filter frequency and just so we all are on the same page the Zout is 50 Ohms. The block which will follow the basic transmitter board is the frequency translation board which performs some very specific functions. The 12.096 MHz SSB signal must be mixed with the local oscillator so that one of the resultant frequencies is on the band of choice. From our earlier discussions the mixing process results in sum and difference frequencies and a second effect is depending whether the Local Oscillator is above or below the filter frequency there may be a sideband inversion. We certainly do not want the case where we have the receiver on LSB and the transmitter is on USB.
The Simpleceiver receiver used an LO at 5.0 MHz so that the one of the resultant frequencies is in the 7 MHz range. Here it is the IF- the LO result we want (12.096 - 5.096 = 7.0). But there also is a component where you have 12.096 + 5.096 = 17.192 MHz which must be filtered from the output. So following our frequency translation we must have another Band Pass Filter centered on 7.150 MHz so that only the subtractive mix is in the output.  Given that we will also be using an SBL-1 for the frequency translation there will be a need for several amplifier stages to boost that signal to something directly usable on 40 Meters. A J310 based Dual Gate MOSFET would serve this purpose nicely for the 1st stage of amplification. Thus we can actually use the Simpleceiver RF amplifier stage as a circuit block following the Band Pass Filter. Again we stated that many of the circuit blocks built for the receiver could be simply reused for the transmitter and so this is what we will do.
I hope in the next week to post an amplified block diagram of the transmitter circuits that will include the follow on boards to the basic transmitter.
Pete N6QW