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Tuesday, 21 January 2014

Minima Bidirectional Amplifier

Well,

Progress on this Minima is going well, I've now built the bi-directional amplifier section. If you look at the schematic here:

http://www.phonestack.com/farhan/minima.html

It's the bit containing Q6, Q7 and Q8 in transmit and Q3, Q4 and Q5 in receive.

The TX amp is giving me about 18.5dB of gain:


Above the yellow in the input and the blue is the output. both 'scope channels are terminated in 50R.

And here is the RX amp which comprises a common base amp plus an impedance transformation, this is Q3, Q4 and Q5:


As above the yellow in the input and the blue is the output. both 'scope channels are terminated in 50R. I have also added a step attenuator to the output of my signal generator to reduce the level of the signals as they will be very small in the RX sections.

The RX amp is giving me about 14dB gain:

Here's what my two boards are looking like now:


I'll try and label the photo above once the audio sections are complete but anyone looking closely needs to be aware that my RF transistors are MPSH10s.

I've also built the PTT circuit and that is a little confusing to simpletons like me. The base of Q23 is held HIGH in software whilst in SSB mode and then forced low in software in CW mode. If you build the circuit without that understanding you will find that the relay energises into TX without any external circuitry connected. Here's what the author said about it:

We were running out of pins. we should have had a separate PTT input and another T/R line as an output. But I had to make do with a single line.  This is quite confusing and i had to keep reminding myself of the plan.  the plan is like this:

  1. In SSB mode, the PTT line is in input mode. it has an internal 10K resistor pulling it up. The ptt switch provides the high/low switching.
  2. In CW mode, the PTT line is put into output mode and pulled down, putting the transceiver into transmit mode. Then, after a delay to let the relays settle, the sidetone is keyed. 

I now have to move on to the CW sidetone generator, and the audio pre and power amps. Once I have done that I can start to build the processor/LO and LCD display sections. They will need to be separate boards to keep all the LCD and processor hash away from the RF sections - I'm currently thinking I will use stripboard for that.

I won't be including the COM port on mine as I will program the processor in my Uno board and then just pull it out and stick it in the Minima. I suspect once I start tinkering with the software I will want to be able to program in-stitu, but that can come later.

I'll keep you posted.

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