LTSpice simulation of Baofeng UV5R band selector

I was quite busy the other day trying to understand how Baofeng is using RDA1846 transceiver in their products. While analyzing the schematic it struck me the overcomplicated way they do the switch between UHF and VHF. This is an excerpt of the schematic:

Baofeng UV5R schematic: band selector block (excerpt)

The entire selector block is made up with discrete components ! Q30 and Q33 PNP transistors work as inverters and reverse logic from input. They did not bother to use digital inverters. I am not sure what is the level of control signals in Baofeng (i.e. 7T and control voltage on P56, to the left), but assuming overall TTL levels, I redraw thus block in LTSpice and simulated the behaviour. Setting value high (1) on P56 for VHF, will put Q30 base to ground and create a high level on LED D2 (high on Q2 base, low on Q3 base, which means Q5 is off). Conversely, a low level on P56 will end in closed Q2, open Q3 (biased by R4) and open Q5 which will generate a positive voltage on LED D. Quite complicated for a switch, in my opinion. I wonder if same functionality could have been achieved in a much simpler fashion. Simulation below:

For the transient analysis data is collected for 10 seconds because I used a DC pulse source that generates 5V pulses with 1μs rise and falltime for a period of 5 seconds. This is the Q30 base voltage as simulated:
And this is Q33 base voltage as simulated:

Q33 base voltageSee the overlapping pattern:

I used generic NPN and PNP transistors. Nothing fancy. But it is always a great fun to reverse engineer and pick the brain of those who created these gadgets. 🙂 Simulating in general is very rewarding too. One can quickly explore an learn without too much hassle. For example, one could explore the effect of adding two large values capacitors in the bases of each invertor like this:

Just for fun, I added C7 and C6, two large values capacitors. The effect on the base voltages for both inverters is shown below:

Have fun !


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