Re: AE7EU Top Level PCB Mod


Diver Martin <diver.martin@...>
 

Hi Mike,

The mixer is used as an attenuator.  As you bias/turn on the diodes, the more they go into conduction (e.g. not an attenuator).  Balance the circuit, and it becomes an isolator, roughly to the tune of 50dB per the datasheet.  This is why I have a negative gain opamp on the output of the log amp.  As audio level goes up, the output of the log amp goes up.  Invert this, so that maximum audio output = low voltage on the mixer (attenuator), increases attenuation and decreases overall RF reception, and thus the feedback loop.

The attenuator is switched in only on RX; obviously you don't want to TX 10W through this attenuator at any attenuation level.


On Fri, Jan 12, 2018 at 11:37 AM, Mike Woods <mhwoods@...> wrote:
Martin

In examining your circuit more carefully, I wonder if you could explain how the AGC works?  The mixer chip appears to be unbalanced by a DC signal from the AGC circuit (derived from the volume control input). However,  I don’t understand why you are injecting this into the IF output of the transformer and why the input from the antenna is taken to the LO input.  It looks like a design idea that could be used more widely on Bitx designs and won’t require track cutting ...

Mike ZL1AXG

On Thu, 11 Jan 2018 at 7:42 PM, Diver Martin <diver.martin@...> wrote:
So, I've finally got a rough draft of what I'm thinking for my uBitx top PCB mod.  Features at this point are:

No modifications required to uBitx board itself (not to say a 470uF anti-pop mod might be bad, or other things couldn't be done)
ATU: 7-inductor, 8-capacitor with Hi-Z/Lo-Z switch.
Selectable Power output (two power input connectors)
Brain transplant:  Teensy3.2 96MHz ARM processor
All connections on side edges - no wiring to enclosure
Front panel with pushbutton switches
DSP Audio (Via Teensy32, would require headphones or amplified speakers)
Solderless AGC modification (Attenuator at the front end switched in during RX)
Realtime clock (via Teensy32)
Linear amplifier control (just a reed relay on a 3.5mm header, sorry, not an RCA, but you could easily change it...)
Top side only construction (would make assembly cost lower)

BoM cost:  Probably way higher than it should be.  Needs optimization, and I still need to enter in costs.
Complexity:  Probably way more than it should be.
Layout:  An ugly mess of routing.  I can't say I like it, but the constraints I put on myself made it really hard to route.  There's lots of open areas to cross with lots of signals going all different directions.

Picture of the layout and schematic PDF attached, along with a 3D view.

I think the one modification that I still need to consider is a microphone preamp.  But, I have no flippin' clue where on the PCB I'd put the gain control knob!  Might have to be an internal potentiometer....

--
Martin Held - AE7EU
http://ae7eu.com/
-------------------------------------------------------------------------------
If there aren't any questions, then what is there to learn?




--
Martin Held - AE7EU
http://ae7eu.com/
-------------------------------------------------------------------------------
If there aren't any questions, then what is there to learn?

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