This is the second installment of the Minimalist SDR Homebrew Transceiver project. Today's "gulp from the firehose" will cover the block diagram and the USB Sound Card. To keep things from going off the rails we will be using the I and Q approach as this topology can be built Manhattan Style and needs no PCB.
Quickie review: The Analog off the air signal is down converted into two audio baseband signal streams 90 Degrees out of phase using two ADE-1 Double Balanced Mixers. The ADE-1 choice versus the commonly seen Tayloe Encoder/Decoder is to keep things minimalist.
The ADE-1 is inherently bidirectional and the Tayloe circuit is not. Plain English, the ADE-1 can be used on receive as well as transmit. This duplicity extends to the Signal Splitter, the Band Pass Filter and the USB Sound Card. This greatly reduces the parts count and the opportunity for errors.
We will start with the I and Q outputs (I = In Phase and Q = Quadrature) from the two Direct Conversion stages which are fed to this jewel, a USB Digital Sound Card.
The "Line In" Stereo Jack is used on Receive and the I and Q Signals are fed to this jack. On Transmit the "Front Out" jack feeds the I and Q transmitted signal stream back through the ADE-1's to the Splitter which now acts as a signal Combiner and on to the Band Pass Filter. Keeping the wiring straight to these two Jacks is the Achillies heel of the project.
The first issue stumbled upon by me was that I had QUISK set up for LSB on receive but found it transmitted on USB because of sideband inversion. My solution was to include a DPDT relay to switch the I and Q channels on transmit and problem solved. Actually, if I had invoked the RTFM process there is built into QUISK a button selection to reverse the transmitted sideband and no relay needed.
The current price of the Optimal Shop sound card is about $18 and the last one I bought was $15 -- thank you what's his name for creating a tariff war. Plain truth no US Manufacturer can supply this same unit for $15.
Now for the "bummer part" for those who must find an excuse for not undertaking a new adventure. There is no overall schematic but instead as my usual practice where required I will supply detailed schematics of modules that are home built. The USB Sound Card is simply a plug in with the Line IN/OUT cables and a USB cable connection to the RPi5. In the QUISK software on the Sound Card selection this is identified as "CM106 Like Sound Card".
The following block diagram was developed over 5 years ago and will be updated with new block titles such as the Pi Pico in lieu of the Arduino Nano and is controlled by the RPi5 and not acting independent as shown in the diagram.
Some other changes as 5 years ago I was not able to directly create two LO's 90 Degrees apart. The trick back then was to create an LO at 4X and then running that through a D Type Flip Flop you create two quadrature (90 Degrees Out of Phase) at 1X. The Pi Pico can do that directly at 1X. This eliminates circuitry!
Right now, in the Junk Box I have a RADIG V.3 board that will act as a test bed for this project. On that board is the Bi-Directional amp stage using two J310's (that will be changed to a BFR106), the Band Pass Filter, the Splitter, the two ADE-1's, Two Modem Transformers, the sideband switching relay and the 74AC74 D Type Flip Flop.
The D Type Flip Flop has been disabled, and I built a 1X quadrature LO using a Seed Xiao RP2040 and Si5351. This is now working on 40 Meters with the Optimal Shop USB Sound Card, the RPi5 and a USB Headset/Mic. It receives as well as transmits.
Transmit Tests ~ Needs Some Work
Haywire But Works!
I cannot overstress the importance of using this testbed as we develop the project. Right now, we have a working Transceiver albeit with some warts. With this as a starting point we can substitute modules that represent the final configuration.
On Tune I see about 120MV PTP at the output of the J310's. This is an important data point as the BFR106 must meet or exceed that level. The J310's have some charm as its gain level is adjustable whereas the proposed BFR106 has a lot less components and a pot in the emitter leg can offer gain adjustment (I have done that on another BFR106).
BTW at 120mv output that is 36 microwatts of power. This begs building an extra stage in the RF Chain -- more data. For those with an innate curiosity, why the 600:600 Ohm Modem Transformers. Tribal knowledge has shown that a Direct Conversion DBM can be subject to noise pickup. The Modem Transformers provide a degree of noise isolation.
Them that know can make it go.
Just a temperature check --- everyone happy with the grocery prices? What I pay for groceries now, just for me, used to feed two people. There are no steaks or booze in those purchases. Something is terribly wrong, and I guess when you are a billionaire the price of eggs is pocket change. Toilet paper is getting there too. Maybe that is behind the push for the 51st state -- lots of trees to make cheap toilet paper.
73's
Pete N6QW