Discussion on the IO Groups BITX20 list has recently turned to how a fast TX/RX switching regime could be achieved for the µBITx.
John K0JD mused that he could probably figure out how the circuit changes to eliminate the relay, but then changing the firmware to let it happen would have to be addressed too.
Madradiomodder suggested that it was easy. He provided us with this example of the front end of the Elecraft K3, that uses PIN diodes.
You just need to add DC to bias the diodes above the lowest AC cycle plus the diode voltage drop (0.6V in the case of the BAV99’s and the 64-04’s they use here). The 7T and 7R (7 volts for Transmit or Receive) used for biasing the diode switches… and the PRE_ON and PRE_OFF voltages (again, 7 volts) to pass or block the preamp Q4.
Argosy II approach to switching using PIN diodes
Jerry, KE7ER found a similar circuit for T/R switching (and SWR metering) in a manual for the Argosy (see page 51):
http://www.tentec.com/wp-content/uploads/2016/05/Argosy_II_Model_525_Manualf933.pdf
The full transmit RF waveform goes through C1, diode D4 ensures the bottom of that waveform is at DC ground, diode D3 then rectifies it to create a DC voltage across C2 that is equal to the peak-to-peak voltage of the transmitted RF. The notes then say that this relatively high DC voltage available during transmit is used to reverse bias diodes D1 and D2 such that no transmit signal goes through them to the RCV connector (which then goes into the receiver front end).
Note that the PIN diodes at D1,D2 are called out as 1n4007’s.
And that this transceiver works up through 30MHz.
Another thing to note, the final remains connected to the antenna during receive, shunting some of the receive energy to ground. Some of the other T/R switches use additional PIN diodes to isolate the final
from the antenna during receive.
Cheap alternative to PIN diodes
Tom, WB6B dug up an old article by Don Brown KD5NDB that relates to 1N4000 series diodes and their potential for use as a cheap PIN diode.
The 1n4000 series of diodes are general purpose rectifier diodes designed for low frequency use primarily 60 Hz power supplies. The construction is the same for the series except the 1n4007. The lower voltage diodes are basic PN junctions and would not work as a RF switch. The 1n4007 however has an extra intrinsic (undoped) layer forming the P-I-N junction. This is to get the 1Kv PIV rating but it also allows the 1n4007 to work as a RF switch like a true PIN diode.
The RF characteristics are not specified on this diode because it is not designed to do this type of work it just happened to work out that way and they are very cheap. True PIN diodes work better than the 1n4007 but are more expensive and not as readily available. Elecraft has designed the RF switching to work with the 1n4007 and making substations of different diodes may not work. Other 1n4000 series definitely do not have the PIN structure so they will not work.
PIN diodes are selected because they CAN’T switch at RF frequencies
Farhan VU2ESE waded in to clear up some confusion … The PIN diodes that are used for T/R are the ones that CANNOT switch at RF frequencies. That’s precisely why they are used.
An RF charge across the diode travels through precisely because the junction cannot switch on and off so rapidly and hence it remains either on or off. A PIN diode is like that, it can handle large currents, but it cannot change the current fast enough. That’s why for switching 100 watts of RF, you don’t need a few amps of bias current, a few milliamps will do.
There are ways of switching T/R with even 1N4148s (for QRP work). W7ZOI has, as usual, been there and done that. Read this paper to understand the concept :
http://w7zoi.net/tr-qrp.pdf
“ElectronicAntenna Switching” by Wes Hayward
This Wes Hayward article could be useful to those intrepid experimenters looking to use PIN diode switching”
W2AEW video
Jim KH6SKY drew attention to W2AEW’s video on PIN diode switching for a good intro to the subject. Using the simple test setup in this video you could check the suitability of the 1n4007s for this purpose. Interesting that the turn on characteristic of the PIN diodes is so strongly affected by frequency.
https://www.youtube.com/watch?v=XpYsCM_Wf50
John Scherer came across an interesting, 
PJRS, the makers of the Teensy family, have just added the Teensy 4.0 to their processor line up. Jack W8TEE says, “It is a beast!”. Perhaps he his contemplating an update to the JackAl board for the µBITx?
