Ruminations on fast TX/RX switching on uBITx

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

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TX IMD measurements for a v5 uBITx

Doug K4DSP just finished measuring the two-tone TX IMD on his v5 µBITx. These are 3rd order products, and they were measured relative to either tone. Since the ARRL states their measurements relative to PEP I have included that as well, for those who sleep better at night secure in the belief that their radio is clean 🙂

This is at 10W PEP using 700 and 1900 Hz tones. Doug’s radio puts out 10W from 80 through 20 meters, and falls off to 5W on 10M:

80m -25.5 dBc (-31.5 dB PEP)
60m -22.5 dBc (-28.5 dB PEP)
40m -22.0 dBc (-28.0 dB PEP)
20m -24.5 dBc (-30.5 dB PEP)
17m -21.0 dBc (-27.0 dB PEP)
12m -25.5 dBc (-31.5 dB PEP)
10m -22.0 dBc (-28.0 dB PEP)

As Doug’s flight instructor used to say after one of his landings, “I’ve seen worse, but I’ve seen a lot better.” This is probably not atypical of IRF510s.

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Powering a larger Nextion screen from the raduino

If your larger (4.3″ or greater) Nextion display is flashing, you are probably overloading your Raduino 5v power supply.  This is because the Nextion does draw quite a bit of current.

Mark AJ6CU found a 7805 with a higher rating on Amazon (maybe 1.5A?). He mounted it on a heat sink attached to the rear panel (using the rear panel as a large heat sink) with a small perf board (to mount the appropriate capacitors).  He then brought in a set of 5V wires to replace the 7805 on the one on the Raduino.  This could be a helpful idea for those of you installing Nextion display units.

MVS Sarma suggested using a 78T05 as a replacement to the 7805. This is  a 3amp TO220 device and available very cheaply .   Managing the TO3  is little dìfferent compared to the TO220 on the 7805.
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Fan cowling

Mark AJ6CU has made a 3D template for a fan cowling for your µBITx. The 3D template for the fan shroud can be found on thingiverse here:

https://www.thingiverse.com/thing:3790450

It basically bolts onto the 80mm fan at the rear of the case (you might recognize the “tan” of a Noctua fan — much quieter than the one supplied). It sits on the uBitx motherboard with two slots straddling the heat sinks.

Marks notes that he has not powered on his µBITx yet, but suspects it will keep things nice and cool even in digital modes. This was designed with Fusion 360. There have been several other suggestions for adding ducts for cooling some of the hotter spots in our radios.

I was inspired in this effort by the work by another cooling shroud for the “shorter” of Sunil’s cases. See:

https://www.thingiverse.com/thing:3360539

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Removing the detent in a 100PPR optical encoder

This optical encoder (widely available on eBay or Aliexpress in black or silver) has been used by some constructors as a replacement to the standard mechanical encoder supplied with the µBITx kit.   It is a 100ppr encoder (whereas the one supplied with the µBITx is 24PPR) so may not work perfectly with either the default firmware supplied by HF Signals or Ian Lee KD8CEC firmware (i.e. you may need to experiment with the code for generating the VFO).

With the detent mechanism removed to allow free turning the encoder gives a very smooth tuning action.   With the encoder able to spin freely the protruding crank handle will always come to rest in the down position so it too has to be removed.

The main complaint with these encoders is the detent feature.  To remove the detent, here are the instructions from Robert GM4CID:

Remove the knob handle, then, using a heat gun or hair dryer heat the silver knob insert to soften the adhesive and carefully pry off that silver insert to reveal retaining screws that can be removed. The mechanism will now come apart and you can remove the detent. Replace everything but not the handle.

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Nextion noise in RX

MVS Sarma VU3SMV  noticed noise on his receiver generate from his Nextion display.   He placed a 2.2mH RF choke in series to the 5v line feeding the  Nextion display.   He also placed a 220µF capacitor across the 5v and Gnd terminals for the Nextion.  The hash was reduced to just 10% of its original value.

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3D printers and the uBITx go together

Doug K4DSP hesitates to say that that is µBITx is actually finished, as he keeps thinking of stuff to add to the hardware and firmware! But the cover is on and he’s making QSOs.

Previously Doug had only used his rig on CW, but he used a 3D printer to make a mic case and got it wired up.   His first SSB QSO was with S51DX.

Doug is pleased with how well the little radio “hears”.   While drafting his update for the BITX20 IOGroups list he was listening to G0EVY on 40M SSB who was booming in.

Doug has yet to install the AGC board so he finds the static crashes are “pretty harsh”.   He says it reminds him of his old Heath HW-16 in that regard.

The mic case  design was sourced from Thingiverse:
https://www.thingiverse.com/thing:3044144

It uses the mic element and microswitch that were included in the uBITX kit. He printed it with black filament and then rubbed some white paint into the front to give it some “character”.

Doug printed the uBITX case from here:
https://www.thingiverse.com/thing:2823172

He only used the front and rear panels.  He designed his own middle section to add mounting holes and a baffle for an internal speaker.   He used FreeCAD to do the design.

Doug calibrated the frequency against a 10 MHz GPSDO. Then, based on advice of others, he used a PC audio spectrum analyzer program (Spectrum Lab by DL4YHF) to adjust the BFO frequency. That worked a treat, and he’s really happy with the way the audio sounds now.

He concludes, “It’s just a very pleasing little radio”

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Technique for removing relays on a uBITx

Gerry recently changed out the relays on his v3 µBitX.  He had trouble getting the wick to remove all the solder, no matter what iron he tried and how much flux he used.   So he tried another method.

There are a number of low-temperature solder products, the most famous/common, being ChipQuik. These are basically an alloy that includes indium or gallium.   When added to a solder joint, this drastically lowers the melting point. You can melt the solder at each leg of a device, and by the time you have gotten to the last leg, the first will still not have set, making it easy to pull free the device. The risk to both the device and the circuit board will be greatly reduced.  If anyone is having difficulty, or is apprehensive about replacing the relays, this represents a good approach.

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New BCI Filter Kit from KitProjects (N8DAH)

David N8DAH from  Kit-Projects.com will have BCI kits out in the next week or so for the BitX radios as well as other direct conversion receivers that suffer from close AM stations.

The drop off of the filter kit is right at about 3MHz.

These filters will cost 5$+ shipping.  They come with 3x toroids and 4x capacitors.  This would be a good kit to learn about toroid construction for those that have not had a chance as yet to try winding your own.

These filters are RX ONLY and should not be placed in the TX path.

David’s website will have detailed specifications and instructions for both the Bitx40 and uBitx radios on it shortly.

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Remote access to your uBITx

There have been quite a few posts recently on the BITX20 list about remote access to your µBITx.  There are a number of ways to achieve this, but Jens (KM6ZJV) has provided a useful summary of how he has achieved this using a v5 µBITx connected around the clock to a Raspberry Pi 3 used as an FT8 monitor.

Jens  can access it from anywhere on any device (laptop, iPhone, iPad) with VNC and OpenVPN.

A big advantage of this setup is that the radio is always ‘ready for action’ – even if you have only a few minutes time to transmit (FT8 or FT4 or FS8).

As others have mentioned, the Raspberry PI comes pre-installed with VNC.

Using OpenVPN

OpenVPN can easily be installed with:

http://www.pivpn.io/

OpenVPN allows access to the Raspberry Pi VNC server with the same IP address / port number used in your local network (without a VPN connection). OpenVPN client applications are available for all major platforms.

Of course, once you have established the OpenVPN connection, you can access any other  part of your home network (fileserver, printer, etc.) as well – not just VNC running on the Raspberry Pi.

Jens does not not recommend directly enabling a port to connect to the Raspberry PI VNC server, instead enable a port to connect to the OpenVPN server.

Using Teamviewer

Frank KJ5WI suggests an alternative in teamviewer host.

Teamviewer host on the Raspberry Pi from the above link will allow a remote connection without opening anything. When the webpage opens, scroll down and watch for the pi logo on the right side.  Frank says that the Sun City Georgetown amateur radio club uses Teamviewer for remote access to their club station radios.

KM4UDX took a bit to get both computers trusted and set up… one running on Ubuntu/AtomicPi and the other Win10/oldlaptop. But he says, “it works!”

The  screen capture above from his win10 laptop controlling the AtomicPi with ubuntu controlling his V4 uBITX running WSJTX and completing a JT8 QSO.

Don KM4UDX is thrilled to have remote control!

After his initial success, he switched to FL-Digi and this works as well sending the keyboard via the link to FL-Digi text input window. However, if the uBITX audio output level is set too low from the last time you have adjusted the speaker volume, then you have no way to change/increase the uBITX output level to make Fl-Digi happy.

Challenges with remote operation on the µBITx

Don sees the real advantage from complete visualization of the transceiver (.e.g. Flex).  If he had all of the uBITX functions on the screen, then the volume controls would be screen sliders allowing adjustment of all functions via the remote control.

The first challenge is how to adjust the uBITX volume levels in software through some sort of interface.

The second challenge is not seeing the RF output levels.  While you can adjust the drive level remotely, the impact on RF power output levels is not known, and could result in harmful emissions.

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