700Hz CW Audio filter board

Nik VK4PLN has yet another board design – this time for an audio CW filter similar to the QCX and HiPerMite.  Nik says “he is  loving the cheap and easy EasyEDA/JLPCB boards and simple web designer”.

He “takes no credit for the design, just copied it and added a switching relay.   He has ordered 10 boards at $10 posted”.

Details of the filter

Center Frequency: 700 Hz
Bandwidth: 200 Hz
DC Power: 5VDC
GAIN: Upto 20dB via R11 and R12 + trimpot.

Resistors 1206 SMD:
R1 33k
R2 33k
R3 1M
R4 47k
R5 47k
R6 36k
R7 36k
R8 10k
R9 750k
R10 10
R11 100K
R12 100K

Capacitors 1206 SMD:
C1 0.1u 104
C2 47n 473
C3 36n 363 (33n + 3n3 mount on side together)
C4 1n 102
C5 39n 393 (33n + 5n8 mount on side together)
C6 1n 102
C7 2n2 222
C8 2n2 222
C11 0.1u 104
C14 220u Electrolytic

Relay OMRON G6S-2-DC5
200K/100k trimpot

R11 and R12 are optional to parallel the 200k trimpot to make it a logarithmic 100k.   Or just use a linear 100k pot.

Use a switch connected to 5V to turn on the filter.

This powers the opamp and switches the relay feeding the audio into the circuit.  When off the relay just passes the audio straight through unfiltered.


Making a PCB? Find the cheapest PCB manufacturer

John, WA2FZW,  noted a suggestion from a friend that if you are designing your own PCB, the PCB Shopper website will help find the cheapest PCB manufacturer:




Kerr Smith confirms this recommendation saying,  “This site is definitely worth using when you are looking to get PCBs made. The last time I used it it suggested ALLPCB which had an offer on at the time so I got 5 boards for $5 with free courier shipping (my PCBs arrived in 5 days).

“I would highly recommend using the site to see which PCB fabricators are currently the cheapest. You can then do a bit of research and get some user reviews and decide if you want to get your boards made with them.

“There are so many different PCB manufacturers this site makes it really easy to get an initial price per PCB for lots of them with you only needing to enter in some basic data such as PCB size and number required etc.”

What sort of soldering iron/station should I use to wire up my uBITx?

Richard WB8YXF asks what sort of soldering iron or gun should be used to wire up his uBITx.   The answer is pretty much anything.

Doug Wilner responded as follows:

“Kind of like asking what car you should buy or shovel or rifle or whatever.  We can be much more helpful if you know your goals.  Are you on a limited budget and this is the only thing you plan to ever build and need the cheapest way to just get this done or do you have so much spare cash you light Cuban cigars with $100 bills and want the best tools that will last for generations?

“Personally my go to iron is a supposedly 60w adjustable pencil type iron that I keep planning to upgrade but I like it and it gets the job done.  I have this one https://www.amazon.com/gp/product/B00Y9UW2XA  that was around $15.  It isn’t available anymore but there are tons of similar Chinese ones on Amazon.  While you’re at it I would suggest picking up one of the copper scrubby looking tip cleaners.”

Perhaps not a soldering gun (first seen in the 1950’s).   You have to press the trigger and wait for them to heat up.  A soldering iron with a thermostat and adjustable temperature is a much better option.

These soldering guns were a source of wonder to the ubitx.net editor in his youth, because his Elmer had one and swore by it.   Of course, said editor earned an income from mowing lawns around the neighbourhood and this meant he simply couldn’t afford it – it was around NZ$60 even then.   Joe KD2NFC said, “I used to watch my grandfather repair tube TV’s with this one.”

The Weller 853D shown at the top of this news item is a full rework station as well and on special can be around US$70.

Chris, KF4FTR, makes the observation, that “Having a hot air gun available for tighter pitch work is important.

“Spark fun has an old multi page tutorial on SMD work located at https://www.sparkfun.com/tutorials/category/2 .

“Is it possible to use just an iron? Sureand again it depends on the work that will be done. However, if you look at the page for removing multi pin SMD ICs you can see the results of trying to this without hot air. You can try and physically force them out of place or use an xacto knife to try and cut the pins. Both are bad because just flexing the part can rip a pad out which means you will need forceps and wire to fix it, or you can cut traces with the knife.

“The best thing to do if any rework or SMD parts need to be placed is to get the right tool for the job which is a hot air rework station. I have seen people suggesting 100$ irons. Well, if you spend the 70$ on a rework station you wont have that problem… This again is all based on the assumption that this person will need to work on the UBitx or mods or possibly other kits. Sure its another 50 bucks or so, but it will save hours of frustration for a person who hasn’t even used an iron in a long time.”

Rich WB2GXM also points out, “There are two very good Youtube videos by Dave Cassler, KE0OG, on soldering irons, accessories and techniques”:

https://www.youtube.com/watch?v=9XW_gkdWJ5U https://www.youtube.com/watch?v=gqz1xGj_m_E


A very nice looking build in a metal case

Joel, N6ALT, has declared that he “is done with his µBITx”.   And we can see why!   This is  a very nice build in a very classy case.

After three attempts at different enclosures, he settled on this one from China. https://www.ebay.com/itm/123008133070?ViewItem=&item=123008133070

The PTT switch is mounted in the pen housing on a small piece of perf board.

It is built very well and super light. Only 4 screws hold the whole thing together yet it is very strong and rigid. Very ingenious.

The radio weighs 2 pounds. Joel did not leave the display mounted to the main board as it would make the display not centred and too low on the front panel. He built a short flat extension cable out of a female and male header.


How the uBITx works: A technical summary

uBITx 45MHz roofing filter (First IF stage)

Here’s a pretty complete summary of what’s going on within the uBitx, under its dual conversion IF system, as explained by Jerry KE7ER.

Actual frequencies are those used in the supplier’s original µBitx code:

Release version dated Dec 6, 2017 at:  https://github.com/afarhan/ubitx

uBITx 12 MHz Filter – 2nd IF stage

Local Oscillators

  • CLK0 (BFO) is fixed at 11996500 Hz, maybe 500Hz below the 12MHz filter’s 2000 Hz wide 3dB passband.
  • CLK1 (second local oscillator) is fixed at 56995000 hz for USB and 32995000 hz for LSB. You can find those three numbers in the file ubitx_20.ino at lines 166, 163, 164 respectively.


The VFO is used to select the operating frequency Fop according to these formulas.

For USB:

Fop  = VFO – (CLK1-BFO)   so:

VFO = Fop + (CLK1-BFO)   where CLK1 is around 45 MHz + 12 MHz

For LSB:

Fop  = VFO – (CLK1+BFO)     so:

VFO = Fop + (CLK1+BFO)      where CLK1 is around 45 MHz – 12 MHz

To receive a 7.2MHz LSB signal (where 7.2 MHz is the frequency of the suppressed carrier), the VFO gets set to:

7200000+(32995000+11996500) = 52191500 Hz.

That formula gives an exact result, not an approximation.


The BFO corresponds to the carrier frequency of the station being received or transmitted. The 12mhz filter is always above the BFO, so within the 12 MHz IF it allows through only the upper sideband.

The VFO is always above the 45 MHz first intermediate frequency, and so always inverts the sidebands.

A carrier at 7200000 Hz would get translated to:

VFO-Fop = 52191500 – 7200000 = 44991500 Hz

A lower sideband at 7198500 Hz would get translated to:

52191500 – 7198500 = 44993000 Hz

In this example we assume the lower sideband is generated from a single audio tone into the mic of 1500 Hz.   We have chosen 1500 hz because it will land in the middle of the 12 MHz filter’s passband, assuming the filter has a 3 dB passband that’s 2000 Hz wide and the BFO is 500 Hz below that passband. The actual range of frequencies passed will be 500 to 2500 Hz. Those assumptions of 2000 Hz and 500Hz might be off by a couple hundred Hz.

Likewise, a high side CLK1 of 56995000 Hz  for USB always flips the sidebands when translating to 12 MHz, however the low side CLK1 of 32995000 Hz we use to receive the 7.2 MHz LSB signal does not.

Our 7.2mhz carrier:

44991500 – 32995000 =  11996500 Hz (exactly equal to our BFO frequency)

Our 7.2mhz lower sideband:

44993000 – 32995000 = 11998000 Hz  (in the middle of the crystal filter passband).

That’s how the original uBitx code works.

There is a problem because where an LSB signal hits the 45MHz filter will be 4khz removed from where a USB signal hits it, resulting in different audio quality between the two.


Metal case from Sunil VU3SUA (InKits India)

Sunil VU3SUA who is an active BITX20 list constructor has a business (Inkits) selling enclosures and electronic components for the Indian domestic and international market.  Sunil is now shipping his µBITx case.  This is a high quality metal case that is available in several colours: Maroon, Black, DA Grey, Siemens Grey, and Blue.

As you can see this is a very professional looking case.  The case will be  in high demand initially and is, therefore, only available on pre order.

Sunil’s website says that the first batch of pre order bookings has now ended and shipping has started (15th March 2018).   

We should see reviews of this product shortly on the BITX20 list.

Bookings for the Second Batch of pre order booking open now.  The shipping of second batch will start by end of March or Starting from 1st April 2018.

The universal case for the µBITx costs  US$34.99 with worldwide shipping via India Post costing US$20.    For a DHL  courier service there is an additional charge of  US$15.

Orders will be shipped in order of bookings.

For international orders from https://amateurradiokits.in/

For Indian domestic orders, go to http://inkits.in

Hiding a light (well a 20×4 screen) under a bushell!

VK3DAN has a µBITx and as soon as he received it he was busy modding it for a 20×4 (2004) screen. This sort of screen is easily substituted on the Raduino, but to use the extra display space requires modding the firmware.

Dan  has added band readout, his callsign, etc.  It also incorporates the CW sidetone fix, CAT system and various fixes written by Ian Lee KD8CEC. This firmware version won’t work with the 16×2 (1602) screen provided with your uBITX kit.   You will need to have purchased a 2004 type screen to replace your stock display.  You will also need to edit ubitx_ui.ino to change the callsign to your own.

More info at http://www.vk3dan.ninja

Download site:  https://github.com/vk3dan/ubitx-20×4