On calibrating up a ubitx, or "why the hell is this so complicated"

 I've built a few of ubitx/bitx40 radios over the last couple years. I built one of each for me, and a couple for other hams.

The biggest gotcha in this whole mess has been figuring out how to calibrate the thing. The ubitx has two oscillators to calibrate - the VFO and the BFO. The VFO is used for tuning, and the BFO is used for the third intermediate frequency (IF) to fall within the passband of the crystal filter.

However, exactly zero (0) of the PCBs I've received have been calibrated even remotely correctly, so I've had to go spend time figuring out how to do it myself. Unfortunately it's not been super easy - the instructions online make it sound way simpler than it really is, and unless you're super careful you're going to end up constantly being off frequency in SSB mode.

Ok, so here goes.

The ubitx radio is organised in a very specifically clever way for CW versus SSB, which makes calibration actually not TOO terrible. In SSB mode all three oscillators are used in a three stage IF design. In CW mode however, the transmit side doesn't bother with the SSB path at all - it directly unbalances the first IF mixer and transmits using the VFO oscillator directly on the transmit frequency.

So when calibrating the VFO, the reason it wants to calibrate it using transmit instead of receive is because it doesn't involve the BFO frequency /at all/. Instead, it's calibrating only the VFO frequency directly.

Now, what's it calibrating? Well, it's calibrating the crystal offset value, not a frequency offset in Hz. Ie, it's calibrating the offset between what you think 10MHz is when you program in 10MHz and what it's actually outputting. This affects all the clocks generated from the si5351 oscillator - not just the VFO!

This is important - when you calibrate the VFO, you're calibrating the whole frequency offset of the si5351, and that includes the BFO. Thus, if you calibrate the VFO, you will muck up your BFO offset.

When you do this stage, figure it out and write it down. I wish the offset was the whole 16 bit number so you could just dial THAT in, instead of how it's currently displayed (as a divided version of that, notionally ppm.)

I calibrate my VFO using a frequency counter and an RF tap. It's the only way to be sure. If you have a known-good receiver, you can tune it in SSB mode and do the math to figure out what the outputted tone will be at the given receive frequency. I've done this a couple times (before I bought a second hand frequency counter) using fldigi to display the audio spectrum, putting the radio into SSB mode and tuning to 9999.20 KHz. The 10MHz signal will show up as an 800Hz audio tone.

Anyway. Once you've done the VFO, it's time to calibrate the BFO. You have to NOT touch the VFO calibration at this point or it'll change your BFO calibration and you'll have to start again.

Ok, so the next oscillator is the USB/LSB frequency oscillator. The frequency is either ~57MHz or ~33MHz depending upon USB or LSB operation, so when the incoming signal mixes with this, the desired mixing product (either inverted or non inverted) shows up in the crystal filter passband. Now, in my v4 firmware I've made this also configurable, but by default it's a fixed value that they hope is in the passband of the crystal filter. I mention this only for completeness, you normally don't have to calibrate this.

The BFO calibration is more fiddly. This is the oscillator that when mixed with the output of the above stage brings the mixing product down to audio frequencies. (Ie, it's an SSB detector!)

When you start the calibration it'll reset to whatever the default is in the firmware, which is WAY off. When you start tuning in the BFO you'll hear the frequency spread of the background white noise shift as more of it comes into the passband, and .. then it'll disappear! And then as you keep tuning past the crystal filter frequency, it'll come back again! What's going on!

What's going on is that when you plot the frequency response of the crystal filter, it may not be exactly one big peak. A good example of it can be found here: https://ubitx.net/2018/04/14/ssb-crystal-filter-response/ .

Now, this is where things get a bit messy. When you change the BFO, it's not just changing the passband on the crystal oscillator. You need to make sure you're on the correct side of the filter so you're not getting an inverted copy of the mixing output.

The ubitx website suggests listening to an existing sideband transmission and figure out if you're on the right side by tuning the filter for maximum intelligibility. You can't use AM for this (eg WWV) because if you get the wrong side you'll hear a mirrored signal that sounds fine because it's AM, but you just won't be able to decode sideband.

What I do, since I have more than one good radio, is I hook up my ubitx to an antenna, my good radio to a dummy load, tune to them to the same frequency / sideband mode, set my transmit power to be like 5W on the good radio, and do a radio check. Yes, into my dummy load. The ubitx will definitely be able to hear it, and you can then set the BFO to maximise intelligibility and correct frequency centreing. You'll know for sure that you're on the right side of the crystal filter / mixing output because you'll definitely not hear yourself clearly otherwise.

After that? Well, I think we need a better way to characterise and calibrate both the USB/LSB frequencies rather than them being static as well as the BFO, and do them together. Ideally we'd be able to tweak both in order to definitely hit the right peak point of the crystal filter to minimise losses, but that will complicate the calibration quite a bit.

Anyway! Good luck calibrating your ubitx!

RFI from crappy electronics, or "how's this sold in the US?"

I picked up a cheap charging cable for my Baofeng UV-9S. (https://www.amazon.com/gp/product/B07TSDSQ4Z/). It .. well, it works.

But it messes up operating my radios! I heard super strong interference on my HF receiver and my VHF receivers.

So, let's take a look. I setup a little antenna in my shack. The baofeng was about 6ft away.

Here's DC to 120MHz. Those peaks to the right? Broadcast FM. The marker is at 28.5MHz.

Ok, let's plug in the baofeng and charger.

Ok, look at that noise. Ugh. That's unfun.

What about VHF? Let's look at that. 100-300MHz.

Ok, that's expected too. I think that's digital TV or something in there. Ok, now, let's plug in the charger, without it charging..

Whaaaaaaaaattttttt oh wait. Yeah, this is likely an unshielded buck converter and it's unloaded. Ok, let's load it up.

Whaaaaaa oh ok. Well that explains everything.

Let's pull it open:

Yup. A buck converter going from 5v to 9v; no shielding, no shielded power cable and no ground plane on the PCB. This is just amazing. The 3ft charge cable is basically an antenna. "Unintentional radiator" indeed.

So - even with a ferrite on the cable, it isn't quiet.

It's quiet at 28MHz now so I can operate on the 10m band with it charging, but this doesn't help at all at VHF.

Ew.

HF phasing noise eliminators, or "wow ok this mostly works"

I have a long standing issue with noise from a neighbour down the road that generates super wideband noise on the HF and low VHF bands. It's leaking out into the powerlines. It's pretty amazing.

Ok, so whilst the FCC goes and figures this out with the house in question, what can I do?

Well, one thing is to use a HF phasing noise eliminator. It's basically a pair of amplifiers and a phase inverting mixer. The "noise" antenna is a low gain antenna that's picking up the noise you want to filter out, but not sensitive enough to actually act as a longer distance HF antenna. You adjust the gain of your main input antenna and the gain of the noise antenna to be mostly the same level and then adjust the phase delay to cancel out the local noise source.

I bought a kit from Russia (http://www.ra0sms.ru/) and .. well, it worked. Pretty well. I used a discone antenna with a vertical radiator for 25/50MHz operation as my "very deaf" noise reception antenna and mixed in the signal from my HF antennas and .. well, it worked fine on 7MHz and 3.5MHz. I think it's a bit too deaf for 3.5MHz operation though; I bet it'd work better on higher frequencies because my noise antenna can start picking up more of the local area noise.

What about 1.8MHz? The 160m band?

Well, this is where the fun begins. The short version is - all I heard was lots of AM radio stations where they shouldn't be. The long version is - I am surrounded by super loud (>10kW) AM radio stations that you definitely need a bandpass filter for. The HF radios I have are good enough to filter them out, but the noise eliminator? It's doing all its mixing before there's any band filtering, so it's also mixing in all the AM broadcast band crap.

So this points out another couple things I need to do - I need to add a little bandpass filter on the noise antenna, and I need a proper HF preselector (ie, a narrow adjustable bandpass filter) on the main radio receiver. That way the transistors doing amplification/mixing in this phaser doesn't get swamped by crap that you're not trying to receive.

(For those who are radio inclined - I'm a few miles from 1100KHz (KFAX) which is a 50KW AM radio station - which shows up as S9+30 or more here. I have a bunch of others that show up as s9+20; so I have a lot of AM noise here..)

Restoring a TS-430S, or "dry joints and stray RF: a tutorial"

I recently acquired a TS-430S HF transceiver. The seller claimed the FM board and full complement of filters worked, but no display, buttons/LEDs or sound. He said it worked until he sent it in to have filters added. I figured it was going to be something simple. Boy was I both right and wrong.

These rigs have a habit of dry joints everywhere. So, I powered it up to see - yes, no display. Ok - step 1 - check power rails. I discovered there was no 5v line. The IF board has the 7805 regulator, so it is time to check for dry joints.

Oh look! Some very dry joints. I bet these were marginal until the tech installing the filters jostled it about. I fixed these any anything else I could find on the IF board.

I then powered it up. One digit showed up - the optional 10Hz digit - but all the lights and buttons worked.

Now, this rig has a separate PLL board for the main VFO which exports a signal that blanks the VFO output and the display. Amusingly it doesn't blank the final digit though. Ok, so it's likely PLL unlock. The PLL board was getting power, but ... no stable 36MHz base oscillator. That's on the control board. I pulled that out to find more dry joints around that circuit and its connector - so, fixed that.

I fired it up again. The PLL board was still unlocked even though the 36MHz oscillator was now working. I spun the dial and measured the other VFO feeding the PLL board - this is the fine grain frequency selection that gets mixed in to the PLL boards four VFOs to output the final VFO signal. It was moving OK - so the control board and the other PLLs were OK. Next - check the four VCO selection lines - nothing.

The PLL board has four varicap diode based VCOs and a PLL loop. The control board outputs the band select data to the RF board which decodes it and drives the PLL VCO, the relay based LPFs and the receiver HPFs. There were multiple issues - the control board bandpass lines were wrong and the VCO select lines were wrong.

Next - the RF board. Dry joints everywhere. Here is one of many that linked ground planes together.

And this one was on the VCO output connector.

I removed the TTL IC that did the BCD to output line demuxing because it was dead and fixed the dry joints. But the control board was still outputting the wrong band info. It turns out the IO expander IC that drives those four lines had two dead IO lines. So, that needed replacing too.

At this stage the control board was OK, the band select lines and VCO select lines are OK, but no PLL lock. Time to diagnose the PLL board.

First up - the varicap VCO was working. Wrong frequency but working. The circuit takes the output of that, buffers it though a transistor amplifier, shapes it into a square wave and divides it down via a pair of TTL chips and feeds it into the PLL control IC.

Next - the 5v line on the PLL board was ... suspiciously low. 5v was coming in OK, but something was dragging it down to 3.8v in places. That is too low for TTL. I checked each chip and... the 75S112N flip flop chip was running hot. Ok, so that needed replacing. Note it is S and not 74LS - the PLL loop runs from 45 to 75MHz, so it needs speed. With that chip replaced the 5v rail was again at 5v. But, no PLL lock.

So I then traced the PLL loop. VCO was OK. VCO though the buffer amp wasn't. I pulled out the transistor there and it was open circuit. I didn't have an equivalent so I found a close enough one for now and ordered a replacement. But then it was sill not working right - the signal level into the TTL NAND chip was super low. I figured either the transistor I replaced it with wasn't biased right or the TTL chip was pulling its input low. Indeed it was the latter - the input side was shorted to ground. I replaced that chip and the rig sprung to life!

I recalibrated the four VCOs now that I had replaced some parts. It was locking OK on all bands.

But - the receive signal was low. I checked the attenuator switch - no go. I disconnected the attenuator control cable to the RF board - RX sprung to life! A little solder reflow on the switch board and that fixed that.

After that I just did the obligatory filter and finals board check and reflow.

One LPF relay clean procedure and finals alignment later and it's all ready to go. The SWR foldback protection needs fixing and I need a 150 ohm dry load to do that, so that's my next week project.

As to how those parts all failed, likely at once? My guess is stray RF fried a path somehow. I'm glad this was the extent of the part damage!

FreeBSD and APRS, or "hm what happens when none of this is well documented.."

Here's another point along my quest for amateur radio on FreeBSD - bring up basic APRS support. Yes, someone else has done the work, but in the normal open source way it was .. inconsistently documented.

First is figuring out the hardware platform. I chose the following:

• A Baofeng UV5R2, since they're cheap, plentiful, and do both VHF and UHF;
• A cable to do sound level conversion and isolation (and yes, I really should post a circuit diagram and picture..);
• A USB sound device, primarily so I can whack it into FreeBSD/Linux devices to get a separate sound card for doing radio work;
• FreeBSD laptop (it'll become a raspberry pi + GPS + sensor + LCD thingy later, but this'll do to start with.)

The Baofeng is easy - set it to the right frequency (VHF APRS sits on 144.390MHz), turn on VOX so I don't have to make up a PTT cable, done/done.

The PTT bit isn't that hard - one of the microphone jack pins is actually PTT (if you ground it, it engages PTT) so when you make the cable just ensure you expose a ground pin and PTT pin so you can upgrade it later.

The cable itself isn't that hard either - I had a baofeng handmic lying around (they're like $5) so I pulled it apart for the cable. I'll try to remember to take pictures of that.

Here's a picture I found on the internet that shows the pinout:

Now, I went a bit further. I bought a bunch of 600 ohm isolation transformers for audio work, so I wired it up as follows:

• From the audio output of the USB sound card, I wired up a little attenuator - input is 2k to ground, then 10k to the input side of the transformer; then the output side of the transformer has a 0.01uF greencap capacitor to the microphone input of the baofeng;
• From the baofeng I just wired it up to the transformer, then the output side of that went into a 0.01uF greencap capacitor in series to the microphone input of the sound card.

In both instances those capacitors are there as DC blockers.

(I'd draw up a circuit diagram but for some reason there's no easy tool here in blogger to do that in-line! Sigh.)

Ok, so that bit is easy.

Then on to the software side.

The normal way people do this stuff is "direwolf" on Linux. So, "pkg install direwolf" installed it. That was easy.

Configuring it up was a bit less easy. I found this guide to be helpful:

https://andrewmemory.wordpress.com/tag/direwolf/

FreeBSD has the example direwolf config in /usr/local/share/doc/direwolf/examples/direwolf.conf . Now, direwolf will run as a normal user (there's no rc.d script for it yet!) and by default runs out of the current directory. So:

$ cd ~
$ cp /usr/local/share/doc/direwolf/examples/direwolf.conf .
$ (edit it)
$ direwolf

Editing it isn't that hard - you need to change your callsign and the audio device.

OK, here is the main undocumented bit for FreeBSD - the sound device can just be /dev/dsp . It isn't an ALSA name! Don't waste time trying to use ALSA names. Instead, just find the device you want and reference it. For me the USB sound card shows up as /dev/dsp3 (which is very non specific as USB sound devices come and go, but that's a later problem!) but it's enough to bring it up.

So yes, following the above guide, using the right sound device name resulted in a working APRS modem.

Next up - something to talk to it. This is called 'xastir'. It's .. well, when you run it, you'll find exactly how old an X application it is. It's very nostalgically old. But, it is enough to get APRS positioning up and test both the TCP/IP side of APRS and the actual radio radio side.

Here's the guide I followed:

https://andrewmemory.wordpress.com/2015/03/22/setting-up-direwolfxastir-on-a-raspberry-pi/

So, that was it! So far so good. It actually works well enough to decode and watch APRS traffic around me. I managed to get out position information to the APRS network over both TCP/IP and relayed via VHF radio.

Fixing up TS-440s rigs

I've been teaching myself HF electronics and antennas lately. I thought I'd go write down some of the things I came across whilst tinkering with this hardware.

First up - I decided to choose the Kenwood TS-440S.

http://www.rigpix.com/kenwood/ts440s.htm

It's a mid 1980's solid state rig with only a few components hidden away in custom ICs. There are some parts that you just can't buy new anymore (mostly these custom parts and stuff in the final RF amplifier section) but by and large it's all a big set of interconnected single-sided PCBs covered in cables and discrete components. There's the occasional bit of 74LS logic too.

It has some pretty clean and sensitive RX paths and TX is supposed to be very good for hours and hours of work. However, these devices are pretty old, and 30+ year electronics can have a large amount of wear and tear.

There are some pretty well known crappy issues too. One of them is the SonyBond compound used in the VCO (voltage controlled oscillator) sections - it's hydroscopic and degrades over time. This ends up throwing the VCOs way off and you end up having to clean it off of the PLL/RF boards. This is pretty well documented and although time consuming, it's not impossible to fix yourself.

I'll try to do a follow-up post with some pointers on tuning the TX path behaviour because I found a lot of really inconsistent suggestions on tuning things. The service manual is also a bit confusing at times on what voltages you should see where.

There are also issues with broken connectors and a lot of dry joints. I found a lot of dry joints in my 440s, which required a lot of tidying up.

Next up in the TODO list for me is tuning the RX sensitivity to make sure it's optimal.

So, what I found with the first rig I fixed up! I wish I had taken more photos; I'll do that soon just for demonstration purposes.

• There were lots and lots of dry joints. Everywhere.
• Sonybond needed cleaning up - and when you do that, you have to tune VCO1 - which is the VCO generating the primary RF frequency for first stage mixing. This is used for both transmit and receive.
• The IF board was putting out a nice, clean 8.83MHz carrier, so that was nice.
• The transmit path hadn't been aligned in a long while, so the waveform on the output of the RF board was splatting overly large signals (> 3v peak to peak) into the finals, and this caused the finals to get very upset.
• .. and then ALC wasn't adjusted, so the filter board was overly aggressively asserting ALC.
• The transmit power control was very touchy - 100% carrier level at like 5% turning of the carrier level knob. Once the transmit path on the RF board was properly aligned, the RF drive output was properly putting out around 2.5v ptp -> 3.0v ptp and ALC was re-aligned on the filter board, the finals were behaving much better and the TX power control was much more linearly controllable.
• Then you have to re-do the finals bias level controls. Careful though, the alignment process says "turn to minimum first" before you re-bias things, but the board is mounted upside down so you may be biasing them to max. :)

I'll go into some more detail later with pictures!

gqrx, direct sampling configuration, shortwave/AM reception, etc

This is one of those articles that I write specifically to, you know, not forget it the next time.

I picked up a v3 RTL-SDR dongle from http://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-guide/ a few weeks ago. It's a solidly looking aluminium can dongle with a much more useful RF connector. But, it still only tunes down by default to the same RTL-SDR limits as .. well, the rest.

So I go digging. I bought this thing because on Windows you're supposed to be able to tune down to a couple hundred KHz. I found out some useful stuff after a bit of google abuse:

• it's called "direct sampling mode";
• it's supported by the rtl_sdr driver/library that you normally use;
• you have to configure things up in a special way to get GQRX, etc to actually do the right thing;
• There's a specific thing called "Q-Branch" that you have to care about.

Ok, so the actual details.

Firstly, the NIC needs to go into direct sampling mode. So instead of being mixed with a VCO, you're bypassing all of that and just acting as a really fast ADC. Which, yes, will alias signals everywhere.

Secondly, you have to tell it to only give you "Q" samples, rather than both I and Q. If it's direct sampling, mixing doesn't come into it, so the hardware is just giving you ADC samples.

Then, you just tell GQRX, etc to ignore enforcing tuning limits (tick "No limits") and you're golden.

The specific string to add to the device string when you set up which rtl_tcp instance to talk to is:

direct_samp=2

(0 = no direct sampling; 1 = direct sampling on I, 2 = direct sampling on Q.)

Ok, so does it work? I'm still testing it. I will need to acquire some RF filters to try and filter out bands of HF frequencies to avoid aliasing as there is limited to on actual passband filtering going on here. But it did tune to AM radio and I picked up some data transmissions in 3MHz.

Baby steps!

gqrx on freebsd

I'm still tinkering with my own SDR code in C, but I decided I'd give gqrx a go. It sports a simple UI to visualise things as well as a simple control network protocol.

FreeBSD has packages for it, so it really was as simple as:

# pkg install gqrx

I already have the rtl-sdr bits installed, so that was pretty easy:

# rtl_tcp

.. to bring up the network service for it.

But running gqrx .. didn't work. It just stood there and did nothing when I ran it - no UI, no useful log output, nothing.

So out came truss. Step one in the "what is going on" showed no useful logging. Step 2 is "ok, what syscalls is it making." It turns out it was trying to connect to the IPv6 localhost address - but rtl_tcp defaults on FreeBSD to IPv4 localhost. gqrx doesn't handle connect() errors gracefully, so it was just stuck in an infinite connect() loop.

So I cleared the config, started gqrx again, and ensured this time it was set to '127.0.0.1' instead of 'localhost'. Voila! It worked!

Now - I need an SDR unit that can go down closer to DC so I can pick up shortwave. This RTL dongle I have doesn't tune below 27MHz.