HB-100 vs Voltage

Having built a DFCW modulator for the HB-100 controlling the operating voltage, I could observe that a difference of +130 mV in the supply voltage causes about +200 kHz shift. The higher the voltage, the higher the frequency.

Besides the lack of temperature stabilization, switching voltages affected negatively the resulting RF frequency, causing all sorts of short-term drifts. Most likely I will switch to FM-modulated CW (audio tone over the carrier) once I want to have a real QSO and not just a reception test.

HB-100 vs Temperature

My simple observation of a single HB-100 module is that the frequency increases as temperature decreases. 

I think it has gained 150 kHz while cooling of 15 °C, so -10 kHz/°C. That's just one test of one specimen, YMMV, even MMMV. In any case the temperature should be stabilized as much as possible, at least for short term variations, by adding thermal insulation.

I wonder if the relationship between temperature and frequency is deterministic, therefore either parameter can be inferred if the other is known: frequency-based thermometer, or temperature-based frequency estimation.

There is life on 10 GHz around here

I am really excited: I received HAM signals on 10 GHz!

I needed to add a T-bias to my RTL-SDR box and the longest part was drilling a hole for the pass-through capacitor.

Here's the quick setup on my crowded desk: can you see the LNB and the RTL-SDR?

 

What did I receive, beaming free-hand the LNB out of the window? Three beacons! And two of them were bouncing off the mountains surrounding Torino.  The recording has HB9EI/B ID signal at 147 km while both the LNB and the RTL-SDR were warming up, so it drifts:

Cool! Next days I will try QO-100 satellite, but I need to move everything on the other side of the house.

Finally measuring the frequency of my HB-100's

Yesterday I wired up the LNB to a TV-spectrum analyzer to check that the preliminary setup would work. Besides an occasional short into the coax that trips the power-over-line circuitry, the LNB noise is visible. Then I fired up my HB-100 transmitter, that has been sitting in a box for months: its modulated signal peaked at 621.45 MHz on the screen.

So, if the LNB L.O. is at 9750 MHz, my TX is at 10371.45 MHz. That's well within HAM allocation.

Then I measured the HB-100 I had used as a receiver, which sat at 694.70 MHz = 10444.70 MHz. This confirms the delta of 73 MHz I observed in May 2017 when doing my first experiments.

Last I measured an apparently untouched HB-100, my third and last sample, and it could be received at 676.35 MHz, meaning 10.42635 GHz. Knowing me, I think I had left this spare radar module untouched, as a reference.

So, if the assumption of 9750 MHz LO is correct, all my HB-100 modules fall within the 3 cm HAM band (10.0-10.5 GHz), even if I was expecting a "brand new" one to be at 10.525 GHz. Hypothesis: I bought off eBay HB-100 modules that were rejected because they did not meet the spec on the operating frequency, and luckily were too low. Unfortunately I haven't found someone that published a measurement of their frequency.

I need a beacon of known frequency to confirm these relative measurements. Fortunately there is a beacon in L.O.S. from home and QO-100 satellite few degrees away. It will not give me accurate frequency values, but enough "ballpark precision" to allow a fellow HAM to quickly tune on my TX frequency and complete a QSO.

Returning to 10 GHz

I am again doing something at 10 GHz.

During the journey to Montichiari HAM fair, fellow Pino JNS showed us how easy it is to receive the geostationary satellite QO-100 with a PLL-LNB and a small dish. The "last mile" is then done with the ubiquitous RTL-SDR.

Now my HB100 radar modules have a valid RX companion, so a 10 GHz RX-TX setup can be built and operated. It will be WFM or CW (A1A), but at 10 GHz and with very simple RF parts. The total cost for RF is about 30 euro.

I will initially try to receive QO-100 and a 10 GHz LOS beacon. Once I confirm the LNB is stable and spot-on frequency, I will tune my HB-100 modules into the 3 cm HAM band to a known frequency. Then again a short distance test, 200 metres "DX", and who knows what will be next.

Thinking out of the box Telecommunication test for millennials in IT

I've just done this at work, using some tricks that were daily routine back in the days of dialup. I wonder how a sysadmin half of my age would tackle this problem.

You are logged into a Linux server through a sort of corporate menu-driven shell proxy and you need to get a 10 MB binary file out of it for local troubleshooting purposes. The shell proxy and firewall do not allow your unprivileged account to ssh/sftp directly into the machine and you don't know its password either. There is a webserver but your unprivileged user has no write rights to its folders. You try to mail the file to yourself with the "mail" utility, but the test message you try before going all the way never reaches your mailbox into the organization. The server cannot do outbound ssh/ftp either.

So, how do you get the file on your local computer? Ask for further details and share your ideas in comments (moderated).

GPS Week Number Roll Over, 2019 edition

I discovered this detail of GPS raw signal in 2010, when my Garmin Geko 201 showed a wrong date. The guys behind GPS (not Garmin) implemented the transmission of a 10-bit weeks counter value that rolled over in 1999, so that some devices began showing a date that was 1024 weeks behind.

It will happen again at the end April 6th, 2019 (UTC).

(Somehow) The week counter has been upgraded to 13-bit, 157 years (but week #0 is still back in 1980-01-06). Probably most devices using GPS location data built in the last 19 years have a software way to handle the rollover, so you will not be affected. Probably. Hopefully. I know too many lazy developers to be 100% sure all my devices will not be affected.

Unless the device is tested with a fake GPS signal ahead in time, all we can do is to wait for the first full weekend of April 2019.

What's worse, the device might have an embedded lookup table "GPS week vs real week" that is 19.5 years long and would roll over any time in the future. And, in most cases, all these devices are unlikely to get a firmware update.

See you in week #0 :)

Weird behavior of ZS-042 RTC modules (DS3231)

First of all: all these ZS-042 RTC modules were paid about 1 USD each and come from not-certified sources. 

One night I decided to measure the 32K output of some DS3231 ZS-042 RTC modules. I wanted to see what difference would make if I retouched the aging register. This experiment lead to few unexpected discoveries.

1) Maxim DS3231 datasheet mentions 32K output and 32.768 kHz interchangeably. Alright, it is a digital device, so 32K equals 32768 if K=1024 as used for bytes. But we are talking of Hz, not bytes, so I think should be K=1000. Proof? Search for "768" in the datasheet.

2) There are two quite different devices that share 6 characters of the part name - DS3231 - and are distinguished by their suffix: M or SN. The former "M" is 5 ppm with MEMS resonator, while the latter "SN" is 2 ppm with TCXO. In simple terms, "SN" is better than "M". But you can't choose when buying ZS-042 modules and you will receive either model

"M", "SN", "M":notice the deeper blue PCB of "SN"

3) All my ZS-042 modules with DS3231M output a frequency close to 32.700 kHz, while those with DS3231SN are close to 32.768 kHz. The "M" would be loosing 180 seconds/day, 1.5 h/month, but none of them does! Actually I have one DS3231M in my longest running Nixie clock and it is within 5 ppm specification.

Measuring "32K" output of DS3231M.

So, let's forget DS3231SN that are most likely second-hand/pulls but legit. How can a DS3231M that outputs 32700 Hz be so accurate as I observed? If it is genuine, Maxim has a way to calibrate the chip once, before it is sold. If it is a fake, then it is microcontroller with proper firmware and calibration to count the correct number of interrupts. I could sacrifice one and decap it, but then I lack the microscope to examine the silicon die.

Conclusion. Buy the DS3231SN if you need an interrupt rate of 32768 Hz. Otherwise for hobbyist use, they are both worth it.