25 April 2017

More on HB100 10 GHz frequency agility

In the previous post I wrote about measuring how much a stock HB100 radar module can be moved in frequency. I did another test to answer "where does the frequency go if the metallic shield is removed?"

This test is purely academic since the exposed DRO is much more sensitive to hand/object proximity and temperature changes, therefore unsuitable for most - if any - HAM radio applications. I got a delta of about 360 MHz, so the naked circuit was transmitting somewhere around 10'165 MHz.


21 April 2017

HB100 10 GHz module frequency agility

Before I can get my hands on two Sat-TV LNB's and work on a 10 GHz WFM transceiver, I wanted to measure the frequency agility of a stock HB100 radar module.

The plan was to use one module as a fixed receiver on the factory preset (presumably 10.525 GHz) and tune a second module. Remember that HB100 is both a TX and a RX at the same time, and it emits a continous carrier. If the two TX frequencies differ, their difference will be visible at the ultra-broadband (unfiltered) I.F. output.

At first I connected the IF output of a receiver module to the frequency counter (100 MHz), and I could read up to 10 MHz delta.

Then I routed the IF output to the oscilloscope (100 MHz bandwidth, 10 MHz probe) and, as long as the signal was above 1 mVpk the sine wave period indicated a signal of about 20 MHz.

Time to turn on the spectrum analyzer, that starts at 45 MHz. Bingo! The maximum delta I could get between an untouched HB100 (let's assume 10.525 GHz) and a fully unscrewed tuning harness was 232 MHz, so it was operating roughly on 10.293 GHz.

Also, the closer the tuning screw is to the DRO, the higher the frequency. Screw in to go up in frequency, unscrew to decrease frequency.

Last but not least, I confirm that the receiving mixer works better if it is terminated on some medium impedance. I will try to visualize the signal on the oscilloscope in parallel to the S.A.

14 April 2017

Measuring frequencies around 10 GHz

While changing the frequency of HB-100 10 GHz radar modules is pretty simple, it is not so easy to understand where exactly it has been set. Unless you have got access to an expensive frequency counter, of course.

A common workaround seems to be using a satellite TV LNB so that the whole 10 GHz band is downconverted below 2 GHz, which is more affordable to be measured even with an RTLSDR dongle! As long as everything sits in the same room, there should be enough signal to do meaningful measurements.

I need to dig out one of those LNB's and do some cut and solder.

10 April 2017

Changing DRO frequency: add dielectric (hypothesis)

In order to bring HB-100 radar modules into the 10 GHz HAM allocation, their frequency has to be reduced. Reports say that the stock screw is enough, better if replaced with one with finer thread.

Then I investigated how Dielectric Resonator Oscillators work (wikipedia, no more no less) and tried few simulatons with the provided formula: the more dielectric material, the lower the frequency.

So, assuming that we have some "extra" dielectric laying around, like from a similar (dead) module, it could be worth trying to add it on top and see the resulting frequency. Besides providing a sort-of fixed frequency, it would reduce the number of factors that are influended by temperature and cause frequency instability.

For first experiments I will stick to the screw method. Reports of success/failure are welcome.

02 April 2017

Just ordered my first PCBs

After fiddling with KiCad for few month, reviewing the same circuit over and over again, I decided to stick with a relatively general purpose PCB for four ZM1332/NL5870S nixies. It is a multiplexed design that includes the decoder IC on-board (7441 or equivalent). Anodes must be multiplexed on the logic board since everyone has his preferred way of doing it (pnp, optoisolator, pmos).

This is the 3D render (by KiCad) of the boards I have ordered on firstpcb.com:
Since I was not satisfied with the result of the embedded autorouter I did it myself. I have never designed a 2-layer PCB, so I used this extra degree of freedom only when I was stuck. I ended up with only two vias, and other transitions were handled at pads when needed.
ZM1332 cold cathode displays are small and not too tall, so I put all components on the back side of the board, leaving only tubes on top. There are no overlapping components, so it won't matter which side I start soldering.
In order to simplify routing I have remapped the outputs of driver IC to actual digits, so this will have to be taken into account in the firmware. The two headers mantain a 0.1" spacing even if they are far apart.

I am really curious to see the resulting boards, and to build them of course! By the way, the size is 10x5 cm.

If all goes as planned, I will publish what is needed to reproduce this project. Fabricator emailed me they should ship on April 7th, 2017. No idea how long it will take to get here!



30 March 2017

Challenges of the HB100 10 GHz module

As listed in the previous post, the HB100 10 GHz sensor module poses some challenges when repurposed as an RTX. Let's list them, and possible workarounds.

1) The RF power is in the order of 10 mW (10-12 dBm).
On 10 GHz it is easy to assemble and use a high gain antenna. Some people report that an IKEA lamp has the perfect shape. Just remember that higher gain means narrower beamwidth.

2) Frequency stability was not a design goal for the original destination of use.
Frequency instability can be tamed with proper thermal insulation of the module. The more the merrier. In any case all narrow band modes are out of question. WFM is the way to go.

3) Receiver is direct conversion.
This one, combined with #2, is a bit harder to tackle. You can't do CW or SSB. You can't do FSK. The solution proposed by a U.S. HAM is to work full-duplex. Transmitters are on different frequencies. The received signal is then at an Intermediate Frequency equal to TXQRG difference. If the difference is about 88-108 MHz, you know which WideFM receiver can be used! Actually an RTLSDR dongle receiver allows more frequency agility and flexibility, thus allowing to operate I.F. outside the crowded FMBC band.

There are other reports that HB100 is sensitive to microphonics (mechanical vibrations are picked up and turned into electrical/RF signals). Not hard to keep under control, either.

29 March 2017

Easy way to 10 GHz

I read it on hackaday, then again on G3XBM's blog: there is an easy way to play with 10 GHz. And (very) cheap too!! The idea is to repurpose something originally meant to be used as something else. Like the RTLSDR TV dongles, the 74HC240 buffer and many more in this wonderful hobby.

The HB100 is a microwave sensor module designed to be used as motion and speed (doppler) detector. It operates on 10.525 GHz and can be retuned below 10.500 GHz into the 3 cm HAM band (Italian bandplan) with a screwdriver. It can be frequency modulated through the power supply (I guess you get some AM too). It features both the transmitter and the receiver, (patch) antennas included.

How much? Less than 3 EUROs including shipping. That's three espresso coffees standing in an Italian bar. Or three of the cheapest burgers in the "M" restaurant (their own definition, not mine).

Drawbacks (A.K.A. "challenges"):
1) The RF power is in the order of 10 mW (10-12 dBm).
2) Frequency stability was not a design goal for the original destination of use
3) Receiver is direct conversion

I have ordered 2 pairs and a spare one. I am curious how far the unmodified version will go.

By the way, I have spotted a similar radar device operating on 5.8 GHz and others on 24 GHz (InnoSent IPM165). Maybe ... ?