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.

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.

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.

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.

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!