Reverse engineering a 23 GHz oscillator module (P/N 408007-601) - Page 1
Forum section: electronics / rf-microwave

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 21, 2024, 08:45:14 am
content: I bought a weird RF module at a ham flea market for $5. After doing some research, I found out that it is a 23 GHz oscillator module, used as part of a PLL system for upconverting/downconverting signals. Unfortunately, all five pins on the underside were ripped out. I was able to superglue 2.54mm pin headers to the back of the module, and carefully solder back the wirebonds to the pin headers. It was quite difficult, as the solder will quickly eat away the gold wirebonds, so you must only apply heat for a fraction of a second. I failed to connect the VCO tune input, so I had to solder a thin wire to a roughly 0.25 by 0.25mm pad on the PCB. Took me a few tries and it was very annoying.Anyways, after successfully soldering all the wires, it was time to reverse engineer the board. I took many pictures with a microscope, and stitched them together to get the first picture. It was fairly obvious which each connection did after looking at the picture. The only thing left to do was figuring out the voltages for the amplifiers.The board first started to work when the 23GHz power amplifier's power was set to 3V and the 11GHz PA + oscillator had 4V. I was able to get the PA + oscillator to 7.5V, but I noticed diminishing returns in the output power after around 7 volts, so I decided to keep that at 7V. The 23GHz power was definitely smaller since the die would probably have to be smaller. I noticed that the power increased up to around 4V, but increasing further to 4.5V led to no difference in the output power. The final power consumption is 177mA for the 11GHz PA + oscillator and 66mA for the 23GHz PA. I got around 6dBm out with these voltages. Given that the original module ran off 8.4V, and there is probably some regulation, 7V seems reasonable. The tuning sensitivity is about 2MHz/V.It might be time to spin a board and try to get everything phase locked.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: July 22, 2024, 04:55:39 am
content: So, they used 11.5GHz series feedback DRO, amplified signal using first balanced amplifier with lange coupler (https://www.microwaves101.com/encyclopedias/quadrature-couplers), filtered second harmonic 2x11.5=23GHz and amplified output again with another balanced amplifier. Looks like a very straightforward approach without putting too much effort into optimizing second harmonic output, except multiple tuning stub patches.Nice photos and it's very educational. We can see that lange coupler "finger" of first amplifier are roughly twice longer, because of half-frequency. Also, it seems that soldered tuning patches of metal used for tuning are placed half-wavelength apart along the line between first amplifier and microstrip filter. Maybe it was easier to achieve tuning this way by distributing one single stub into smaller ones at impedance repeating points. Instead of sildering one large stub which is very sensitive to positioning and has a nasty air gap, soldering smaller stubs seems reasonable. Plus, after soldering each patch we still have a freedom to tune phase position of each consequent patch. Or maybe there are some other reasons.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 22, 2024, 09:01:37 am
content: I believe that there are active devices involved in doubling the frequency.I think that a microstrip to slotline transition is used to act as a balun, and that there is a diode rectifier on the back of the board. The same trick is probably used with the mixer (I am 100% confident that the top left board is the mixer). This method is a lot more efficient than other multiplier designs, and there is not a lot of harmonics.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: July 22, 2024, 03:10:58 pm
content: you are 100% right, there is something much more effective used for frequency doubling. Thanks for your explanatory drawings