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Found a new useless electronics project to waste time in 2022. Sinewave leveling to 0.3 dB flatness. Likely won't be successful - $10,000 oscilloscope calibrators exist for a reason. Even verifying its flatness is a challenge. But hopefully I'll ended up knowing more than when I started...

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The variable RF attenuator turned out to be a more difficult problem. Ideally it needs to work to DC (50 kHz is often used as a reference signal so you can calibrate the calibrator accurately), but it also has to support 24+ dBm of input power. There is almost no suitable IC for that. Perhaps the solution is to solve the problem in reverse - not to attenuate a strong signal but to amplifier a weak signal. Then distortion becomes the new problem...

It turns out, it's really valuable for this application to target a signal generator with AM input that supports DC, so you only need to implement a power detector and an error amplifier, amplitude can be controlled at the source. Otherwise, you need to do some stupid signal conditioning with your own variable gain amplifiers and attenuators... At this point you have basically already created a convoluted version of a signal generator's entire output stage. And at this point you may as well put a DDS synthesizer into it and convert it to complete full signal generator...

When you see the circuit you are working on in a paper hosted at LIGO.org, you know you are messing with something you're not qualified to do...

The high power is not just a problem for attenuaton, also RF amplifiers. I found The only type of high power, 1 GHz monolithic RF amplifiers that covers both RF and DC is a distributed microwave amplifier, which is extremely expensive (they can work at 6, 10, or even 20 GHz) and definitely overkill. The output also requires a user-supplied bias-T - true DC operation is still not practical.

A two-path design looks sensible now. Ideally if the power meter is at the output side, the frequency response in the entire input chain should not matter.

I just realized I miscalculated the power requirements due to Vp and Vpp confusion :blobfacepalm: . The actual power level I need is only 18 dBm, not 24 dBm. More margin, and even brings the power within the absolute maximum of the variable gain attenuator candidate. But still not enough to solve the attenuator P1dB issue in a "leveling down" design, still need amplifiers to "leveling up".

IDT/Renesas has a high power RF variable attenuator from 1 MHz to a few GHz. A two-path design looks ideal here, the attenuator does 95% of the job, removing the need for an expensive and error-prone RF gain block, a simple opamp handles the rest of the job in the DC-LF range. The only nuisance is two independent Automatic Level Control loops are needed.

Hmmm, modern RF power meters are quite accurate. The measurement uncertainty is around 0.05 dB within +20 to 25 degrees C. Renting a calibrated meter for a few weeks can be the test plan, but I need to be really careful... Blowing the meter up is equivalent to burning $2000.

For these kinds of measurement devices, what people are really paying for is not the hardware, not the software, not the technology or perhaps not even the service. It's all about a single piece of paper called a NIST-traceable calibration certificate...

The legendary LT1088 thermal RMS-to-DC converter chip was like 1% accurate to 50 MHz and 2% to 100 MHz, that is 0.2 dB assuming ideal match. Even today it's still a really accurate RF power meter. If I can get an LT1088 and shows it agrees with my leveled sinewave generator to 1% that would be the end of the story, at least to 100 MHz.

Too bad this chip never sold well and Linear discontinued it decades ago.

Nope. The IDT/Renesas attenuator's 1-dB compression point is also too low below 50 MHz. Still have to go back to a 2-path LF/RF design...

Block diagram of the proposed leveled sinewave generator. Basically just an ALC/AGC loop found in all signal generators and radios, but must be accurate enough within a fraction of a decibel. The two-resistor splitter looks like a joke but it's really how an RF ALC loop works.

The log amp actually requires some calibrations to reach the accuracy needed in my leveled sinewave generator design. And what do you need for the calibration to begin with? A leveled sinewave generator...

I think a VNA can solve this problem. The beauty of a Vector Network Analyzer is that you don't need a perfect signal generator and signal path, it measures and removes all errors by itself via two-port calibration.

Finally decided the architecture of the RF gain stage. After much flip-flopping between an one-path or two-path design, ultimately I found no MMIC simultaneously satisfies the requirements of high output power (5.5 Vpp, or 19 dBm, this basically means the majority of MMICs, the standard 5 V parts, are simply not an option) and DC-to-GHz operation (50 MHz and below are not covered by suitable MMICs). A separate Low/High Frequency path is inevitable.

Oh no, the PHA-202+ MMIC produces 4.5 watts of heat from a tiny 6 mm x 5 mm QFN package. The circuit board is gonna be a frying pan and it would also change the operating point of the RF power detector, this is not going well...

This layout sucks. But the MMIC (U8) gets extremely hot in operation, the sensitive detector need to stay as far from it as possible. I'm not even sure if it's enough. I need to get one of those MLX90640 IR imagery sensors...

Evil RF circuit board hack - When you can't get the ideal impedance for the given stackup, just remove copper in layer 2 and force the RF signal to use layer 3 as the reference plane.

First logarithmic converter RF power meter prototype, to be used in the feedback loop of the leveling sine wave generator. Performance is underwhelming. There's up to 17% variation in frequency flatness in the uncalibrated frequency response.

But I cannot be sure whether it's really coming from the RFIC. My radio analyzer itself is out of calibration. Time to get an expensive RF power meter to find out... 17% looks like a lot but it's just 0.7 dB, not really bad, but not good enough for me.

Ah, I think I've found a cheap alternative to the RF power meter to solve this problem - crystal detectors. HP used to sell RF diodes with guaranteed specs as coax plug-ins for simple RF power control or demodulation applications. Unlike a specifically designed power sensor, these bare-metal diodes don't have any accuracy guarantees or calibration. But absolute accuracy doesn't matter, I only need relative flatness, which is just 0.25 dB for the HP 423A/B. That's great for my purpose.

Last time I evaluated my ADI log converter board and found its frequency unflatness was up to 17%. But I couldn't be sure whether it was really coming from the RFIC since my radio analyzer itself is out of calibration.

The HP^H^H Agilent diode detector bespeaks the truth. It measured a similar response. The variation is up to 60%, but it's power not voltage. Taking the square root gives 26%, close enough. My PCB and the ADI chip have acquitted on all counts...

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