Thermocouple detectors are definitely more linear, RMS, and have been around very long time. Some of the peak power meters use the diode versions. These typically have lower dynamic range. The HP8900D uses diode detectors for peak reading. Analog Devices bought Linear Technology but they are still selling independent product lines so the power detector IC's are there. We designed a detector based on one of their log-amp based power detectors that reads linear voltage for dB RF. It was a mistake and so I found the ADL5511 which has both voltage for dB in and also envelope. It also has temperature compensation to first order. Envelope is perfect for peak reading. We have built over 60 of these in a small pod, that has a PIC to take care of curve fit and they track our Keysight and Boonton meters on single frequency (which is what we needed). There are numerous companies now making USB power heads that go direct into a PC, like R&S, LadyBug (ex HP), Keysight, MiniCircuits. They all use the new generation ICs.
