RF RMS to DC Converter - Page 1
Forum section: electronics / rf-microwave

author: Qmavam
author_rank: Regular Contributor
timestamp: January 16, 2021, 04:46:39 pm
content: Hi all,For many of us measuring RF voltages is not something we can do unless we have a scope and then measuring low voltages with high impedance sources is still beyond reach.What I would like to see developed is a high input impedance amplifier with an RMS to DC converter that we could connect to our Digital meter.I compare it to a project like Dave's µCurrent device.It probably needs to be voltage level switchable. If it worked to 10MHz, that's fine with me but, I think it would have a bigger market if it went to 30MHz.I suspect it needs a X10 scope probe input, but that means it needs an extra X10 gain, besides what we need to drive the RMS to DC converter.I have a little trouble thinking about a minimum voltage level, but smaller is better (a 5 micovolt setting?), and as to maximum, I'd rather trade low voltage measurement for higher voltages.Whatever is developed, the final voltage reading would be displayed on your Digital meter.Your thoughts?Mikek

author: radiolistener
author_rank: Super Contributor
timestamp: January 16, 2021, 07:06:59 pm
content: Quote from: Qmavam on January 16, 2021, 04:46:39 pmhigh impedance input is very bad for RF, it will be hard to do impedance match.The better way is to use 50 Ohm input.You can use cheap Chinese logarithmic detector modules to use it with DMM.If you need high impedance input, you can use active FET probe, something like that:https://www.radiolocman.com/shem/schematics.html?di=163113

author: coppercone2
author_rank: Super Contributor
timestamp: January 16, 2021, 08:21:33 pm
content: If you want a project the way to do this is jim williams themral RMS rc circuit, the problem is the IC is 35$ new old stock, but it does everything you want to a higher bandwidth.If you want it easy you buy a thermal RMS meter on ebay, that offers 2 digits of resolution. I have 1, mr carlson has like 5 lol .There is a hard core model that is like 5.5 digits and it a boat anchor also.But this is the older way to do it, the modern way is Delta-sigma converters. Might be less accurate in some cases (its accepted though), harder to do or more difficult to understand, but much cheaper. I don't know of a good ready made project for DSRMSC, but for thermal this is popular, at least to talk about,. The LT1088 is hard to get though.35$ from china on ebay, but the circuit is really cool. I want to say I got the thermal rms meter I got for... like $40. Hard to beat old.https://www.analog.com/media/en/technical-documentation/application-notes/an22.pdfand the attempt at counterfeithttps://www.eevblog.com/forum/projects/anyone-ever-attempt-a-diy-lt1088/What I want to see is a higher bandwidth thermal meter, say more accurate at 200MHz. But then you have nice RF power meters so its almost 'why bother'. Higher frequencies tend to run on ohms and power not voltage and current, other then the supply rails, unless you are doing something with slots. I.e. at 400MHz your waveguide size is only 23x11 inches, so thinking about voltage measurement approaches being not very useful.

author: radiolistener
author_rank: Super Contributor
timestamp: January 16, 2021, 09:34:41 pm
content: Here is logarithmic detector module for 4 USD:https://www.aliexpress.com/item/4000532223547.htmlit works up to 440 MHz. Usually AD8310 has 95 dB dynamic range, but this Chinese module use simplified input circuit, so dynamic range will be reduced a little, but still good enough.You can put it's output to a DMM and calculate dBm or RMS voltage value in excel.Max input is about 10 dBm. Absolute maximum rating is 18 dBm.You can use standardSMA attenuatorto measure more high power.Another way is to attach some cheap STM32 Chinese module with a cheap OLED display and use STM32 12-bit ADC to measure logarithmic detector output, calculate dBm and RMS voltage and show it on the display.Total cost will be about 10 USD:- 4 USD logarithmic detector module- 2 USDSTM32F103C8T6module- 3 USDOLEDdisplayFor more wide frequency range you can use AD8318 module, it works up to 8 GHz, but has less dynamic range:https://www.aliexpress.com/item/32798085351.html

author: coppercone2
author_rank: Super Contributor
timestamp: January 16, 2021, 10:07:09 pm
content: this is taken from a forum post on AD's websitehttps://ez.analog.com/rf/f/q-a/72271/log-detector-or-rms-detector

author: mawyatt
author_rank: Super Contributor
timestamp: January 17, 2021, 06:34:20 pm
content: We have an AD8307 (also a AD606) Successive Detection Log Amps. Just did some quick tests with the AD8307 using a AWG (Siglent 2042X) as the signal source (10MHz). The AD8307 has a ~0.300 volt output without any input, and a ~2.184 volt output at 0dBm input. The slope factor is ~25.2mv/dB and a simple straight line y=mx+b fits well over an input range of -56dBm to +15dBm, with b=2.184 and m=0.0252. This yields an intercept of -86.7dBm which agrees with the data sheet, as does the slope factor.The AWG allows sweeping the input amplitude with the AM modulation function which can be ramped, pulsed or use an arbitrary waveform. Using a linear ramp sweep revels the log relationship of the log amp as shown in the 1st screen capture. Using the AWG built-in Exponential falling waveform as amplitude modulation should linearize the log amp output and it does well over most of the modulation range as shown in the 2nd capture.Sweep in AWG frequency from 100KHz to 100MHz is shown in the 3rd capture. I'm not sure if the small variation over this frequency range is due to the AWG or the AD8307, or maybe some of both.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: January 17, 2021, 07:27:23 pm
content: Did the same testing with the AD606, it has a slope factor of ~38.3mv/dB and 0.122 volts without any input. The 0dBm output is 3.224 volts and the intercept point is -84.2dBm. Here's the linear and log amplitude sweep using the AWG AM function. Note how noisy the output is in the 2nd capture, this is because this PCB doesn't have much low pass filtering on the output, like the AD8307 PCB has.BTW the data sheet for the AD606 shows a simple RF Power Meter (Figure 7), it uses the ICL7136 3-1/2 digit DMM chip.Best,