RF signal level meter

[stilwellj1983]

I have built a simple RF signal generator to create a sine wave at 10 MHZ and 65 dBmV. I currently use a off the self JDSU DSAM to measure the signal. I would like to build a analog or digital device that when plugged into the same cable will give me a dBmV reading so that I can measure the loss in the cable and determine if there is a problem. I am having trouble getting started with this I have been doing some research on line and I can only find information on build of 46 MHZ or higher. Any help would be greatly appreciated.

[KJDS]

From what I posted in this thread

https://www.eevblog.com/forum/testgear/homebrew-return-loss-bridge/msg401143/#msg401143


Use BAT63s, use one for RF detection and the other with an LTC1051 or similar low offset op amp for temperature correction.
https://www.infineon.com/dgdl/bat63series.pdf?folderId=db3a304314dca389011518104e5d0df2&fileId=db3a304314dca38901151817843c0df4

So from the RF input, a series dc blocking cap and then one half of the dual BAT63 shunt to ground, put the cathode to ground as bias current from an op-amp generally flows out of it*

Then just put the other half of the diode pair in the feedback path to give temperature stability, otherwise you'll have something that's a better temperature detector than it is for RF power. I'd then use another LTC1051 to provide a gain of 100 or so to provide a useful output voltage.

Having typed that out, I've just checked and found the TLC2652 is still available, I thought it had gone EOL years ago and that gives better results than the LTC1051. Use the A version if you can get hold of it.

[JackOfVA]

What is the range of expected cable lengths? Balanced or unbalanced? Impedance?  What accuracy to you expect? Do you have access to both ends of the cable?

[stilwellj1983]

The cable length will vary,  there may or may not be splitters in line. I will have access to both ends of the cable. I am looking for close but not exact. I will be using these two devices to find out the rough cable length and if there are splitters in between the two ends.

[JackOfVA]

I would be inclined to use an AD 8307 as the detector then.
http://www.analog.com/en/rfif-components/detectors/ad8307/products/product.html

It's a log amp with reasonably good linearity extremely high dynamic range (80 dB or more). The output is a voltage linear in terms of dB input. Assuming the signal is not modulated, then the output is DC, 25 mV output (DC) per dB of input signal.

If the cable will also have other signals on it then a frequency selective input filter at 10 MHz would be in order.

The AD8307 is available in a PDIP format, so it's not hard to build a one-off circuit on a breadboard, and if you Google the terms AD8307 and "power meter" you will find many circuits and ideas how to use one to measure power with.

[Jay_Diddy_B]

Hi group,

A little while ago I designed a power meter shield for the Arduino.

Here is the schematic:


Here is the component side of the board:


And the solder side:



I have also attached a pdf file with the test results.

Regards,

Jay_Diddy_B

[ivaylo]

@jay_diddy_b, sorry don't mean to change the topic but what CNC machine do you use for these beautiful boards?

[Jay_Diddy_B]

@jay_diddy_b, sorry don't mean to change the topic but what CNC machine do you use for these beautiful boards?

I have a LPKF Protomat 60. This is a dedicated PCB milling machine.

Jay_Diddy_B

[Neganur]

Would an oscilloscope not be doing a great job here? 10 MHz sine should be a piece of cake for it. Is it really 65 dBm (~400V in 50 ohm?) or -65 dBm ?

Otherwise, the log detector someone suggested sounds like it could work for you, have a look at this circuit. It's based on the AD8306 and you basically feed your signal in on one side, and plug the other side into your multimeter. The RSSI dynamic range is -90 dBm to +10 dBm (3 MHz to 160 MHz), with a DC output of +0.4 volts to +2.4 volts. 50 ohms input impedance.

This is the log detector stage of Scotty's spectrum analyzer project and is a proven design.

[stilwellj1983]

I do not want to use an oscilloscope,  I will be in other people's homes or businesses working with there cable network. I want to be mobile.

[KJDS]

Would an oscilloscope not be doing a great job here? 10 MHz sine should be a piece of cake for it. Is it really 65 dBm (~400V in 50 ohm?) or -65 dBm ?

Otherwise, the log detector someone suggested sounds like it could work for you, have a look at this circuit. It's based on the AD8306 and you basically feed your signal in on one side, and plug the other side into your multimeter. The RSSI dynamic range is -90 dBm to +10 dBm (3 MHz to 160 MHz), with a DC output of +0.4 volts to +2.4 volts. 50 ohms input impedance.

This is the log detector stage of Scotty's spectrum analyzer project and is a proven design.

It was 65dBmV which is just under a couple of volts. Any of many log amp chips would work, or any of many temperature stabilized diode detector designs. All off them would probably benefit from some attenuation before the detector.

[Neganur]

my bad, I blindly read dBm, don't see dBV or dBmV much in my field of work

that's 18 dBm then, I'd grab the oscilloscope for that. 1 dBm loss in a cable would be about 0.25 Vrms less and is easily measured. Although I doubt you'll see a lot of loss at 10 MHz in a cable that you couldn't have measured with an ohm meter?