Modulation / Deviation Meter - Page 1
Forum section: electronics / testgear

author: German_EE
author_rank: Super Contributor
timestamp: October 21, 2014, 05:32:15 pm
content: I am looking for the schematic of a modulation meter / deviation meter that I can build rather than buy. After spending a couple of hours on Google all the designs I can find are reasonably old and based around dozens of transistors rather than a few ICs. Receive range needs to be HF to VHF and I need the ability to measure both AM modulation depth and FM deviation. Output to a meter rather than digits is preferred.Any ideas?

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 21, 2014, 05:55:20 pm
content: I would first look to amateur ("ham?) radio sources for circuits like that.The basic function is pretty straightforward.But it is not as clear how one calibrates such a circuit.

author: edavid
author_rank: Super Contributor
timestamp: October 21, 2014, 05:59:54 pm
content: You can check out the Boonton 8210 manual to see how a well designed commercial unit works:http://www.davmar.org/pdf/Boonton8210.pdfMaybe after you read it you will just buy one

author: German_EE
author_rank: Super Contributor
timestamp: October 21, 2014, 06:41:13 pm
content: Thank you for the pdf link. One of the things I cannot yet understand is how a meter differentiates between a low level signal and a signal with a low level of modulation then displays the correct result on a meter. Until now if I wanted to measure AM modulation depth I would use a scope and use a spectrum analyzer for FM deviation but I have decided to add to my test gear so that I can do the job properly.

author: w2aew
author_rank: Super Contributor
timestamp: October 21, 2014, 06:42:11 pm
content: Here's a link to the manual for the old Heathkit FM Deviation meter. I have one of these, and it works quite well. The manual includes the schematic, theory and calibration instructions. The schematic is a bit hard to follow because it is scanned onto several pages, but you can still get through it.http://tubularelectronics.com/Heath_Manual_Collection/Heath_Manuals_IM-IN/IM-4180/IM-4180.pdf

author: w2aew
author_rank: Super Contributor
timestamp: October 21, 2014, 06:44:03 pm
content: Quote from: German_EE on October 21, 2014, 06:41:13 pmFor what it's worth, I have two videos on my channel that discuss measuring FM deviation. One shows how to do it with a spectrum analyzer (by properly setting RBW and using min/max hold traces. The other shows how to measure deviation using the Bessel Null method.

author: Rory
author_rank: Frequent Contributor
timestamp: October 21, 2014, 07:42:25 pm
content: Quote from: German_EE on October 21, 2014, 06:41:13 pmMeasure the transmitted signal first with no modulation, adjust the input level for zero deviation (full quieting), adjust frequency for zero discriminator offset and then add audio to measure deviation. Check both modulation limiter (mic gain) and peak deviation levels.

author: German_EE
author_rank: Super Contributor
timestamp: October 21, 2014, 08:37:43 pm
content: A deviation meter using the Bessel Null would be an interesting project. Generate a series of fixed accurate audio tones within the instrument to feed to the transmitter and have a meter circuit where correct deviation for that tone (null carrier) is center scale.

author: G0HZU
author_rank: Super Contributor
timestamp: October 21, 2014, 08:59:30 pm
content: In my experience a very basic standalone VHF/UHF mod meter has limited value and probably isn't worth building in 2014. I know this isn't what you want to hear but unless you want to make something 'modern' and very accurate using (high speed) DSP/SDR technology then I see little value in making a basic and crude dinosaur that can be outperformed by a 35 year old classic like the Racal 9008M. These old Racal meters are available for very little money these days.The classic commercial mod meters will also offer high quality demodulation with an AF output that can be fed to a scope or soundcard and this feature has been far more useful to me over the years than being able to accurately measure FM deviation or AM depth. The demod function in the Racal 9008M is very linear and can be used to measure the mod distortion performance of all but the very best sig gens.In a few short years you will be able to buy wideband SDR based stuff from the far east on ebay for very little cost that will outperform everything mentioned so far on this thread

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 21, 2014, 09:51:20 pm
content: Quote from: G0HZU on October 21, 2014, 08:59:30 pmOr just write some software for a $10 SDR dongle. With a selection of readout options limited only by your imagination and the Windows API.

author: G0HZU
author_rank: Super Contributor
timestamp: October 21, 2014, 11:03:28 pm
content: Yes, for many simple applications a little SDR dongle could prove very usefulHowever, I think we are not that far away from seeing a significant step up in performance in low cost SDR technology and we will benefit from all the versatility something like this offers as a high quality RF test unit. Not for $10 but maybe $100 will buy a high grade unit.