Going bonkers trying to measure frequencies - UK Vintage Radio Repair and Restoration Discussion Forum

avocollector · 14th Jan 2016, 3:18 am

Header says it all - I'm only really interested in audio to low frequency radio but I cannot seem to get 2 consistent measurements from 2 digital frequency meters - I own about 6 or 7 - and am seriously considering resurrecting a W122 hetrodyning device out of sheer desperation. I understand the readings may differ as much as 10% but I keep on getting utter nonsense - one meter will say 50hz and the other will say 1 megahertz

What do you wise ones do? do you just use leads with .00001 muf caps in or similar to measure say the 455khz if in a radio, do you have any standard way of screening out harmonics (which I suspect cause some of my problems). Any advice would be great as I'm rapidly tearing out what little hair remains.

Mind you the other day some DMM's also joined in the fun - they told me my mains ac voltage was 537 and 792 volts ac respectively - fortunately the old analogue meters told me it was still 235v ac.

Note to mods if there's a better place/category for this question please feel free to move the discussion. Thanks folks.

G4_Pete · 14th Jan 2016, 7:51 am

My Osciloscope has an output for channel 1. That way I can see what I am measuring , that is noise, distortions etc and use the scope controlls to set a good threashold for the frequency meter triggers.
That way if you are getting Jittery readings you can normally see why and also you can often just put the probe near what you are measuring and get minimal loading.
However note also that any lightly capacitivly conected input is a high pass filter which will preferentialy pass any spikes and noise which can make worse any attempt to trigger on lower frequencies.

You may want to consider a low pass RC filter.

p.s. Being mindful of earth loops I presume you have got the Frequency meter ground connection common to the circuit being measured??

Pete

Radio Wrangler · 14th Jan 2016, 8:46 am

When you're measuring a low audio frequency, the input voltage slews across the comparator threshold of the frequency counter VERY slowly and so any noise on the signal (or from the comparator stage in the counter) will lead to a burst of random-ish alternating 1's and 0's going into the logic part of the counter.

One answer to this is to add a little hysteresis to the comparator stage, and this is often done particularly on low frequency counters. Some well-equipped instruments have controls so you can set the threshold voltage, switch in coupling capacitors, filters, extra hysteresis and all sorts of things to clean up measurements. Usually these counters don't go very high in frequency - say 50MHz at best

Other counters are designed for high RF frequencies, hundreds of MHz and their input stages are optimised to go to hundreds of MHz and work with small sinusoidal signals into 50 Ohm input impedance. Thse get only a tiny bit of fixed hysteresis because hysteresis prevents them working on small signals and the manufacturers were competing on 'how low can you go' in the input level. Thee counters are usually fine at RF, but if confronted with LF signals can count pops clicks and noise.

Usually, top range models have multiple inputs, with a couple of wide-application low frequency, high impedance, ports which can do all sorts of fancy counting, plus one UHF range RF 50 Ohm port.

David

Dickie · 14th Jan 2016, 9:27 am

Most quality counters also incorporate some form of AGC in their amplifiers. This works to maintain the signal at the input to the comparator to a level just greater than the hysteresis over a wide range of input levels. Hopefully the noise is then too small to trigger extra counts.
If you are expecting an audio frequency and you only measure 50Hz then either the signal is too small or there is a grounding problem. If you are measuring 1Mhz then the signal is too large or has a lot of noise on it, probably both. In that case a simple resistive attenuator may help.

llama · 14th Jan 2016, 12:02 pm

You could record the signal you want to measure on a PC and then open the file in (eg) Adobe Audition and use the frequency measurement function.

I haven't searched but I expect there are real-time PC-based frequency measuring programs about. Saving it as a file will show you any spurii though.
Graham

avocollector · 14th Jan 2016, 2:11 pm

Many thanks for all those thoughts/help folks - actually the 'scope idea is the most practical for me and it must be a sign of old age that I never thought of it. I must admit with the amplitude of the mostly audio signals I was/am measuring being so high (typically 3-4 volts ptp), I never thought the loading of the meter or similar would/could be a problem.

14th Jan 2016, 3:18 am	#1
avocollector Heptode Join Date: Jul 2010 Location: Wellington, New Zealand. Posts: 653	Going bonkers trying to measure frequencies Header says it all - I'm only really interested in audio to low frequency radio but I cannot seem to get 2 consistent measurements from 2 digital frequency meters - I own about 6 or 7 - and am seriously considering resurrecting a W122 hetrodyning device out of sheer desperation. I understand the readings may differ as much as 10% but I keep on getting utter nonsense - one meter will say 50hz and the other will say 1 megahertz What do you wise ones do? do you just use leads with .00001 muf caps in or similar to measure say the 455khz if in a radio, do you have any standard way of screening out harmonics (which I suspect cause some of my problems). Any advice would be great as I'm rapidly tearing out what little hair remains. Mind you the other day some DMM's also joined in the fun - they told me my mains ac voltage was 537 and 792 volts ac respectively - fortunately the old analogue meters told me it was still 235v ac. Note to mods if there's a better place/category for this question please feel free to move the discussion. Thanks folks.

14th Jan 2016, 7:51 am	#2
G4_Pete Hexode Join Date: Feb 2014 Location: Southampton, Hampshire, UK. Posts: 419	Re: Going bonkers trying to measur frequencies My Osciloscope has an output for channel 1. That way I can see what I am measuring , that is noise, distortions etc and use the scope controlls to set a good threashold for the frequency meter triggers. That way if you are getting Jittery readings you can normally see why and also you can often just put the probe near what you are measuring and get minimal loading. However note also that any lightly capacitivly conected input is a high pass filter which will preferentialy pass any spikes and noise which can make worse any attempt to trigger on lower frequencies. You may want to consider a low pass RC filter. p.s. Being mindful of earth loops I presume you have got the Frequency meter ground connection common to the circuit being measured?? Pete Last edited by G4_Pete; 14th Jan 2016 at 7:57 am. Reason: Ground conections

14th Jan 2016, 8:46 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Going bonkers trying to measur frequencies When you're measuring a low audio frequency, the input voltage slews across the comparator threshold of the frequency counter VERY slowly and so any noise on the signal (or from the comparator stage in the counter) will lead to a burst of random-ish alternating 1's and 0's going into the logic part of the counter. One answer to this is to add a little hysteresis to the comparator stage, and this is often done particularly on low frequency counters. Some well-equipped instruments have controls so you can set the threshold voltage, switch in coupling capacitors, filters, extra hysteresis and all sorts of things to clean up measurements. Usually these counters don't go very high in frequency - say 50MHz at best Other counters are designed for high RF frequencies, hundreds of MHz and their input stages are optimised to go to hundreds of MHz and work with small sinusoidal signals into 50 Ohm input impedance. Thse get only a tiny bit of fixed hysteresis because hysteresis prevents them working on small signals and the manufacturers were competing on 'how low can you go' in the input level. Thee counters are usually fine at RF, but if confronted with LF signals can count pops clicks and noise. Usually, top range models have multiple inputs, with a couple of wide-application low frequency, high impedance, ports which can do all sorts of fancy counting, plus one UHF range RF 50 Ohm port. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

14th Jan 2016, 9:27 am	#4
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Going bonkers trying to measure frequencies Most quality counters also incorporate some form of AGC in their amplifiers. This works to maintain the signal at the input to the comparator to a level just greater than the hysteresis over a wide range of input levels. Hopefully the noise is then too small to trigger extra counts. If you are expecting an audio frequency and you only measure 50Hz then either the signal is too small or there is a grounding problem. If you are measuring 1Mhz then the signal is too large or has a lot of noise on it, probably both. In that case a simple resistive attenuator may help. __________________ Regards, Richard, BVWS member

14th Jan 2016, 12:02 pm	#5
llama Octode Join Date: Jun 2006 Location: St Osyth, Nr Clacton, Essex, UK. Posts: 1,482	Re: Going bonkers trying to measure frequencies You could record the signal you want to measure on a PC and then open the file in (eg) Adobe Audition and use the frequency measurement function. I haven't searched but I expect there are real-time PC-based frequency measuring programs about. Saving it as a file will show you any spurii though. Graham __________________ Half my stuff is junk - trouble is, I don't know which half!

14th Jan 2016, 2:11 pm	#6
avocollector Heptode Join Date: Jul 2010 Location: Wellington, New Zealand. Posts: 653	Re: Going bonkers trying to measure frequencies Many thanks for all those thoughts/help folks - actually the 'scope idea is the most practical for me and it must be a sign of old age that I never thought of it. I must admit with the amplitude of the mostly audio signals I was/am measuring being so high (typically 3-4 volts ptp), I never thought the loading of the meter or similar would/could be a problem.