The AM Forum

After getting an e-mail from a station that is about 11 miles from me stating that he heard my CQ loud and clear but he could not get me to respond I decided to check out my radio. ( I know there are a lot of reasons why I might not have heard him but I thought it may be time to give the 870 a check-up).

Reading the spec's on my TS-870 the RX sensitivity is suppose to be .2mmv or better.
Ok...so I set up my 8640B signal generator w/audio tone.
Both radio and generator are set and locked on freq.
I feed the radio with .2mmv and I can hear a faint tone through the speaker/headphone, yes increasing the output of the generator makes the tone louder.

Seems like that is all there is to it....I guess....Yes/No ?? ???

But how does one quantify this?
Should the tone be barely heard or should it move the s-meter to a particular level...S9?

I do seem to be having a hard time hearing some stations...I want to know if it's the radio or my ears... :-\

Thanks
Ken
N4zed

I be no expert, but I would expect that the voltage cited would be WRT some other parameter like an S-meter reading...  they ought to give a spec for voltage to produce S9 somewhere...

But unless there is something really wrong with the receiver the antenna is going to make the most difference. Even pretty "deaf" receivers sound pretty good on good antennas. Conversely, even a good receiver sounds "dead" on an antenna that doesn't do a good job... for example none of my receivers sound particularly good when listening to 40m on my 80m dipole. Worse still as I go to higher up bands...

Also, your ambient background man-made noise level will effect what you can hear and what you can not. If you have a pretty quiet location, you can hear much more, further down into the noise... I'd question the antenna and local noise conditions mostly.

                         _-_-bear

The answer is 'Read The Funny Manual".  The test procedure will specify the equipment needed, the setup, and performance specifications and the acceptable limits.

So whatever the manual says the criteria are, that is the answer.  Whether it is 'audible to the ear' or centibels per furlong (cB/F'lg) that is the answer.

[edit] Let me add that some manufacturers will use a Minimum Discernible Signal standard, while others may define S9 as xx microvolts, with 20dB over as XXX microvolts.  It all varies.

Maximum Sensitivity /Minimum Discernible Signal is almost never a limiting factor on HF.  Most radios made in the last sixty years have more than enough sensitivity.  Natural Noise is the limiting factor (and maybe man made noise too these days).  You can check by disconnecting your antenna.  If the radio gets quieter then the radio has plenty of sensitivity.

More elaborate testing can get into measuring Siignal to Signal + Noise ratios at different frequencies.

Oh yeah, he could hear you, but you couldn't hear him?

lessee, are you running 100 - 300w of plate modulated AM?
...is he running 20watts of ricebox AM?

His antenna is what? Radiating nil? :D

That also might account for something?

       

Agree with the above on if you hear a noise level increase when the antenna is connected you likely have all the sensitivity you can use on that band.  The exception would be if you have a lot of LOCAL noise and a better placed antenna would reduce the local noise pickup and then check again.  But unless your radio has a major fault sensitivity isn't going to be an issue on the lower bands.

But the IF and audio systems make a big difference in intelligibility with weak signals, high background noise, etc.  Some receivers have a much "cleaner" sound than others under a given set of conditions.  Two of my most often used receivers are a Hallicrafters SX-88 and a Drake R-8.  They both do a great job under various conditions but I find the SX-88 provides a little better performance when the band noise is very high.b

Sensitivity specifications are based upon so many microvolts for a stated Signal to Noise Ratio, usually 10dB. So a .2uV AM signal should be 10dB above the noise and perfectly readable preferably with headphones at such a weak level.

If you dont have a calibrated audio meter to plug into the headphone or external speaker jack then switch to SSB, shut off the 8640 modulation and see how weak a signal you can barely hear using the output level meter on the 8640. Thats called MDS, is usually rated for 3dB above the noise and varies by receiver bandwidth.

KW uses 50-100uV for S9 and if it passes that test the other guy has a problem.

Carl

This article tells it all:

http://www.monitoringtimes.com/html/sensitivity.html

Thanks guys.

First, I don't have an audio volt meter, that's what the "Funny" manual wants. Probably the only piece of test equipment I don't have.

"So a .2uV AM signal should be 10dB above the noise and perfectly readable"

I'll probably use one of my scopes to measure the voltage at the speaker hole, probably the most accurate way any-who.

I'll try the other recommendations as well tomorrow it's my Friday off.

I really don't think I have a problem but I thought this might be a good exercise. I'm one of those people who really want to know how well/bad my equipment is.

Thanks again, I'll let you know how things turn out...

Ken

Most any of the old VOM's with a dB scale and input will work fine even tho their input is for 500-600 Ohms, all you care about is a relative difference between noise and signal anyway. I did pick up an old GR meter for peanuts at a surplus shop since it looks more "professional" than an old 630 ::) and I can dial in any impedance I want.

Carl

The GR 1840, which is what I think Carl mentioned,  is a very handy little meter.   Power range is 1mW to 20 Watts, and can match impedances  from around half an Ohm, to 32K Ohms.

(http://i670.photobucket.com/albums/vv62/mikeinkcmo/testgear/GR/1840%20Aud%20Pwr%20Mtr/GR18401.jpg)

If you are simply concerned whether or not the receiver is sensitive enough to hear the station, you don't need any test equipment at all.  Simply tune to an unused portion of the band, or just outside the ham band to the middle of an unoccupied swathe of frequencies at least several times the bandwidth of the receiver, preferably in the absence of lightning static or power line noise.  Turn the rf gain to maximum and note the background hiss.  Connect and disconnect the antenna.  If the background hiss noticeably increases with the antenna connected and decreases with the antenna disconnected, you already have achieved ultimate sensitivity and no amount of tuning adjustments at the receiver or adding preamps will bring in the signal any better.  A pre-amp or additional rf gain will bring up the signal strength but will also bring up the atmospheric background noise, resulting in no change in signal-to-noise ratio. All you need is to get the noise floor of the receiver beneath the atmospheric noise floor at the received frequency.

Thats way too modern, dainty and femine looking Mike ;D  I use a GR-583 which looks better in the basement lab of a mad scientist ::)



Don, I dont completely agree with your test. First of all the human ear can hear below the noise and some ops have trained themselves to go well below. Secondly your test is additive, noise + noise = more noise.

This becomes important with low signal level antennas from Beverages to the various mini loops, etc. They need a preamp in many cases and it has to have a very low noise figure so as to amplify the signal + noise to a level that overcomes the radios noise figure. A preamp with a 1dB NF and 20-25dB of gain is often required to peel away another layer and dig deeper.

Moving up to say 10M the literature often claims a 12-15dB NF is adequate. That couldnt be further from the truth. My TS-940 has a measured 12dB NF on 10M and a 1dB NF preamp with 11dB gain gets me down several layers since the system NF is now 3.9.
The reason is that those published figures are done with a dipole or vertical, no directivity worth mentioning and most likely during prime sun noise hours. A yagi compresses the azimuth and elevation beamwidth so it hears only a slice of the total. So does a Beverage and many wire arrays for the low bands. And then to further confound and confuse there are those extremely quite days and nights ...such as sunrise during a high sunspot number and working pileups of JA's and other exotic DX Long Path over Africa. Or 160 when the exotic Asians are coming thru in darkness and the S meter band noise reading is at zero.

My 75A4 has a 10M 4dB NF, down from a 6.5 after the earlier mods and 18dB as original. It hears stations that even the latest high dollar sets cant hear on any band since as soon as they plug in an antenna the broadband white noise mixes with all the synthesizer crud and raises the noise floor even more.  My modified HRO-60 hears almost as well. The QST reviews are in a controlled lab enviroment and the published MDS data is artificial.

Take a typical boatanchor with a 20dB NF on 40-160 and 20-30dB front end gain (1-2 RF stages), a good part of the losses are in inductors, switches and wiring which adds to the tubes basic NF. Now add a preamp with 15dB gain and a 1dB NF which could be a 2N5109 set up well. The resultant system NF is now 6.4 and you just may be able to hear that pip squeak Ranger that has been trying to break in for an hour ::). You can back down the radios gain a bit to prevent overload with minimal effect. Or do as the HRO-60 where the 2nd RF stage gain is factory set to just overcome circuit losses while improving image rejection a lot.

Now, OTOH, if all you listen to are the louder stations on 160-40M AM then most anything will work as is.

Carl

Thanks for all the ideas guys. I've decided, as many of you suspected, that I don't have a problem.

As I stated in my original post I can hear a faint tone through the speaker/headphone when I feed a .2mmv signal to the receiver. And, except for some obscure modes that I am not aware of, what I hear or am able to hear is what is important to me.

That being said I will be looking for a really cheap audio level meter at the hamfest next month..

I've seen this stated many times but have never seen any actual measurements. Can you point to a source?

Don's test can easily be done with any antenna.

I took a test at Dayton in the 80's and watched early EME pioneers copy things I could not hear in the 60's. It was Sam Harris, W1FZJ/W1BU, that convinced me of the legitimacy and I spent years developing it.


Thats why so many believe it without question. It "appears" so simple and for most it is sufficient as they are not interested in working over 300 countries on 80 and approaching it on 160. For EME they let a computer program do the work these days.

So you have no actual verifiable and repeatable measurements for review. I was hoping for something more than apocryphal tales and chest thumping. Thanks for playing.

Hearing signals below the floor?

Heck, my Bevs can hear 'em before they're even sent - being composed in the other op's brain, caw mawn... ;D

T

Mine is so sensitive, I don't even need to turn it on.  ::) I've worked 3000 countries, 27 0planets, two galaxies and a partridge in a pear tree. And the Tron with one watt.

But seriously, specialized computer software can hear below the noise floor. I would think that a human brain could be trained to do it too, but to a limited degree.

On CW, copying might be based on the change in noise character or mixing as the weak pulses key up, I dunno.

T

You forgot the cardboard shack
http://www.youtube.com/watch?v=Xe1a1wHxTyo

Typical response from you when you want something but forget about it the other way around.

Here Ive done your work for you, it should at least get you started.
http://nw7us.us/jt65a/2010_10_JT65A-Part-1_Compressed.pdf

It's not about you Karl. You missed the point completely. I asked for real measurements on the human auditory system's ability to hear negative SNR. That's it. You haven't provided it. The article at the link says:

"Speaking strictly in terms of detectable signal-to-noise ratios (SNR), a CW signal that is “encoded” at twelve words per minute (12 wpm) is generally held to be copyable at an SNR of –15 dB, whereas a phone transmission that sends information at 250 wpm requires an SNR of +6 dB. (These ratios are typically calculated based on a 2.5-kHz channel bandwidth.)"

Generally held by whom and on what basis? What seminal measurements were performed on listening panels of humans to obtain these numbers? I'm not saying humans can't hear negative SNR stuff, I'd just like to see some numbers. Since you chose to throw out the premise, I thought maybe you had some data. It wasn't s challenge to your premise, your technical skills or your manhood. Lighten up. If you don't have real data, it's OK. Once again, it's not about you, it's about data. This is the technical section after all.  :)

I missed nothing since you made no point, just got snarky in your reply. I know of no test that would pass a standards commitee since Ive never heard of an approved test procedure.

The only time I saw any equipment was

1. At Sam Harris' place. There was a chart recorder hooked up to the R-390 used as the IF for 1296 EME. The recorder was better than my ear by far until I got lots of practice back where I was living. Id go back about every week of good moon time to compete with Sam and the recorder during EME skeds. I dont remember the exact details but eventually I was hearing as well as Sam who was much older and confirmed by the recorder. Ive no idea the dB involved but eventually I was hearing better than the other club operators who showed up at times and were also older. The purpose was to work and confirm some of the earliest ham EME contacts, not write a text book.

2. At Dayton sometime in the mid 80's at the Contest Corral where a contest was held. These were tapes recorded in 1dB steps and I was good for -8 to -10dB MDS since they were off the air with varying noise and QSB. It was good for 4th place among about 30.

Again nothing scientific and how that compares to that article Ive no idea. Id suspect W1JT has all the answers and suggest if youre that interested then contact him.

I think thats technical enough for this forum anyway based upon the replies ::)

I don't have any figures about how low you can hear below a noise floor, but experienced Sonar operators could hear and count turns on stuff that most people couldn't hear. It's all about training your ear (and your brain) to sort out signal from noise. It drives the audiologists a bit crazy that I can hear stuff way down there, despite quite a bit of hearing damage and the noise floor provided by tinnitus. I think I would test with far more hearing loss without my ears being trained by Sonar and lots of listening to weak signals as a kid with the old Heathkit GR-91.

Of course all that doesn't help in picking out conversations at a noisy party. I think that that is the difference between minimum discernible signal and minimum intelligible signal.

Back when I was servicing audio equipment many moons ago, and if my memory servers me right we would test FM receiver sensitivity using an FM signal generator, 1000hz tone and a distortion measurement system and the sensitivity figure was based on 3% distortion figure, I think. Maybe some of the audio guys can verify that as it was a long time ago.  Not sure how other receivers would be tested though.  There must be some established method for comms equipment.

There was no standard in the boatanchor days but a 6 to 20 dB SNR was often used in ads. For AM that was with a 30% 400Hz modulation. I use the 10dB in my receiver testing certification and on 10M; thats enough signal to be good copy on a quiet band with sufficient modulation.

Later MDS became a quasi standard but even that is open to interpetation. Some refer it to 3dB above the noise and others to what they can reliably copy in whatever bandwidth, neither is scientific ;D

Carl