noise to signal ratio

[G0HZU]

For a quick example of how the generator operates you can look in the user manual for an example of how to set the generator to 25uV rms into a 50R load.

In the image below you can see that my little emulator app agrees with the service manual for a level of 25uV.

i.e. you need to select 72dB attenuation.

Note that on other generators you would have to dial up 50uV EMF on the generator display to get 25uV into 50 ohms (-79dBm)

eg my Marconi 2024 signal generator shows -79dBm as 50uV EMF.

But the 50uV EMF refers to the voltage from the generator when UNTERMINATED. As soon as the generator gets connected to a 50 ohm load then the 50uV EMF will drop to 25uV rms into that 50 ohm load

 

[RADIOOMAN]

For a quick example of how the generator operates you can look in the user manual for an example of how to set the generator to 25uV rms into a 50R load.

In the image below you can see that my little emulator app agrees with the service manual for a level of 25uV.

i.e. you need to select 72dB attenuation.

Note that on other generators you would have to dial up 50uV EMF on the generator display to get 25uV into 50 ohms (-79dBm)

eg my Marconi 2024 signal generator shows -79dBm as 50uV EMF.

But the 50uV EMF refers to the voltage from the generator when UNTERMINATED. As soon as the generator gets connected to a 50 ohm load then the 50uV EMF will drop to 25uV rms into that 50 ohm load

That post was done well..... I attempted to do something like that but gave up.
Ended up typing the information from the manual, manually.
It has been my experience that the older tube radio was -73 dbm (50uv) for S9 and the solid state is -67 dbm (100uv) for S9.
I try to go by what the service manual says.
I could be mistaken.

 

[KG6YGE]

Here's a simple spreadsheet where you can change the dBm (power level) to see what the equivalent uV is.

 

[longhaireddwb]

WOW!
Thanks for the pics and video. I now know what I'm doing to get the proper S-9 on my rig. And with all the other graphs and spread sheets maybe I'll be able to figure out what setting I need to put the generator on to get a 1mV output level for the best RX I can get (signal to noise ratio).

You guys put a lot into this thread and I appreciate it a lot. Thanks for all the info and when I make it back home I'll give this stuff a try again.

BTW, I did print out the manual and see to do the first adjustment in the CW mode for AM on the radio. I will need to read it some more to get the full understanding of it but its making some sense now.

I found the website the other night that I used to set up the generator for an extra output for a frequency counter and how to change over the output jack to a BNC. This is what I did to it a couple years ago. On the site it explains what to do to fix leakage of signal. I'll have to see about getting a ferrite core to wrap my BNC cable around and check on a couple other things it said to do.

The site also has a mod to change it to a 1KHz tone instead of the 400Hz tone. I need to see if this has been done as from listening to it I bet it the 1KHz and has been changed already.

Does this tone really matter? Does it need to be 1KHz to testing and setting RX? I would think a tone is a tone but I've been wrong before.

Thanks for the safe driving wishes as I can use those all I can get with all the 4 wheelers on the road trying to get to places way to fast and causing me problems. Its part of the job but sometimes I just want to bump a couple off the road.

I'll keep my eyes on this post this weekend and until I get home encase someone post up some more. Thanks again for the help and 73 to all!

BTW, Here is the site:
E-200D Info Page

 

[RADIOOMAN]

WOW!
Thanks for the pics and video. I now know what I'm doing to get the proper S-9 on my rig. And with all the other graphs and spread sheets maybe I'll be able to figure out what setting I need to put the generator on to get a 1mV output level for the best RX I can get (signal to noise ratio).

You guys put a lot into this thread and I appreciate it a lot. Thanks for all the info and when I make it back home I'll give this stuff a try again.

BTW, I did print out the manual and see to do the first adjustment in the CW mode for AM on the radio. I will need to read it some more to get the full understanding of it but its making some sense now.

I found the website the other night that I used to set up the generator for an extra output for a frequency counter and how to change over the output jack to a BNC. This is what I did to it a couple years ago. On the site it explains what to do to fix leakage of signal. I'll have to see about getting a ferrite core to wrap my BNC cable around and check on a couple other things it said to do.

The site also has a mod to change it to a 1KHz tone instead of the 400Hz tone. I need to see if this has been done as from listening to it I bet it the 1KHz and has been changed already.

Does this tone really matter? Does it need to be 1KHz to testing and setting RX? I would think a tone is a tone but I've been wrong before.

Thanks for the safe driving wishes as I can use those all I can get with all the 4 wheelers on the road trying to get to places way to fast and causing me problems. Its part of the job but sometimes I just want to bump a couple off the road.

I'll keep my eyes on this post this weekend and until I get home encase someone post up some more. Thanks again for the help and 73 to all!

BTW, Here is the site:
E-200D Info Page

Not 1mv, I think you mean 1microvolt.
You want the least amount of signal that will allow the SINAD meter to work properly.
I have not examined the SINAD meter software as provided by ROBB to see if it has other frequencies as an option so I don't know if you can alter it to work with 400cps. (maybe this is a good time to alert ROBB to a private message from myself)
But I do know that a SINAD meter will not function properly if the tone is not exactly what it is supposed to be.
If you do the mod for the 1kc modulation tone you have to make sure that it is dead nuts on frequency, (1kcps) as the SINAD meter will not tolerate that being off frequency.
After considering what information you have provided I believe you are good to go with the generator.

 

[longhaireddwb]

Thanks radioman. And i do understand about the tone being exactly correct because the sinad is looking for the tone in the noise. Now i get it.

And i thought 1mv was 1microvolt?

 

[Robb]

1 mV = 1 millivolt (one thousandth of one volt)
1 uV = 1 microvolt (one millionth of one volt)

 

[KG6YGE]

The 400 Hz tone is at the low end of the designed in frequency response for the TRC 458, and the upper end is at 3500 Hz. I didn't make an exact fall off plot for mine, but the spec is 6dB down at a nominal 450 Hz on the low side. If you had, with a given volume control, RF gain, and modulation %, a 1 watt rms ac at the speaker at 1 Khz, it would decrease to 1/2 that vaule for 3dB down and by 1/2 again for 6dB down, for a nominal .25 w rms output, all else left as is.

Kind of limited in frequency response like POTS (plain old telephone system). Optimized for voice, but not as good as an AM clock radio. It may pass 400 Hz, but be down on audio. Upper end around 3500 Hz or so. Anything beyond on either side requires more bandwidth.

 

[longhaireddwb]

The 400 Hz tone is at the low end of frequency the designed in frequency response for the TRC 458, and the upper end is at 3500 Hz. I didn't make an exact fall off plot for mine, but the spec is 6dB down at a nominal 450 Hz on the low side. If you had, with a given volume control, RF gain, and modulation %, a 1 watt rms ac at the speaker at 1 Khz, it would decrease to 1/2 that vaule for 3dB down and by 1/2 again for 6dB down, for a nominal .25 w rms output, all else left as is.

Thanks robb for straightening me out on that one. I knew it but...

So now i dont know what KG6YGE is getting at?

Wait a minute i think it just came to me. He us saying that if a radio wants 1khz and you use 400hz instead, it wont come out right. Yes, no, or am i way off base here?

 

[RADIOOMAN]

Thanks robb for straightening me out on that one. I knew it but...

So now i dont know what KG6YGE is getting at?

Wait a minute i think it just came to me. He us saying that if a radio wants 1khz and you use 400hz instead, it wont come out right. Yes, no, or am i way off base here?

He is talking about the combined audio response after it is filtered in the RF domain and then is filtered in the audio amplifications stages as it makes its way to the speaker.
Interesting enough but nothing to be concerned with since you must do the SINAD with the AM carrier with 1 kc modulation.

 

[KG6YGE]

The 1 KHz (or 400 hz) signal I'm referring to is the one you measure the S/N ratio on. Thats the post detection audio signal, which is present at the speaker terminals, and back at the right side of the am detector diodes. The pre-dectection C/N ratio would be measured in the I/F stages, for instance, where the signal is at 7.8 MHz.

When Robb made his measurement, did he not use a 1 KHz test signal (I thought he did)? Also, thats why the intrest in moving from 400 Hz to 1 Khz.

Some radios will recover 400 Hz audio, so your mileage will vary. Mine starts to die there, so I moved up to 1 Khz. Caught me by surprize, so I started exploring the audio freq response and looking for the specs on it. You will most likely get it to work at 400 Hz, but you could be at the lower edge of the audio pass band, which will degrade the SINAD. Give it a try anyway, press on. !!

For instance, the Kenwood TS-120 HF radio has an audio frequency response of 400-2600 Hz (the band edges of the audio). Your modulating signal on the carrier needs to be in the 1 Khz range to characterize the best possible SINAD.

 

[longhaireddwb]

Makes sense. In the middle enstead of the edge. I now know why in the manuals it states a 1khz tone.

 

[longhaireddwb]

Got home tonight and thought I'd look into all this before I go to bed.

According to the service manual I need a 1uV signal from the generator at 30% modulation to set the signal to noise level. And looking at the charts I see that it would be -106dbm. So that means to set all my attenuator switches in the IN position and the fine adjust at -10.

At this point I and the radio don't hear anything!

Maybe I'm not understanding this as well as I thought. I'll go read everything again and see where I messed up but...

Can someone agree or disagree with these numbers for me?

OK, I am hearing it and the radio does too. I Googled a 1khz tone and its not the same tone my generator is putting out. And its also not a 400hz tone. Its between them. I looked at the program Robb posted up and it has the ability to change the tone so maybe I can fine tune it to be the same my generator is putting out.

Tomorrow I will open up the generator and see what has been done to it as far as tone mods. Maybe I can adjust it. We will see.

BTW, I did it by ear again but don't think it helped. I'll compare it to the program tomorrow and post what happens.

 

[Robb]

Turn off the ANL/NB and turn up the RF Gain knob to full open before you do any of this - BTW..

Yes; ~-106dBm is approximately 1uV.
But this number is used for two reasons 1) it is a signal that any radio can pick up and hear 2) it is the minimum amount that can still be heard in the receiver used so that the 1st IF stage can be adjusted to maximize signal vs distortion ('SINAD').

If your radio cannot hear 1uV; then the receive is far more out of adjustment than you first thought and/or there is a failure in that 1st IF circuit.
More likely that it is just out of adjustment.

Get enough output from your signal generator so that you can just barely hear the signal weakly (with the radio's volume up) but not enough to move the radio's S-meter. Re-tune the receive circuit for max output. Then turn down the sig/gen output to -106dBm and listen if you can now hear a signal. It doesn't necessarily have to be -106dBm because some radios can't hear that; but most can/do. The weak signal is pivotal in re-tuning the 1st IF can correctly. Once the 1st IF can is tuned at that level, go through the receive circuit once again (except for the 1st can) for max sig/noise ratio ('SINAD').

Once you are satisfied with the results; then turn the sig/gen up to 100uV (-67dBm) and then adjust the radio's S-meter VR to read 9 S-units.
That should get you really close - so long as your sig/gen is fairly accurate.

 

[RADIOOMAN]

Got home tonight and thought I'd look into all this before I go to bed.

According to the service manual I need a 1uV signal from the generator at 30% modulation to set the signal to noise level. And looking at the charts I see that it would be -106dbm. So that means to set all my attenuator switches in the IN position and the fine adjust at -10.

At this point I and the radio don't hear anything!

Maybe I'm not understanding this as well as I thought. I'll go read everything again and see where I messed up but...

Can someone agree or disagree with these numbers for me?

OK, I am hearing it and the radio does too. I Googled a 1khz tone and its not the same tone my generator is putting out. And its also not a 400hz tone. Its between them. I looked at the program Robb posted up and it has the ability to change the tone so maybe I can fine tune it to be the same my generator is putting out.

Tomorrow I will open up the generator and see what has been done to it as far as tone mods. Maybe I can adjust it. We will see.

BTW, I did it by ear again but don't think it helped. I'll compare it to the program tomorrow and post what happens.

You're close, but no cigar.
Refer to my post (#20).
When your generator is calibrated at the "0" mark on the meter and you have all attenuation out, according to the manual for your generator, you have 100,000 microvolts.
0 dbm is 223,000 microvolts.
Your generator is already -7db below the standard of 1 mw into 50 ohms = 0dbm.
(square root of (50 ohms x 0.001 watts = 0.223)
You need only add 99 db of attenuation to establish -106 dbm.

Put all attenuators in. ( 20,20,20,20,10,6)
Then dial the meter down from "0" (the calibrate mark) to "-3".

The signal level required to get a certain S/N reading on the SINAD meter is going to vary from radio to radio (even with the same model number and even if they have consecutive serial numbers it will vary a bit)

Just use a signal level that is sufficient to allow the SINAD meter to function properly.

( I should note that the frequency accuracy of the 1kc tone is very imporatant so that if your results vary drastically from what is expected then the frequency of the 1kc tone should be investigated, if it is off by too much the SINAD meter will not work properly)

If it requires more signal level than the standard of 1uv then use more and make your alignment and if your alignment improves the sensitivity of the radio then turn down the signal level from the generator and touch up the alignments till you get the best S/N with the least amount of signal level from the generator.
If you end of with 0.3 uv for 20 S/N SINAD then you got a radio that will have great receive.
If you end up with 2 uv for 20 db S/N SINAD then you got a radio that will have a little less receive sensitivity.
Once you have aligned the radio for the best S/N according to the SINAD meter then turn down the generator and take note where the S/N drops below the standard level.
This will be the specification for that radio in your domain.
It may well be different if performed with different equipment.
Even with laboratory grade equipment there will be minor variances.