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MDS practical measuring?

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alsopb

MDS practical measuring?

It was time to drag out the test gear and give my older K3 a going over.

One thing of interest was Minimum Detectible Signal.

There is a WIKI definition for it but the equation doesn't help me a bit.

I just thought I'd attach a calibrated signal generator and keep
reducing the level (for a 500 Hz bandwidth) until I couldn't hear it.
That doesn't seems subjective since the signal generator is a constant
tone and not information to decode.

Then I tried looking at Spectrogram and defining a minimum S/N ratio
which would define MDS. The MDS values derived this way were much lower
for a 6 dB S/N ratio. In fact, I couldn't hear the signal at this S/N
ratio! The integration time constant used for averaging clearly was
helping with detection. So what time constant would be appropriate for
normal CW?

How indeed is MDS measured quantitively?

73 de Brian/K3KO

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John Oppenheimer

Re: MDS practical measuring?

The XG3 manual
http://www.elecraft.com/manual/XG3%20Owner%27s%20Manual%20Rev%20D.pdf

Documents a technique to estimate MDS on page 15.

John KN5L

On 05/04/2014 07:34 AM, Brian Alsop wrote:
> How indeed is MDS measured quantitively?
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Don Wilhelm-4

Re: MDS practical measuring?

In reply to this post by alsopb

Brian,

The traditional way to measure MDS requires the use of an audio
voltmeter (True RMS voltmeter) a low level very well shielded oscillator
with a known and calibrated output level and a well shielded step
attenuator. Any oscillator leakage will produce erroneous results.

Turn the receiver AGC off, and using the attenuator, reduce the signal
generator output to a barely perceptible level.
Then turn the generator off and measure the receiver noise.
Now turn the generator on and adjust the attenuator to produce a
voltmeter reading 3 dB above the level that the noise alone produced.
The input level from the generator and attenuator combination is the MDS.

If you do not have a low level signal generator and step attenuator with
sufficient shielding to prevent leakage, a good approximation can be
obtained with a signal generator such as the XG3 (or XG1 or XG2) with an
output of -107 dBm - the calculation is in the XG3, XG1 and XG2 manuals.

While not an exact MDS measurement, I routinely check the sensitivity of
all receivers I work on. My process uses Spectrogram to view the audio
output. I set my HP8640 to -130 dBm and measure how many dB on the
Spectrogram screen that signal is above the receiver noise floor.
Caution: My HP8640 has all internal shield covers installed and has no
detectable leakage at -130 dBm. Some of those generators have had the
right rear shield cover removed and discarded to make servicing easier,
and those generators will have significant leakage. To check a signal
generator (and other test setup gear) for such leakage, start with a
higher level, then add attenuation in 10 dB steps. If the audio (as
observed by Spectrogram) does not drop 10 dB with each step, there is
leakage from something.

73,
Don W3FPR

On 5/4/2014 8:34 AM, Brian Alsop wrote:

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Alan Bloom

Re: MDS practical measuring?

In reply to this post by alsopb

I have always disliked the term "Minimum Discernable Signal (MDS),
because it really is a misnomer. Obviously different people can discern
a signal at different levels, and what does it mean to "discern" a
signal anyway?

The technical definition of MDS is simply the effective noise level in
the receiver. It is typically measured by injecting a low-level CW
carrier. When the signal plus noise is 3 dB (twice the power) compared
to the noise alone, then the signal level equals the noise level, which
is the MDS.

A receiver's MDS is a strong function of bandwidth. If you increase the
bandwidth by 10x, the MDS goes up (gets worse) by 10 dB. The standard
bandwidth for receiver testing is 500 Hz.

Since an SSB receiver is nothing more than a frequency translator from
radio to audio frequencies, you can measure the signal to noise level at
the audio output. However, theoretically you need a true RMS voltmeter
to do an accurate measurement. A typical multimeter reads the average
voltage of the rectified AC waveform, which is a little different from
the RMS value. The error is small, however. You can correct for it by
adjusting the signal generator for a 3.2 dB increase (1.445 voltage
ratio) instead of 3.0 dB.

A bigger source of possible error is the automatic gain control in the
receiver. Disable it if possible to make sure it doesn't affect the
measurement.

Recent editions of the ARRL Handbook (2012 or later) have a thorough
explanation of how to measure MDS, in Section 25.5 "Receiver Measurements."

Alan N1AL (author of Chapter 25)

On 05/04/2014 05:34 AM, Brian Alsop wrote:

> It was time to drag out the test gear and give my older K3 a going over.
>
> One thing of interest was Minimum Detectible Signal.
>
> There is a WIKI definition for it but the equation doesn't help me a bit.
>
> I just thought I'd attach a calibrated signal generator and keep
> reducing the level (for a 500 Hz bandwidth) until I couldn't hear it.
> That doesn't seems subjective since the signal generator is a constant
> tone and not information to decode.
>
> Then I tried looking at Spectrogram and defining a minimum S/N ratio
> which would define MDS. The MDS values derived this way were much lower
> for a 6 dB S/N ratio. In fact, I couldn't hear the signal at this S/N
> ratio! The integration time constant used for averaging clearly was
> helping with detection. So what time constant would be appropriate for
> normal CW?
>
> How indeed is MDS measured quantitively?
>
> 73 de Brian/K3KO
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2012.0.2247 / Virus Database: 3722/6937 - Release Date: 05/03/14
>
> ______________________________________________________________
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>
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>
>

Lyle Johnson

Re: MDS practical measuring?

Of course one of the nice features of the K3 and KX3 is that a true RMS
audio voltmeter is built in to the radio.

Lyle KK7P
> ... However, theoretically you need a true RMS voltmeter to do an
> accurate measurement. A typical multimeter reads the average voltage
> of the rectified AC waveform, which is a little different from the RMS
> value...
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