[Moved]: lm3915 scale question

"The LM3915 is a monolithic integrated circuit that senses analog voltage levels and drives ten LEDs, LCDs or vacuum fluorescent displays, providing a logarithmic 3 dB/step analog display. "

What does the logarithmic 3 dB/step mean when a 10 led bargraph display is driven?
I mean, how can it be logarithmic and each step (LED) to display 3 db, isn't that linear?

Also, will the logarithmic scale variant be useful to make an S-meter for a radio, or the linear or the VU-meter scale variant of the chip family?

Re: lm3915 scale question

The steps are set by a log value chain of resistors in its internal comparator. The 10 steps it produces are from reference 0dB to -27dB, the chart in the data sheet shows the actual voltages for each threshold along the scale. For linear readings, the LM3914 is the device to use, it has equal value resitors in its comparator chain so the steps are the same size.

Yes, you can use the LM3915 for an 'S' meter but other factors such as receiver AGC have to be taken into account to get an accurate signal level reading. Typical 'S' meters read up to +60dB so it makes sense to use two LM3915s (or one LM3914 for the low end and an LM3915 for the high end) to get an extended range. The 'VU' scale is slightly different as it is intended to be used where clipping or compression takes place above about +3dB as in 'pro' audio mixing.

Brian.

Re: lm3915 scale question

betwixt said:

Yes, you can use the LM3915 for an 'S' meter but other factors such as receiver AGC have to be taken into account to get an accurate signal level reading.
Brian.

Click to expand...

Thanks!
I only want a bare bones S-meter, but it needs to be tiny, so no LCD display. 9 S-units as the maximum reading, is ok, I don't need more (intended to handheld use).

So I guess a single LM3915 is ok. This should have a greater resolution on the low level signals (beginning of the LED scale) and lower resolution on the high level ones, am I right?

Any ideas how to calibrate it? Maybe compare it against the meter of a commercial receiver and draw a scale next to the LEDs?
Or the LEDs would present more "fixed" values, so no calibration is needed (assumming no input attenuator)

Re: lm3915 scale question

So I guess a single LM3915 is ok. This should have a greater resolution on the low level signals (beginning of the LED scale) and lower resolution on the high level ones, am I right?

Click to expand...

Exactly correct!
I have done this in the past using 3mm green LEDs for outputs 1-8, a yellow one for 9 and a red for 10 so the red one indicates S9+.
It is easiest to calibrate against a known meter as there seems to be little correlation between S units and actual signal levels on many receivers anyway.

Brian.

Re: lm3915 scale question

What does the logarithmic 3 dB/step mean when a 10 led bargraph display is driven?
I mean, how can it be logarithmic and each step (LED) to display 3 db, isn't that linear?

Click to expand...

No, it's not linear in input voltage. It means that the LEDs have the threshold voltages specified in the datsheet:



If you want a linear characteristic, use LM3914 with uniform threshold steps.

Consider that moving-coil S-meters have also linear scale. So even if the receiver characteristic isn't really linear, it's a better estimation than the LM3915 log curve. Although an S-meter is showing field strength in a log scale, the log characteristic is already achieved by the AGC transfer function.

Re: lm3915 scale question

FvM said:

No, it's not linear in input voltage. It means that the LEDs have the threshold voltages specified in the datsheet:

View attachment 138309

If you want a linear characteristic, use LM3914 with uniform threshold steps.

Consider that moving-coil S-meters have also linear scale. So even if the receiver characteristic isn't really linear, it's a better estimation than the LM3915 log curve. Although an S-meter is showing field strength in a log scale, the log characteristic is already achieved by the AGC transfer function.

Click to expand...

Oh, it does not have an AGC. But I got the idea. Thank you all for the info!

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betwixt said:

Exactly correct!
I have done this in the past using 3mm green LEDs for outputs 1-8, a yellow one for 9 and a red for 10 so the red one indicates S9+.
It is easiest to calibrate against a known meter as there seems to be little correlation between S units and actual signal levels on many receivers anyway.

Brian.

Click to expand...

PS. I am QSO-ready, I have tested UK with a mobile station. When you are ready let me know Brian.