Just when I think I am beginning to understand things ...

So, a few days ago I was tvfool, trying to figure out what channels I could receive. There were some with positive db values and some with negative values. I could in fact get the ones with positive values (they were marked "good") and did not get any with negative values.

Today I am playing with antennas on my cellular modem. I see it is working but slow ... my readings are negative 108 to negative 117.

I did some reading and they said "five bars" .. would be around -50 db and at -120, it would stop working.

Hmmmmm ...

So why with the TV was I looking for a positive value (say 30 to 80 ) and with a cellular modem negative 50 is considered 5 bars ?????

Thanks .... Mike

 

So why with the TV was I looking for a positive value (say 30 to 80 ) and with a cellular modem negative 50 is considered 5 bars ?

Because dB values are always relative to some reference level (which is likely difficult to determine here), such as dBm, which is dB relative to 1mW of power.
So you can't compare dB values until you know what reference is used for the dB readings.

 

Any number expressed in dB is a ratio. For voltages, a 2:1 ratio in value equals approx. 6 dB. So a 1:2 ratio is -6 dB. But ratio values have no meaning without a reference. In broadcasting, 0 dB (the reference value) is 1 milliwatt dissipated in 600 ohms, or 0.7746 Vrms. Once you know this, other signal levels expressed in dB can be converted to volts, and vice versa.

For your readings you have to know the reference being used for each system before you can manipulate and compare the readings.

ak

 

In broadcasting, 0 dB (the reference value) is 1 milliwatt dissipated in 600 ohms, or 0.7746 Vrms

600 ohms is typically used as an audio impedance reference, but I think dBm is referenced to 1 mw, independent of the resistance.
For RF work, 50 ohms might be used for the working impedance level, giving a 1 mW reference voltage of 223.6mV.

 

So just talking RF ... why would I "need" a positive 30 - 80 db signal for TV and a -50 to -120 signal for cell?

Thanks ... Mike

 

why would I "need" a positive 30 - 80 db signal for TV and a -50 to -120 signal for cell?

As we stated, since we don't know what reference level each of those dB values used, we can't really know what those values mean or why they are so different.

 

This may be relevant

Arbitrary Strength Unit (ASU) is an integer value proportional to the received signal strength measured by the mobile phone.

It is possible to calculate the real signal strength measured in dBm (and thereby power in Watts) by a formula. However, there are different formulas for 2G, 3G and 4G networks.

In GSM networks, ASU maps to RSSI (received signal strength indicator, see TS 27.007[2]sub clause 8.5).

In UMTS networks, ASU maps to RSCP level (received signal code power, see TS 27.007[2]sub clause 8.69 and TS 25.133 sub clause 9.1.1.3).

In LTE networks, ASU maps to RSRP (reference signal received power, see TS 36.133, sub-clause 9.1.4). The valid range of ASU is from 0 to 97. For the range 1 to 96, ASU maps to

The value of 0 maps to RSRP below -140 dBm and the value of 97 maps to RSRP above -44 dBm.

 

Got it!

I made the bad assumption that the 0 reference would be the same for all RF signals ... I see I am wrong.

Thanks for the help!

 

Hello,

Read the attached PDF's for the decibel.

Bertus

 

Great!

I just did a quick look ... I will sit down with them tonight.

Thanks so much !!!!!!!!

Mike