|National Radio Company|
Tecsun's use of dBµ (a funny-looking, backwards 'u', which is the Greek letter µ 'mu', meaning micro, or one-millionth) is really an improperly-used, shortened version of the term dBµV. Warning! You may have also seen the term "dBu" (lowercase "u") written in various publications, associated with field strength also. It refers to something different (actually the E-field of the passing wave). We can handle that one in a separate article, so be sure not to confuse Tecsun's dBµ with dBu!
Back to Tecsun's dBµ. Let's break it down:
dB = decibels, or simply a way of expressing magnitudes of a value, like voltage, logarithmically
µV = microvolts, or millionths of a volt
Consequently, dBµV is a voltage expressed in dB above (or below) one microvolt. This is measured across a specific load impedance, commonly 50 ohms. Important! Here we have a real received voltage measured across a specific load impedance like a tuned circuit!
The 'dB' or decibel measurement is a logarithmic ratio as you may know. In terms of voltage, an increase of 6 dB is a doubling of voltage. So, if our little Tecsun receives a signal at 28 dBµ and it increases to 34 dBµ, the received voltage has doubled. Coincidentally, this is also an increase of one S-unit! Now we are getting somewhere.
Let's translate our received dBµV into actual received voltage:
dBµV µV(millionths of a volt) ------------------------------- 94 50000.0 84 15810.0 74 5000.0 64 1581.0 54 500.0 44 158.1 34 50.0 (the S-9 of old!) 28 25.0 22 12.5 16 6.3 10 3.2 4 1.6 -2 0.8 (less than 1 µV sends the dB ratio to a negative value!) -8 0.4 -14 0.2
The following formula is used to convert dBµV to millionths of a volt:
µV = (10 ^ (dBµV/ 20))
To convert millionths of a volt back to its decibel representation:
dBµV = 20 * Log(µV)
(Log is the common logarithm, or base 10).
The modern DSP receivers like the Tecsun PL-380, 310, etc. which employ the Silicon Labs chips, measure and display dBµV as received at the tuned front end across a load. They call it the RSSI indicator. Our radio's antenna, the iron core ferrite rod, is basically a signal concentrator. The longer the rod and thus the more iron ferrite, the more the concentration, and the greater the signal voltage transferred to the radio's tuned input.
So what exactly is this so-called dBµ indicator on our DSP radios telling us?
Some time ago, more than a year ago, I posed this question to Scott Willingham, who was on the design team for the SiLabs DSP receiver chips used in these radios.
"The RSSI (dBµ) readings are referred to the pins of the chip, which are the inputs to the LNA. In the Tecsun radios operating in the MW band, this is also the voltage across the loopstick. In SW bands, the Tecsun ULRs use a preamp/LNA on the circuit board between the whip antenna and the Si4734. In that case, the RSSI readings reflect the signal at the output of Tecsun's external LNA."
Essentially for mediumwave, the received signal is measured in microvolts right off the loopstick and then converted to dBµ, which is decibels above a base of one microvolt. Remember again, dB is just a logarithmic ratio. Of course a PL-380 is not going to read the same dBµ as a PL-310 or a PL-398, etc., because the antenna setups (loopstick lengths, whip extension, tuned circuit efficiency) are different and each will induce different received voltage levels to the radio.
A curious measurement, yes, but there is also some meaningful information here in comparing signal strengths within the same radio just like an S-meter did, and in fact there is a direct correlation to the S-meter.
The analog S-meter us old guys remember in now ancient receivers was based on S-9 indicating a 50 µV (microvolt) input signal to the antenna circuitry, at a load impedance of 50 ohms. That is, the S-meter read S-9 if the receiver S-meter was calibrated right, as the meter was further down the IF chain, and usually responded to the AGC (automatic gain control) level. Each S-unit is 6 dB apart, meaning a signal reading S-9 is 6 dB stronger than a signal reading S-8. S-9 +10dB is 10 dB greater than S-9, or one S-unit plus 4 more dB.
What does this mean? An S-9 signal is twice as strong as an S-8 signal. The received voltage is double. An S-9 signal is four times as strong as an S-7 signal. The received voltage is doubled twice.
Some direct correlation can be attempted with the SiLabs DSP chip dBµ readings used in the Tecsun radios.
S-unit µV dBµV --------------------- S9+60 50000.0 94 S9+50 15810.0 84 S9+40 5000.0 74 S9+30 1581.0 64 S9+20 500.0 54 S9+10 158.1 44 S9 50.0 34 S8 25.0 28 S7 12.5 22 S6 6.3 16 S5 3.2 10 S4 1.6 4 S3 0.8 -2 S2 0.4 -8 S1 0.2 -14
Look at the S-unit and dBµV columns. As can be seen, a 34 dBµV signal (again, the Tecsun DSP radios label it dBµ) is essentially equivalent to an S-9 signal on the old S-meter setup. The 25 dBµ signal shown in the picture below represents a signal halfway between S-7 and S-8.
On the Tecsun PL-380 (at least the version I own, which registers from 15 dBµ - 63 dBµ), somewhere around 15 dBµ seems to be the signal detection threshold which translates to just below the old S-6, at 6.3 microvolts of signal. As noted elsewhere, these modern drug store consumer radios are not as sensitive as the old communications receivers we remember. S-6 on an old vacuum tube receiver was virtually "arm chair" copy. This is where an FSL or passive loop brings up the weak received signal to similar levels in the DSP radios.
So there you have it. Keep this chart handy and you can convert between Tecsun's dBµ and S-units.
Stay tuned for the second article in this series: The dBµ Versus dBu Mystery: Signal Strength vs. Field Strength?
|25 dBµ, or between S-7 and S-8|