Decibel accuracy TM5402 M3 \ E2 STB's \ Sat meters.

[Manikm909]

does anyone know if the DB readings on the TM5402 M3 STB are accurate? i.e has anyone compared the STB reading to a sat meters reading for example (my cheapo but goodO meter doesnt give DBs just %)

i find that the TM receiver either OVER states, or the E2 STB's understates

for me on 1w sky news, going through a VBOX, my Edision MIO 4K reports 17.0db - without the VBOX about 17.3db - however, the TM5402 (no vbox) reads 17.7db.

i wondered which is the more accurate

 

[Terryl]

The only way to tell is to check them on a known good source, An RF generator set to a certain RF level at the frequency under test would be the right way, but who has one lying around.

The dB readings come from the tuners RSSI voltage, then a software program converts that to a dB reading or % reading, they will all be different from tuner X to Y.

 

[moonbase]

does anyone know if the DB readings on the TM5402 M3 STB are accurate? i.e has anyone compared the STB reading to a sat meters reading for example (my cheapo but goodO meter doesnt give DBs just %)

i find that the TM receiver either OVER states, or the E2 STB's understates

for me on 1w sky news, going through a VBOX, my Edision MIO 4K reports 17.0db - without the VBOX about 17.3db - however, the TM5402 (no vbox) reads 17.7db.

i wondered which is the more accurate

If I remember correctly, the TM5402 (M3) overstates the value compared to a Dr HD F15 receiver (my benchmark receiver). If I get time I will hook up a few receivers with dB signal level readouts and compare them to my Promax Ranger Neo+ which is a decent meter.

I might be cynical but I would not put it past Technomate to tweak their firmware to deliberately overstate the signal level as justification for "look at how good our tuners are"

 

[moonbase]

The only way to tell is to check them on a known good source, An RF generator set to a certain RF level at the frequency under test would be the right way, but who has one lying around.

Forum member "s-band" might be the man for this, he has all manner of wonderful stuff at his gaff.

 

[s-band]

i wondered which is the more accurate

Those figures are calculated by the demodulation process not from RSSI. Their relationship to the actual signal level is complex. e.g. if the phase noise of the LNB is poor the SNR will be reduced. This would be especially so for narrow signals. Fractions of a dB SNR variation are not really important at high SNRs due to the law of signal level vs SNR. At high levels you may need 3dB extra RF signal to get 1dB improvement in SNR. There will be a point were the SNR doesn't get any better. This plot was done years ago on a 30W TP and shows the SNR reaching a limit at around 18dB. You can see that it would be best to use a lower SNR TP to tweak a system.


I would expect a dB or so spread between receivers so the <1dB difference, at 18dB SNR, is insignificant. The important thing is the point at which you lose lock. A simple signal generator can't be used to check these values. You'd need a vector signal generator and set up for the actual modulation you're interested in but it's not worth the cost/effort for our purposes. The simple way is to turn the dish, to reduce signal, and see how the receivers track.

@moonbase is best equipped to compare receivers as he has a decent dedicated meter.

Here are a couple of related links:
BER MER OR BIT ERROR RATE MODULATION ERROR RATIO (QAM related but the principle is similar)

https://www.rohde-schwarz.com/file/n178_snr-mer.pdf