MiniTiouner Power Level Indication
As with any receiver, the MiniTiouner USB receiver needs the input signal strength to be within a defined range to achieve best results.
The Serit 4334 NIM or tuner is designed to be used in a satellite receive system fed by an LNB on the dish with ~55dB of gain To compensate for this, MiniTiouner must always be used with external preamp followed by a satellite line amplifier when used to receive terrestrial signals.
The exact amount of gain and correct input level to the tuner has been determined by tests carried out by G8GKQ.
Summary of Results
My conclusions were:
- For the MiniTiouner to operate at its best and be as sensitive as possible, the RF Power indicated on the MiniTioune RF power meter with no incoming signal should be between -70 and -65dBm. No advantage was observed with higher indicated RF Powers.
- The indicated RF Power on MiniTioune is useless as a means of peaking weak RB-TV signals for maximum signal strength, as the power is measured across about 10 MHz of bandwidth (3dB) and the noise power in this bandwidth swamps any marginal increase in signal power.
- It is very difficult to measure the RF Power using a digital tuner and the indications on MiniTioune are neither linear nor absolute (labelling as dBm is misleading). Having said that, the indication is very useful - thanks to F6DZP!
Input Power Measurement in the FTS-4334 Tuner
The received signal in the FTS-4334 Tuner is amplified, down-converted to a near-zero IF (using a local oscillator near signal frequency) and then digitised in an analogue to digital converter. All the demodulation takes place in the digital domain after the analogue to digital converter. For the best performance, the full range of the analague to digital converter needs to be used, without overflowing. Weaker signals will still be digitised, but with a lower resolution (which appears as more noise), leading to a lower MER or failure to decode at all. Signals that overflow will be distorted leading to errors, again affecting MER and decoding performance.
The tuner has at least 2 AGC loops to adjust the input level and it is thought that it is these that are monitored to display the RF Power in the MiniTiouner software. The Portsdown is able to monitor AGC1 and AGC2, but not the RF Preamp gain, these 2 AGC values are more than adequate for our needs, giving 70 dB of range.
Measurements of the received power indication were made using MiniTioune Version v0.9beta8_9 while receiving the QO-100 beacon. A line amplifier and a switched attenuator were used to input a wide range of power levels. Using the same test set-up similar measurements were made using modified LongMynd software to support development of input level indication on the Portsdown.
Three areas of operation were observed:
- At low input levels (below -70 dBm indicated RF Power) the tuner runs at maximum gain, but is unable to use the full range of the analogue to digital converter. This leads to lower MER or failure to decode.
- At optimum input levels, the internal AGC loop adjusts the internal gain to provide the optimum amplitude of signal to the analogue to digital converter. This range, which is indicated at between -70 dBm and -27 dBm on Minitioune, provides the optimum decoding performance.
- At high input levels (-26 dBm indicated RF Power and above on MiniTioune) the LNA on the input to the tuner has its gain reduced to maintain reasonable levels into the analogue to digital converter.
The test results are tabulated at the bottom of the page.
To measure the power measurement bandwidth, broadband noise was input to the tuner at 700 MHz and the level adjusted to an indicated RF Power of -65 dB. A 700 MHz carrier was then added to the noise to increase the indicated level by a real 10 dB (as previously measured in the signal tests above). The frequency of the 700 MHz carrier was then moved HF and LF to find the offset where the indicated level dropped by a real 3 dB. This was at + and - 5 MHz, so a 10 MHz 3dB bandwidth is estimated for the power measurement.
The 2 AGC levels are available as output parameters in the LongMynd software. Using a look-up table it is possible to translate this to a dB figure with the same reference of -70 at the point that MiniTiouner reaches its optimum performance. This means that all the user has to remember (if using MiniTioune or Portsdown) is that the quiescent input signal level should be above -70 dB. Portsdown will display sensible values up to -35 dB, which should be adequate range for most applications.
These tests were only conducted with a single signal source (the QO-100 beacon), but they do tie in with my experiences of using MiniTioune and LongMynd software for a number of years. Assumptions have been made about the operation of MiniTioune software (which is not open-source), and also the operation of the tuner (for which the full documentation is unavailable).
The results are tabulated below.
|Input Lev (dB)||MT Indication||AGC1||AGC2||Portsdown Indication||Remarks|
|-4||-26||37700||148||> -35||LNA Gain reduces to 12.3|
|-3||-26||37700||148||> -35||LNA Gain reduces to 11.2|
|-2||-26||37700||148||> -35||LNA Gain reduces to 10.1|
|-1||-26||37700||148||> -35||LNA Gain reduces to 9.1|
|0||-26||37700||148||> -35||LNA Gain reduces to 7.9|
Dave, G8GKQ, July 2021