Portsdown 2018 Band Switching

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The Portsdown band switching has been extended in the Portsdown 2018 software to allow 8 or 9 discrete bands. There are 2 types of bands, the direct bands and the transverter bands. By default, the direct bands are used, and the specific band is defined by the transmit frequency. Transverter bands can be selected from the "Band/Tvtr" menu on the main menu, and can also be stored and recalled in the presets. The settings used for each band (including the transverter bands) can be selected from the "Set Band Details" button on Menu 3.

The bands are listed in this table.
Band Freq Range Band
Tvtr Line VCO Filter
Switching Signals
8-band RF
Switch Output
71 MHz < 100 MHz 0 0 0 0 0 71
146 Mhz 100 - 250 MHz 0 1 0 0 1 146
437 MHz 250 - 950 MHz 1 0 0 1 0 437
1255 MHz 950 - 2000 MHz 1 1 0 1 1 1255
2400 MHz > 2000 MHz 1 1 0 1 1 1255
Tvtr 1 Any 0 0 1 Set by user on 8-band decode PCB T1
Tvtr 2 Any 0 1 1 Set by user on 8-band decode PCB T2
Tvtr 3 Any 1 0 1 Set by user on 8-band decode PCB T3
Tvtr 4 Any 1 1 1 Set by user on 8-band decode PCB T4

The frequency-defined bands are used for direct transmission on the frequency generated. If switching of outputs to transverters is required, the 8-band decode board and 8-band RF switch can be used to switch outputs to transverters independent of the frequency. In this case, the frequency that the Portsdown-filter modulator board is operating at must be programmed on to the DIP switches on the 8-band decode board as described here: 8-Band_Decoder#Setting_the_VCO_Filter_for_Transverter_Bands

Characteristics of Each Band

For each band, the following discrete parameters are stored:

  • A label for the Band button, which can be edited using the touchscreen keyboard
  • The attenuator level that should be applied for the band
  • The DATV Express Transmit power level
  • The DATV Express and/or LimeSDR port settings for the band. This is a number between 0 and 15, corresponding to the binary configuration of the ports as described here Controlling_a_DATVExpress#Port_Switching_From_the_Touchscreen.
  • The Lime Gain for the band
  • The LO frequency that should be applied when calculating the final output frequency if a transverter is being used. Enter a negative number if it is a high-side LO.
  • The contest numbers to be displayed in Contest mode

Note that, even for the bands not designated as a "transverter band" an LO frequency can be entered and it will be used in the calculations. The final output frequency should be entered on the keyboard (in an effort to save mental gymnastics).

The DATV Express settings are not used unless DATV Express is selected as the output mode and a DATV Express board is connected.

Relationship Between Presets and Bands

The 9 Bands are set up on Menu 3, “Set Band Details”. For each Band, the software stores:

Band Title (Text displayed on the “Band” button)
Attenuator Level
DATV Express Output Level
DATV Express Port Settings
Lime Gain
LO Frequency (used to calculate Portsdown Output Frequency when Transverter Output frequency is entered)
Contest Numbers

When one of the 4 presets is stored (by selecting “Store Preset” and pressing a preset button), it stores the current:

Encoding Mode
Label (added at time of storage)

These values are all recalled when a preset is pressed. Then the Attenuator Level, DATV Express Output Level, DATV Express Port Settings, LO Frequency and Contest Numbers are automatically set according to the recalled band.

LO Filter Switching

The transverter bands are identified by setting GPIO pin 7 high, and then using the binary code that would normally be sent to the LO filter. So, to enable the use of the transverter switching, some extra logic needs to be put between the GPIO outputs and the LO filter. This extra logic must also have knowledge of what IF frequency is to be used for each transverter.

The 8-band decode board provides a PIC-based replacement for the current 4-band output switch and the logic, together with 8 DIP switches, allowing the correct IF frequency (and hence LO filter) to be programmed for each transverter band. The design is shown here.

Transverter Switching.jpg

If fewer transverters and bands are required, simpler designs are obviously possible.

Attenuator Control

The same attenuator design that was recommended for the Signal Generator can be controlled to provide a different attenuation for each band. The attenuator should be inserted in the RF line between the Filter-Modulator board and the 2-way switch on the diagram above.