Phase 1 of my 2 year HF Pre-Selector project is finally complete. The final step was to install the front panel label. My initial goal was to build a pre-selector that would prevent ADC overload on my Duo radio. A secondary goal was to measure the NPR performance improvement by adding a pre-selector to the front end of a SDR receiver. Please see attachments.
This pre-selector performs the same function as an Elad SPF-08 pre-selector, with some additional functions. The SPI signals from an Elad Duo or S2 receiver automatically select the correct ham band, according to the programming in the SW-2 band table setup. The control board in the enclosure takes the S2/Duo SPI signals and converts them to BCD control signals that feed the band pass filter board. Once the band table has been configured, a Duo can operate in the stand-alone mode without the need to use the SW-2 software. The covered bands are 160M to 12M. I converted the 10M band to a bypass function. More on that later. Five volt power to operate the pre-selector is obtained from the 5 volt rail on the S2 or Duo. Total current draw is less than 70 ma. For the actual pre-selector filters, I used the Ukraine HF Band Pass Filter board. Total cost of the box is about $160, and $100 of that cost is for the HF band pass filter board.
When used with a S2/Duo radio, the band switch control should be in the AUTO position.
The pre-selector can also be used with a non-Elad radio. For this mode, the operating band must be manually selected using the band-switch control. In this mode, a 13 volt DC source must be connected to the Power Werx cable. The front panel power switch connects power to the control board. The front panel red LED in ON whenever the pre-selector is connected to a S2/Duo or an external power source. Steering diodes prevent both power sources from trying to supply power at the same time.
The Bypass position on the band-switch bypasses all filters so that the antenna input is directly connected to the radio. Please note that this pre-selector cannot pass TX power. So one needs to insert the pre-selector in the receive only path of the radio.
The following table shows insertion loss of the pre-selector when attached to my Duo radio:
When used with an IC-7300, the pre-selector adds 20 – 40 db of additional adjacent band rejection. Ultimate rejection of the filters is 60 to 70 db.
The pre-selector was designed for ease of testing/trouble-shooting. The HF band pass filter board and the SPI control board are installed in card slots of the Box3 enclosure. All wiring to the HF band pass filter board and the SPI control board is connectorized. No SMT parts are used on the control board.
Phase 2 of the project is to add an Antenna TR Switch. This switch will allow one to use a magnetic loop antenna such as the Wellbrook or W6LVP units. On the switch board, there will be an AM band notch filter that can be switched in/out of the receive signal path. The switch will also allow one to insert the pre-selector in the antenna TX/RX feed line.
If one is interested in building the pre-selector, I can provide the bare control board.