QO-100 Experiments – New Down-converter

Probably like most people starting out with QO-100 operation I initially used the Goonhilly QO-100 WebSDR to listen to the satellite down-link. This inspired me to experiment with an LNB, a 65cm offset dish, and a NooElec SDR module with SDR-Console. Modifications to the LNB soon followed for improved frequency stability. The net result was a receive setup that heard well, and with the use of Omni-Rig and SDR-Console, operation was very easy as my Elecraft  K3 transceiver (used for transmit) automatically synchronised with SDR-Console. A mouse-click on the waterfall or turning of the K3 tuning knob resulted in both TX and RX tracking and both being tuned to the same frequency.

Having an interest in both QO-100 and 3cms operation (rain-scatter and EME) I decided to look at making a down-converter to take the output of the LNB (~739MHz) and convert this to a lower frequency suitable for input to the K3 either at 28MHz or 144MHz (via the Anglian transverter). With a switched LO (Local Oscillator) the K3 would be able to tune across either the 10368Mhz or 10489MHz sections of the 3cms band.

This resulted in the following idea for a down-converter board:

The down-converter board would ideally provide the following for QO-100 operation:

    • Mix the 739.500MHz output from the LNB with 595.500MHz LO to give a 144MHz RX IF.
    • Provide an attenuated 739.500 MHz output (from the LNB) to the shack SDR.
    • Switch the 144MHz IF from the K3 to the Up-converter on TX.
    • Two GPSDO 10MHz references for the RX and TX ADF4351 LO boards.
    • A 25MHz reference to the LNB, derived from the GPSDO 10MHz reference.
    • A fused 12V supply to LNB
    • TX/RX switching for PA bias, relays etc.

By switching the LO frequency to 474MHz the 144MHz IF output of the down-converter would tune the 10368MHz segment of the 3cms band.

The LO for both my Up and Down-converters use a switched ADF4351 PLL board that’s locked to a 10MHz GPSDO reference as described here.

I developed the down-converter schematic and board layout using KiCAD. It’s been a great learning curve and new interest that has helped during these lock-down periods that most of us are unfortunately experiencing at this time. Nothing particularly special was used in the way of components. I used a SRA-173H for the mixer, ERA-2 MMIC devices were used for amplification, and a 74HC390 to divide the GPSDO 10MHz reference by 2, followed by a 7-pole Xtal filter to provide a clean 25MHz reference signal for the LNB.

A 3D render of the board design resulted in:

Below is a photo of the (almost completed down-converter (with the wrong type of regulators fitted!) being tested on the shack bench. The ADF4351 LO board can also be seen in the bottom right-hand corner:


Lessons learnt:

    • There were a few minor undocumented features!
    • KiCAD is great, but I’ve still got a lot to learn.
    • Pay more attention to the mounting holes… they should have been earthed!
    • Pay more attention to the Silk-Screen, it’s a DOWN-converter. Right…
    • I added an onboard bias-T for PTT change-over switching via the 144MHz coax feed. The relays switch when 12V is supplied on TX. Had I reversed this so that the relays are powered on RX then the external LNA power function that’s available on a number of popular radios could have been used to switch from RX to TX.
    • The board provides 12V (via an onboard bias-T) to power the LNB via its coax feed. Had I made this a switchable 12V/18V supply I could have selected Horizontal or Vertical polarisation for 10368MHz terrestrial / EME reception.

Hindsight is 20/20, fortunately none of the above were major show-stoppers. For QO-100 operation the board is working fine. With some very minor changes I should be able to change the way the Bias-T operates both on the IF PTT side and the switchable 12/18V power to the LNB. I have used the down-converter with the K3 in 144MHz transceive mode and with a TS-2000 in satellite mode with the RX IF operating on 432MHz.


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