My 2.4GHz Transverter Project

After having done a 1296  transverter, the next logical progression from this is to do a 2.4GHz Transverter.

The Local Oscillator

I am using a Butler oscillator design as in EME65.

As an aside, one can check for “free air” resonance in this Butler design by swapping the crystal for a 47 ohm resistor and 100pF in series. The osc will then run at its own frequency. This I a good thing to do as I found that the reliability to start was an issue if off the chosen resonance. The unit seemed to “snap” in and out of oscillation if too far off. 

 I followed with  a quadrupler board that I made up from an old VHF comms design but modified to my needs. ZL1QF also uses something similar in his design., I proceded to get this bit running first as it is the prerequisite for a transverter to be able to run.. I am aiming for 2279 for a 145MHz IF.

The X4  Board with  BFR96 and ERA-5

 The Output from the above board.

Other attempts are with a 3x3x3 multiplier that so far is spectrally un clean  and lacks output level.

 The 3x3x3 with pipecap filter on the end.

Pin Diode Switching

I had a bit of a dilemma when it came to deciding how to split out the LO signal to the TX and RX converters. Several options existed.

1)  Use a Wilkinson Divder network with 2* 1/4 wave 75ohm sections that have a 100 ohm resistor across the ends. Basically a "Y" section with LO fed at the one end and split signal is fed out both legs of the "Y", across the "Y" is the 100 ohm chip resistor. A google search will go into more details of how these work.

2) Pin diode switching. This is the way I chose as I could switch all of my available RF from the LO to make sure I have plenty of drive for the mixers. I was aiming for about +7dBm  minimum  for safety.  As my LO chain delivers +12dBm, the loss wasn't too bad at +10dBm after the switch. Leakage was minimal.

With PIN diode switching, a margin of resilience is built in due to the fact if  there was a fault on one side of the TX or RX mixers, loading would be only present in the mode selected and would leave some semblance of operation still. This is not the case of the Wilkinon divider. 





The  TX Mixer.

The TX mixer is from a Stratex DXR 200 data radio. These already run at 2.4 GHz and it seemed a good prospect for the Ham bands. These units have two converter modules - TX & RX. I have used both of them together to form the transverter.

There is a mixer on each board so that meant splitting or PIN switching the LO signal. As mentioned before, I opted to switch the signal.

The unit has fairly simple connections to make. They are:-

LO IN
RF OUT
IF IN
POWER
GAIN CONTROL

Reverse designing the MMIC stages, it looks that the unit needs 13.8 volts to run.

I have a 10 volt supply to feed the power GasFET stages so I run the converter off that as I would need another power switching relay otherwise.

On first test, I find I have the 2424MHz signal out for a 145MHz TX IF input. All looks good so far. I did, however, notice that the LO signal at 2279MHz was present at about 20 dB down from the 2424.

Playing around with the LO drive level reduced this to about 35 odd dB down. I put a 470 ohm preset in series with the 68 ohm resistor feeding the ERA-5 at the end of the multiplier to alter the output power.

Tweaking the pot, I found I could get the LO down to the 35dB figure I mentioned above. Much past that, I started to lose output at 2424.

I have set the levels for the TX IF at 1mW (0dBm) and the LO drive level at about 2.5mW (4dBm). This seems to produce the best output with lower LO level at the output.

The TX gain control I found best at about mid supply rail so set it with a potential divider of 2k7s from +VCC to Ground and pick off at the middle.

 I had  fiddled the TX hairpin filters so that the LO falls out of the pass band of the filters but it seemed to knock out the 2424 slightly if you went too far with only a slight reduction of the 2279 LO.

My other option was to make a small external filter out of hardline and the copper U shaped case as shown below.

The Filter made from a design from G0ORY's website.

The TX Mixer and filter feeding the PA.

Here is the output - left peak is 2280 LO 48dB down here from the 2424 in the middle and the small peak to the right is the 2600 odd image. 

RX Mixer

The RX side has the same sort of connections to it, they are as follows:-

LO IN
RF IN
IF OUT
POWER
GAIN CONTROL

The front end has a Gas MMIC (MGA-86576) that is located after the first hairpin filter. After that there is a dual gate MOSFET that controls RX gain. RX gain is as what I have done for the TX gain control.

I added a little attenuator pad at the back of the RX IF port of the converter to aid in protection of the output MMIC as it is possible that it could see a short burst of RF from the 2m exciter IF. Across the output I put a couple of 1N4148’s back to back across part of the pad to limit input voltage to the MMIC. I have yet to see if it affects the noise floor.

I also removed the ceramic resonators that lie in the output path of the RX IF output as well as the trimmer caps. This increases 2m IF dramatically as the filters were used with a 400MHz IF before. I added a couple of 240nH coils to work as a filter as the mixer products seem to make the agc of the FT817 cut in without them.

In its current state, I can hear the WLAN system in my house at about S8 and could hear my 2.4GHz signal source from my AO 40 days.

I will get a sig gen and take some measurements when I can..

TX/RX Switching…

Switching the DC as needed to the relevant sections just  takes caution against noise or  potential instability issues, not that there should be any of course!

So far , that is it but more will come as the project nears completion.

More Pictures of the unit....

The top side showing the LO (top left) - TX RX Sequencer below & 10v LDO regulator for the PA, PA all covered up and TX mix + filter.


Bottom side with TX RX antenna relay, RX mixer and Circulator.



The X2X2 multiplier o get to 2279MHz  with Pin switch to the right.

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