John's homebrew pages

A 13cm (2.3GHz) transverter with 432MHz IF

I decided to try to move up the microwave bands one by one - so 13cm comes after 23cm. Whether or not I'll be able to resist jumping straight to 3cm at some stage we'll have to see.

After using multiplier designs for the local oscillators of my 23cm transverters, I decided to try a PLL-synthesiser approach for 13cm. I had decided to use a 432MHz IF some time ago so had obtained a suitable VCO - I now have ideas for doing some home brew VCO experiments as well. I had also obtained a couple of the Analog Devices ADF4113 PLL-synthesiser chips.

I have done the design for the local oscillator, with the loop filter design from the Alanlog Devices design tool. It uses a 20MHz TCXO (temperature compensated crystal oscillator - a bit like a micro crystal oven) as the frequency reference, and the synthesiser will be programmed by a PIC on start-up, which then goes to sleep. I have added an MMIC buffer amplifier to give enough drive for diode mixers, though at the moment I plan to try some mixer ICs.

Here's the component side of the LO board - it measures 60mm by 37mm. The board layout was drawn out using PCB again, and the board produced optically on pre-sensitised 0.8mm FR4.

PCB compponent side

The PIC will be inserted in a socket on the groundplane side; to avoid stress on the PCB tracks, the PIC socket is epoxied to the board surface (as are the 12V and "PLL lock" pins - the holes drilled are a touch too large!).

PCB groundplane side

It's taken a while to get the circuit built because we've been busy in the house - but here it is ready for testing. The 20MHz reference oscillator has been tested, but nothing else yet. The VCO is mounted at a slight angle because of a small error - failure to read the data sheet carfully enough! The connection locations on opposite sides of the VCO package are slightly offset, rather than lined up as I had assumed. Anyway it still fitted. The 470k wire ended resistor is there because I didn't have a surface mount type, and couldn't even find one on my collection of old boards! The output buffer amplifier is fitted, and should give an output between about 6dBm and 9dBm.

PCB compponent side - populated

The groundplane side has leads to connect the +5V and +3V to parts of the circuit.

PCB groundplane side - populated

The small amount of code needed for the PIC is now written, so that the PLL-synth is programmed on power-up. The oscillator now works, but at this stage with a test external 20MHz reference oscillator. De-bugging took a little while - the main problem was extremely simple, discovered by examining the ADF4113 on the board very carefully. The pins for the serial programming interface (clock, data and latch) showed connectivity with the board tracks using the test meter, but looked like possible dry joints. I re-soldered them and found that the charge pump reference voltage is, after power-up, now at the correct voltage (it was zero), and after the programming words are loaded the oscillator is set to the required 1890MHz.

It was a while before I had time to return to this and complete the LO. Finally I got there; first the clock slowdown in the programming code (put there to check the serial line signals) was removed, and the progamming stopped by making the PIC sleep; these both worked fine. Next I tried the on-board TCXO 20MHz oscillator instead of the external reference, and it also worked. This was something of a relief as the signal level from the TCXO is only marginally in spec (it's a bit low in amplitude) for the reference requency input for the PLL! It's now being boxed up; here's a photo of the board partly boxed, with an SMA output connector and the power input (just visible over the top of the box) in through a feedthrough capacitor in the side of the box.

PCB partly in box

Now it's working, here's the circuit diagram as built. The loop filter doesn't quite have the right values in (as given by ADIsimPLL) but it locks fine. Note that I have removed the pull-up resistor on the lock detect line, and used a logic output rather than the open drain output. Click on the diagram to get the full size version.

circuit diagram

The next step is to build the main board.

For the design there were a few new ideas I wanted to try. First, commercial passive double balanced mixers for the microwave region are expensive, whereas I had found some IC mixers costing just a pound or two each. That was clearly an idea worth trying. Secondly, I needed some selectivity at 2.3GHz, and commercial helical filters are also rather expensive; so I thought I'd try some stripline tuned circuits (and pipe cap filters are another definite possibility).

I also had quite a lot of A06 and A03 MMICs on surplus boards from commercial equipment, so thought I'd make use of what I have there as well. With these design ideas in mind, I drew up a circuit using the gEDA Schematic Editor.

Here's the PCB after etching; my most successful optically produced PCB so far!

part populated board

With the receive and transmit sections soldered up, I have been doing a few tests. From home this was not very successful. Here's the test setup outdoors; the transverter board still isn't boxed so it's rather a lash-up.

Rx test setup

I'd set up for a Tuesday evening SHF (2.3GHz and up) activity contest, but heard no-one. I'm not surprised, my home location is far from ideal for microwaves, with a clearish direction only to the north. However, with my home brew 12 element DL6WU Yagi (still to be written up) I was able to detect RF "mush" from a communications mast about 2km away, and also my little mini beacon (just a 25.1750 MHz block oscillator with a 30mm bit of wire attached to the output pin; I detect the 92nd harmonic in the 13cm band!) was detectable from the garden. Here's the antenna:

Rx test setup

The antenna certainly seems to behave very well indeed, and it will be great for /P operation.

Having failed to detect any amateur signals, I decided that I should go somewhere I would have an excellent chance of doing that, providing the receiver was working as I hoped. So I set out for Cairnpapple Hill in West Lothian, which is line of sight to the GB3CSB beacon cluster. Once the test setup was assembled, I could hear the 13cm beacon easily - even before the antenna was pointing at it. In fact, I could hear it without an antenna! (Since the transverter still isn't boxed.) With the antenna pointing at the beacon, there was a very strong signal indeed - S9+.

The beacon tone was absolutely clear (very musical in fact with the JT4G signal alternating with the CW), so there are not going to be noise problems resulting from my use of a PLL/synthesiser local oscillator. In fact, the 20MHz TCXO reference oscillator I have used seems to be very stable and also very accurate; the beacon frequency indicated on the FT-817 was only about 60Hz out, not bad at 2320MHz!

The next step was to finish the transmit side, then put the whole thing in a box. The transmit side seems to be a little more problematic than the receive side; it seems that the filters have rather more loss than I'd expected, since the output level is not at all as high as I'd hoped. However, given large filter losses (6dB in each?) the observed output (which might be as much as 0dBm!) is not too surprising, given the MMIC combination I'm using. So I decided to go ahead, box it up, and add the PA (a salvaged commercial board which should have 40-43dB gain!) to see if it works.

Here's the underside of the main board, with the ends added to take SMA connectors, and screws in the regulators to bolt them to the sub-chassis for cooling.

Main board ground side

Here are the circuit diagrams. First, the power switching which is on-board:

power switching circuit

Next, the receive side of the transverter. I forgot to change the grey background on this one!

receive circuit

Finally the transmit side:

transmit circuit

The bottom of the box has the sequencer board (controlled by a PIC - details will follow eventually), the coaxial relay (obtained at a rally) and space for the PA board. There will eventually also be a bandpass filter between the antenna relay and the antenna N connector. A heat sink bolts to the other side of this chassis under the PA board.

Assembly - bottom level

The main transverter board and LO are mounted on a sub-chassis; this makes for a more compact overall system than my mark 2 23cm transverter. I didn't know at this stage whether or not I needed a lid on the main board box.

Assembly - top level

Under test, with the PA board added, I found that the system took off (oscillations) under transmit, even on SSB, with the main board open. A bit of anti-static foam over the box stopped that for low levels of drive but not higher levels; and with the main box lid on it was not stable at all on transmit.

Here's a photo of the system under test. You can see the anti-static foam just placed over the transverter board box. The bench could be tidier!

Under test

I made a proper tinplate lid for the main board box, and that cured the oscillations completely with the overall box lid off, so I felt we were ready for some on-air tests.

The monthly RSGB UKAC for SHF was due, so I went out to a local hill (the Braid Hills) which is easy to access and gives me a much better location than at home. This turned out to be really successful; I could even hear the GB3CSB beacon, and had four good contacts over 40 minutes (I tend to chat!) including 93km to Jon GM4JTJ, with some good signal reports. I thought this was not bad for probably 4 Watts (as estimated from the DC power input to the final stage and the device efficiency) and a 12 element Yagi.

The next steps were firstly, to put in a proper screened output N connector (which unfortunately made no difference!), then secondly to box in the PA board, to try to stabilise the whole system under transmit. I made up a screening box from tinplate which fits between the box base and within a few millimetres of the sub-chassis; it fits tightly round the PA board. It's a bit rough but I was in a hurry!

PA screening box

Here's the screening box fitted around the (surplus commercial) PA board. I was quite surprised and absolutely delighted when I reassembled the system with the sub-chassis to find that the unwanted oscillations had all gone away; I can now operate the transverter with the overall lid in place. I thought that I might need to add some braid around the bottom of the screening box, to make a better contact with the box floor, and to add a proper lid. I was even prepared to add absorbing foam inside the screening box! However these steps proved not to be needed.

PA boxed in

So here's the whole transverter with the lid on. It still needs some rubber feet! The two power connectors allow two external 12V batteries to provide both 12V and 24V easily.

completed transverter

I still want to add a bandpass filter of some sort, and the transverter would probably benefit from being tuned up again now all the lids are done, but it works and gives me a few Watts on 2.3GHz. I'm ready to take it out on the next RSGB SHF UKAC now! 

That's pretty well the end of the project - I'll add anything further if and when it happens. Down the line I would like to build my own PA board but that's really a separate project.