A Cheap 925 MHZ transverter.

I am quite keen on the VHF and UHF bands and have delved into various microwave bands. 2m an d70cm are pretty mainstream and I had a hankering in getting on the microwave bands.

The most common progression is to get on 1296MHz, this band can offer extremely good contacts with relatively little power. After having conquered the first challenge of 1296, I then had a crack at 2424 and 5760MHz.

While I have a 10gig project on the boil, I came over an old cell phone 5watt transceiver unit that had some of the parts missing. The box was screaming out for a project to fill its innards. It had a nice bit of heat sink fin area and plenty of room for boards and nice shielded compartments to put RF bits in.

With that, I decided to create something that filled the box. One band I am missing is 925MHz (32cm) band. There are not many kits around apart from the Down East Microwave (DEM) units, so I decided to have a challenge and make one myself. The challenge was twofold, firstly, build a unit of my own design, and also make it out of junk parts around the shack if at all possible. To what degree you can do this, is how diligent you are at collecting junk of the right sorts! I must be quite good at this as the house is nearly reaching critical mass and could blow at any time!!

This box I had, appeared to come from a Japanese emulation of a Motorola unit but as it has no markings, one can�t be sure. All that I know, is that it has great compartments for RF. This is especially good when making Local Oscillators that are multiplied up. Any change in nearby capacitive influences can affect its frequency stability so enclosing it in the most solid enclosure will help in making it mechanically robust as well as aiding in LO stability.

Design.

Perhaps the hardest bit of the transverter is getting a stable LO source at a low cost. That includes procuring a crystal of suitable frequency. If one can avoid it, have a look in the junk bin or find a crystal denominator that is easily obtained or a common frequency in use. This is not always practical in real life but one can try.

Sitting down with the calculator, I found that a 65MHz crystal would finally give 780MHz LO. 925 � 780 = 145.

Very good since I choose to have my transverter IFs at 145MHz due to problems one gets in a contest situation with nearby 144.2MHz contest bands. Not so bad if you are a single operator running several bands but in a club station or multiple ops, poses a problem with IF breakthrough that can affect both parties with weak signal reception.

Luckily I had a couple of spare minikits LO boards from VK5EME, www.minikits.com.au . These are based on a G4DDK butler oscillator design which provides low crystal loading and very good performance.

In its base form, his board uses 3^rd or 5^th overtone crystals with a X3 and then a X2 multiplier that will take the output up to 390MHz. Output should be in the order of about +10dBm or so.

From there, I used a Waikato VHF group filter board in a X2 multiplier form to give 780MHZ out. These boards are available from the Waikato VHF group at a modest price. These filters are based on the G4DDK article in the VHF/UHF manual, and are very useful and versatile. They are quite frequency agile as well. I have used these up to and past 1296MHz.

With a transistor driving this filter, it will give anywhere from 0 to 2dB gain and a very clean output spectrally. I found I could get +13dBm out of this unit.

I was able to procure a minicircuits SIMA-5h mixer at a very good price. It is very similar in size to the common SBL-1 mixer unit. While it came cheap, it needed quite a high LO drive of between 14 -17 dBm. I missed out with the raw LO drive level since in a good design, a mixer needs to see 50 ohms at each port so it ensures low intermod products and good linearity.

Click here to magnify

I chose to put a 3dB pad at each port of the mixer as I couldn�t be sure that the following filter was necessarily at 50 ohms.

The mixer was to be used as a bidirectional setup so that one mixer would service both TX and RX functions.

Here is the block diagram of the unit..

For a nicer looking PDF - click here...

TX and RX parts are PIN diode switched to the relevant functional blocks.

MMICs are used as gain blocks and work well with a simple config. I used MAR-8 devices as I had them in the shack but these devices are inherently unstable and need taming down.

The filter sections are the same as what is used on the multiplier stage but there are 3 on the transverter board. One for post mixer filtering and one each for TX and RX filters respectively. This project is a little trimmer capacitor hungry but I wanted to make a board that was not monotonic in frequency and that is easily reproducible by home etching methods as this unit could be used as a basis for a 1296 unit as well.

The board is made up from standard double sided G10/FR4 fibreglass board. All components are surface mount apart from the mixer and the IF port as well as the LO / TX and RX ports, depending on your box construction. The above mentioned units mount from the ground plane side of the board.

RX Path.

This passes through the RX filter and into an MMIC device to be amplified to a level that will overcome the insertion loss of the PIN switch, filter and 3dB pad as well as enough level to drive the mixer RF port.

I have not include a front end amp as the board space was limited and also it gives an option to put on a pre amp of the readers choice, be it a GaAsFET, GaASMMIC or another standard MMIC.

Click here to magnify

TX path.

A mixed up product of the LO + IF generates a 925MHz plus its image that is filtered by the post mixer filter to pass 925MHz and eliminate the LO and image products from the output. This passes through the PIN switch and is amplified by another MMIC which then feeds another TX filter. From there, it will be further amplified by a MAV-11 MMIC to give approximately 50mW output. From there, it can be amplified further or just left as it is and feed it to a high gain antenna.

I plan to modify an old cell phone hybrid power block to see if I can make it run on 925MHz.

Construction.

Firstly, and the most logical place is to start with the LO. Best place to start is to procure an EME65 LO kit from Minikits. There is a later model of this board called an EME65B that improves low level phase noise and spurs due to improved power supply regulation.

This is a simple kit to build and should be able to be built easily within 2 hours. The instructions are good and is quite easy to get going. If you have a mate with a spectrum analyser, all the better.

Kevin ZL1UJG has some very good tips regarding this oscillator that will make things easier as well. Please contact the author for this info.

Once this is up and running, start on the multiplier stage.

This multiplier section is commonly used by Kevin ZL1UJG so no reinventing the wheel here. Follow the construction from the schematic and picture here and build with proper RF build practice. Put plenty of vias under active devices and at the end of the filter elements on the ends close to the centre of the board.

	Note - to increase efficiency, you may need to add approximately 2pF across the base to ground.

The PCB I have in protel format and can be easily reproduced by the well known laser printer transfer method. The information will be on my website at http://www.qsl.net/zl1sww under construction projects. Just make sure that when you have transferred the pattern on the board, that you can read the writing correctly.

I coated the groundplane side with a clear lacquer to protect it from tarnishing as well as offering an etch resist in the Ferric Chloride etchant. Do this AFTER ironing on the pattern as it will boil the paint off the other side if you do it the other wasy around..

Once the board is etched and cleaned up, spray with a thin coat of lacquer to stop tarnishing.

Drill holes in the groundplane close to the active devices and at the base of each filter element. Put vias in these holes by short bits of TCW and solder. I have found that if you leave about 0.7mm each side, you can use a ball peen hammer to have a quick tap to create a blind rivet effect that can be soldered over without sharp protrusions.

Run copper tape around the edge of the board where there is a top ground plane connection.

I started the construction of the transverter section in order of the signal path. That is, LO amplification into the mixer, Post mixer filter, TX amp and filter chain and finally the RX filter and amp section.

Doing it his way makes for a progressive construction method where you can fault trace and verify each stage works correctly. This was especially important for me in designing this unit as I need to get all power levels satisfied through all the sections. This is very important around the mixer section as low LO level will impact conversion gain throughout the system, possibly resulting in low RF output and poor RX sensitivity.

The circuit for the unit is supplied at this link here as a pdf.
Note that the ERA numbers aren't stated on some devices as other versions could be used with changes in bias resistors etc. I used MAR-8 devices as I had them, but not really reccomeneded unless you use MAR-8a.

Tuning. Up.

As mentioned above, I did the tuning on a progressive basis as I was building. This aids in having a system that should basically work at the final stages of board construction/.

LO Amp.
I used an ERA-5 to get +16dBm or thereabouts. Seeing I had heaps of drive. I put in a 10dB attenuator here to reduce the level to stop the ERA going into compression and possibly killing the device as I had about +13dBm from the LO section. I was after about +3dBm into the ERA input.

Check that it works to this point.

Start with the LO drive and verify that there is +15 - +17 dBM present at the mixer input port. (Measured without the mixer in place. Once happy with level, install mixer.

Post Mixer Filter.

This is the same filter type used elsewhere in the project. One as the post mixer filter and one for TX as well as the other for RX filter.

The filters here a just adapted from the Waikato VHF group boards and are placed on a common PCB for ease of construction.

Apply a low level IF drive at 145MHz to the IF port, drive with about 6-10dBm.

Peak the filter to get a clean 925MHz signal at its output.

Note that the centre trimmer cap in these filters are quite sharp when tuning.

Install the PIN Diodes

PIN switching is used for switching the TX or RX sections to the common filter and mixer. These are BA379. These little diodes I have used at up to 2.2 gigs. The PIN diodes are just forward biased along with the respective TX or RX supply that has its DC rail connected by switching relay.

The PIN diodes are fed by 120nH SMD inductors.

On TX the PIN switch directs the filtered Mixed LO and IF product to the input of a MAR-8 MMIC ( used MAR-8 as that�s what I had, would personally recommend MAR-6, more stable). The MAR-8 is heavily damped on the input by 220 ohms to maintain stability.

When Soldering MMICs, it is important to keep a very low impedance to ground for the ground pins of MMICs. Solder small via wires under each MMIC before installing.

Instal the TX MMIC and filter components. Do the peaking procedure by enabling the TX power feed and hopfully the PIN diode will be forward biased and RF will pass through to the MMIC. Peak up the TX filter.

The amplified signal passes through another filter and amplified again by a MAV-11.

The MAV-11 should get about 50mW or more.

Install the RX components � MMIC and filter bits. Repeat peaking as per the TX filter but in this case there will be a Sig Gen at the RX input and the IF port of the mixer is now connected to a 2m RX on 145MHz. Make sure the RX PIN diode is forward biased and MMIC has the proper voltage on it.

In its present state, I found the unit had -87dBm sensitivity, a bit deaf in my view so I preceded the board with a GaAsFET or HEMT
pre amp made with devices from a surplus LNB. Now it hears down to -130dBm or better.

I amplified the TX out with an MRF559 amp that now give about 300mW out.

I have had two contacts so far with great success.