ARAC2 transverter project

[G3WIE]

Hello everyone,

As the title suggests, I am in the final stages of the design of a 4m transverter - the ARAC2. The circuits have been designed, breadboarded and tested as far as possible in that environment. The purpose of this post is to describe the project, and collect feedback and last-minute ideas before I commit the design to printed circuit. The timetable to complete the project is still a (hopefully small number of) months.

I will put the design (including PCB data files) in the public domain so anyone can build one or, more likely, groups of builders can arrange a joint project. Apart from the first batch of PCB for alpha builders/testers, I will not be manufacturing PCBs or kits. You should be able to send the design data (eg gerber file) to one of the online services in the UK or abroad and they will make the PCB.

Background
Back in the 1990s, the Andover radio club ran a club project to build 4m transverters with apprx 5W output - the ARAC design. The idea was to encourage newcomers on to the band by giving them a pcb/kit to build, guidance on how to use ex-cb rigs as a driver, and a dipole design which could be built from DIY store bits. The design used commercially made coils, now unobtainable, and this is the primary reason it has lapsed. Somehow, I seem to have volunteered to do an update. Principally, I blame G4NNS (the original designer of the ARAC), plus an unremembered quantity of alcohol and general bonhomie at a microwave round table meeting. Ted, G3XUX, and the others who spend a lot of their time and effort running FM nets must also take responsibility.

The ARAC2 will follow the same principles as its parent. Additionally, avoiding obsolescence is a primary aim of the new design. There are pockets of 4m FM activity which will be a target audience, as well as clubs. The aim is to give them SSB/CW to expand their horizon. Chinese 10m transceivers can replace the cb radio of ARAC1, and many more people have FT8*7 or equivalents than in the '90s. We expect initial activity by many of these builders will be on vertically polarised SSB, perhaps as extensions to the FM nets that run regularly (that's the key word) in various places around the country. Vertical antennas are cheap to make or buy, simple to erect and have low visual impact; the FM community have them already. Once the bug has bitten, antennas can become horizontally polarised and expand to suit, rather than being required before contacts can be made.

This is not intended to be a cutting-edge, high performance design for dx-chasers. It will be as straightforward to build and set up as possible, and if that compromises the last bit of performance, then so be it. The design could be altered for 6m and probably 2m. Someone else can do that.

Here are the project aims in more detail:

Straightforward to build
------------------------
Generous pcb dimensions and mostly through hole components.
Double-sided pcb with plated through holes. More expensive, but more repeatable performance. You could probably make a pcb with the laser toner method as one side is a ground plane.
A prepunched aluminium case is under consideration

As foolproof as possible to set up
----------------------------------
Built in measurement circuitry for local oscillator setup, power output, and to indicate overload of the transmit mixer

Resistance to obsolescence
--------------------------
Inductors are air-wound, or on toroidal cores. Yes, the latter is betting on the survival of Micro-metals Inc, but I saw little alternative for larger values of inductance. Generous clearance around toroid locations allows future substitution if really necessary.

Mmics for amplifiers. There's a huge array of these from many manufacturers. They are relatively power-hungry, but dc power is easy to come by.

The transverter will produce about 100mW output. A power amplifier design on a separate board will provide about 18 watts output. A *possible* alternate PCB design will accommodate whatever you can find in the way of power transistors.

Multiple footprints for device substitution. Eg the mixers which can be the widely available SBL-1 or the newer standard for 6-pin SMD mixers.

Comments and ideas are welcome - please append to this topic, after my next post which has more details.

Chris G3WIE
(continued in next post....)

[G3WIE]

Not much of the design is original. My sources for the transverter board include the the classic transverters from G3WPO, G4JNT, G4FRE and others. The power amplifier design is by Geoff GI0GDP, who has applied a lot of time and effort to characterising modern RF power devices. The performance goal is: adequate for day-to-day contacts without annoying other band users or the neighbours, and to give a reasonable performance in contests. If you'd like something better, all the design data will be freely available under the Creative Commons licence or something similar.

This is a nominal 12volt 18W PEP design. It should operate from 12 to 14 volts.

Local Oscillator:
-----------------
- Jfet oscillator with back-to-back diodes across the tank circuit to define the output without device saturation. This should have better phase noise than the usual oscillators - that's my one concession to "improved performance"! I spent a long time on this circuit so that it oscillates reliably with four crystals from different manufacturers.
- Tested with 42MHz for 28MHz IF, or 49MHz for 21MHz IF. Or choose your own frequency.
- Buffer amp, then splitter/attenuators to give each of the diode mixers the required 7dBm from a 50ohm source

Receive chain:
--------------
- Dual gate mosfet preamp. BF998etc/SOT143 or BF981etc/SOT103 or 3N211,40673etc/TO72 (on the other side of the pcb). Luckily, all have the same ordering of pins.
- 3-resonator bandpass filter of "mixed form" from the book Experimental Methods in RF Design. These have a symmetrical frequency response. Transformed (not transformeR) coupling so that the smallest capacitor is ~15pF. The usual top coupled filters require tiny, unrealisable capacitors or rely on strays, and they have a poor response on the hf side (think: Band II broadcast protection).
- Double balanced diode mixer. The ubiquitous SBL-1 or Minicircuits ADE range of standard footprint components.
- Mmic if amp. I'm using Minicircuits ERA-5 everywhere in the design. Others could be substituted
- 2-resonator IF filter at 28/21 MHz.

Transmit chain:
---------------
- As with ARAC1, the input attenuator starts with a 50ohm power resistor, bolted to the case. This provides the driving rig with a decent match. Input level should be a maximum of a few watts, and the case must be big enough to dissipate the heat from this!
- A built-in level detector to help you can set the driver rig output level so it doesn't overdrive the mixer.
- Double balanced diode mixer as Rx
- mmic amplifier
- PIN diode attenuator with a front panel control to set output power over a range of about 10dB
- 3-resonator filter as per the receive chain
- another mmic
- a bipolar Class A amplifier to bring the output up to about 100mW.

Power amplifier
---------------
This is another area plagued with obsolescence, and fraught with difficulty when choosing devices for linear operation as most manufacturers do not specify them for linear operation.

There are (at least) 3 ways to provide a PA for the transverter:

1) Use a module. That should be "use THE module" as there's only one that's generally available, the Mitsubishi RA30H0608 30W module. There's no design here for one of these; they're simple enough to use, follow the data sheet. They cost ~£60 and I'm trying to keep the cost down.

2) An 18W Power amplifier by Geoff GI0GDP (this is the preferred design):
Geoff GI0GDP has designed amplifiers for some of the newer Mitsubishi RD-series power mosfets, and has measured gain and the all-important intermodulation distortion. His measurements show that the low-priced devices (eg the RD15HVF1 15W device at £6) have appalling IMD performance at output above one tenth of their rating. However, the higher power devices, starting at the RDHVF30 seem to be from a different design. The RD30HFV1 is capable of two-tone IMD performance of about -30dB if its output is restricted to about 18W. They cost $23 from rfparts.com. A separate PCB design for a single stage amplifier using this device will be provided. It will have a built-in SWR power/indicator

3) Scour the surplus market
Rather than specify what's available today (but not tomorrow) a further design MAY be done to accommodate the widest range possible - essentially this is a generic PA pcb. The appearance of this PCB depends on how much time I have, or if anyone else volunteers.

Many suitable devices are sporadically available. Quite a few older power transistors are specified to run at 28 volts, and there is evidence that these have better linearity than their 12 volts cousins. I have bought step-up voltage converters on a little PCB from China for a few pounds which provide 28 volts at a couple of amps. Buy your own and add one.

Outline specification is something like:
- A separate general purpose PCB with two stages. 50ohm input and output for each.
- Attenuators at the input of each stage. Levels can thus be set to suit each device, and attenuators reduce tuning interaction between devices.
- The first device will probably be a Mitsubishi "15watt" device, running at about 1 watt output for reasonable 3rd order intermodulation performance. They are cheap and have lots of gain so you can use heavily damped matching to keep performance reproduceable and stable.
- The second device will be whatever you can find: bipolar or mosfet. I hope to arrange the PCB to accomodate a couple of the most common packages.
- Regulated bias supplies for both stages.
- Built-in power/SWR indicator

ALC
---
There isn't any, so you can't blindly rely on it and radiate a horrible signal. The builder is expected to set the input attenuator and driver rig up using the overload indicator, and then not touch them!


Control circuits
================
The intention is to accommodate a wide range of driving radios, with the assumption that they can be turned down on trasmit to a maximum of about 10W.
- a simple sequencer controls timing of power switching and relays, including the PA.
- Transmit/receive can be controlled by PTT line or, if really necessary, by sensing rf input from the driver.
- Some Yaesu radios put 5v on the antenna socket during transmit. That can also be used.
- I'll include G4JNT's circuit that controls the driver's ALC input (if it has one) to stop rf surges on changover from RX to Tx
- There'll be links so parts of the control can be missed out or changed around to suit particular circumstances

Chris G3WIE

[G6NJR]

Hi .

How much trouble would it be to move the IF I/O to 21Mhz


Pete G6NJR

[G3WIE]

21MHz IF is covered - it's mentioned in the Local oscillator section. You'd use a 49MHz crystal oscillator (I've tested with this), and a 21MHz if bandpass filter. Personally, I'll be using 21MHz; 28 is provided for the Chinese 10m rigs

Chris G3WIE

[LA9XNA]

Sounds to be an exiting project.
If possible I'd like tu buy one of the prototype boards and activate JO28ux.

Regards
Geir

[G6NJR]

Ok .

will have to have a look at that shortly .once i got this move sorted that is .

Pete G6NJR

[G3WIE]

Geir,

I'd be delighted to help you put JO28UX on 70MHz, and to have another alpha-tester. I will let you know when we are about to make the prototypes. I hope these PCB will be right first time, but I cannot test for such things as leakage from the LO to the output until we make one and measure it. You may have to modify the board...

Chris

[G3WIE]

After a long pause, here's an update.

I have rebuilt the breadboard transverter (a couple of times since the last post) and just finished laying out the PCB for it. I will first home-etch a couple of prototypes so that the design can be proved in a more realistic environment. There was little point in offering the breadboard up to a spectrum analyser. Various bits have also been tested in isolation as all stages are 50ohm in/out. For example the rx preamp and bandpass filter were repeatable with a nice symmetrical passband.

The reality is quite close to the "spec" at the top of this post. Principal changes are

- for the majority of components you have a choice of through-hole or surface mount, you can mix as you wish. The pcb footprints accept both. That made my brain hurt.
- the mixers are SMD only. These are cheap and have 0.1"-spaced pads. The layout was too gruesome to provide for both, so your SBL-1 will stay in the drawer.
- there is one tiny TSOT-23 SMD chip to solder which is the power detector; this is used as an alignment aid and also as the overdrive warning and RF-actuated PTT circuit
- the control and sequencer circuits are on a separate board. I will design a PCB for this, but you could build it on veroboard. You could also roll your own to fit in with your own station requirements. You could use a microcontroller like I have done on my current transverter, and also spend a long time keeping its noise out of the rx!
- it has sprouted a receiver output after the rx bandpass filetr so you can feed it into your SDR/Funcube dongle/DVB-T dongle.

The "standard" PA will be the GI0GDP design; Geoff has kindly provided his prototype which happily produces 18W. We discussed availablilty of devices - his design uses a Mitsubishi part that seems only available from rfparts in the USA. I sourced some D1001UK 28v FETs from Semelab (British manufacturer - yay!!) and Geoff has also built and tested a PA with one. They are available from RS and Farnell. If you don't have 28v to hand, you can get step up converters on fleabay from China for a few pounds. You could also use a Mitsubishi module.

In the spirit of the original ARAC project, I have also done some work on simple antennas to get folk going. A simple end-fed dipole made from coax is on the stocks. Those of you in the RSGB may have seen mention ofthis in G3LDO's column on CFD dipoles last year. A friend has tested on of these for six metres and managed to work a couple of hundred km on 20W with it strung across his garden at eaves height. So you can get going on 4m ssb without loads of aluminium in the sky

I repeat my commitment to put all the data in the public domain. I'm not going to sell kits; group buys are the way forwards. A commercial company can sell kits if they wish but they can't charge anything for any of my designs which will be licenced under the creeative commons share-alike licence.

I'm going to recruit a small number of alpha-builders to validate the first batch of pcbs, which I will buy. If you'd like to join in, please let me know, but you must commit to building it! I'd like a mix of ability and experience from beginner upwards. This won't happen for a month or so while the prototypes get built. If we're lucky, we can get this up and running in time for this year's Es season. And I might have time to join in instead of designing a transverter....

73

Chris G3WIE

[G3XUX]

Hi everyone
G3WIE & I conducted an ARAC2 transverter project review yesterday after the Didcot rally. Having seen Chris's working papers, I can tell you that this transverter will be ideal even for faint-hearted constructors (which I was until very recently).

Anyhow, if you are at all interested in this project, please DON'T HESITATE to post something here to give encouragement to the team.
Cheers
Ted