AOR DDS-2A Product Review

John Wilson's partiality for the older valued receivers is well known. In his offering this month he looks at an interesting add-on unit designed without a thought for profit - just because it was needed.

It's reassuring to find that even amongst the dedicated market chasing manufacturers there are those who will produce something just for the hell of it, and to follow their own particular enthusiasm. The founders of AOR, Mr. Oshima and Mr. Takano fall into this category and because they are admirers and users of Collins equipment have decided to produce something which may not bring any profits but will bring a great deal of delight to all the other Collins enthusiasts around the world. I have been privileged to try out this new device, largely because I own two KWM-2 transceivers, but my comments will apply equally well to the "S" line receivers and transmitters.

The KWM-2 and "S" line equipments, for whatever design reason Collins may have had at the time, tune in bands only 200kHz wide, and because the first conversion oscillator uses a crystal to convert the incoming signals down to a tunable i.f., a separate crystal is required for each 200kHz band covered. Whilst this may not be a problem for a purely amateur band receiver where 200kHz covers the bit you need at any one time, it's a real drawback if you want to have general coverage because you will need upwards of 130 crystals. Not only would 130 crystals cost a lot of money (I just had a quote for a single crystal at œ13 per unit), there aren't enough spaces in the equipment to fit them, so it would be "up with the lid" every time you wanted a band change. For non - aficionados of Collins, you should know that the r.f. sections of the KWM-2 and "S" line are capable of operating anywhere between 3.4 and 30MHz, so only the first conversion crystal needs to be changed for operation between these limits.

What AOR have done is to design and manufacture a solid state external unit which generates all 132 spot frequencies at 200kHz intervals to replace the first conversion oscillator, and also a DDS v.f.o. which replaces the 2.695 to 2.495MHz v.f.o. in the Collins. By clever electrickery and use of microprocessor magic, the DDS-2A effectively turns the KWM-2 and "S" line units into fully tunable general coverage receivers/transceivers and at the same time introduces a range of modern features such as memories, twin v.f.o.s, digital readout to 10Hz and so on. Given the seemingly eternal appeal of Collins equipment, the concept of having continuous frequency coverage makes the DDS-2A a "must have" accessory; I certainly want one for myself as soon as possible.

The DDS-2A is packaged in a rugged but stylish box measuring 80mm high, 180mm wide and 181mm deep, weighing 1.9kg. It's made to match Collins style and colour, and you can see from the photograph that AOR have done a good job. As with other AOR products, a great deal of thought has gone into the smaller details, such as the fact that the main dial shaft has been specially fitted with a quarter inch shaft rather than the commonly encountered metric shaft so that the owner can replace the supplied knob by the classic Collins spinner knob to make the match complete. Taking a look inside reveals a beautifully made unit which oozes quality (and costly) manufacture, whilst wide use is made of surface mount techniques, and all sensitive sections are separately screened. The absence of flexible wiring shows a design awareness of the potential problems encountered when cable harnesses are used. Needless to say, I was eager to get the DDS-2A connected sol found a suitable 12V d.c. supply and got to grips with the handbook.

I couldn't have wished for anything easier: all I needed to do was take out the "VFO Power" plug inside the KWM-2, connect a ready terminated lead from the DDS-2A and connect another ready made lead from the DDS-2A to the "Ext VFO" phono connector on the rear panel. That took care of the v.f.o., but how were AOR going to replace the h.f. crystal oscillator, because Collins had made no provision for this in their design. The answer was brilliant; AOR supply a small tubular unit with a B9A plug on one end and a B9A valve socket on the other. Take out the first receive mixer valve, replace it by the little unit and plug the valve back into the socket on top. From the side of the unit comes another coaxial lead which goes back to the DDS-2A, and this carries the 132 different injection frequencies into the receive and transmit mixers in the KWM-2. How neat. I have to say that the 75S series of receivers have to have a tiny modification done to them but as Collins owners are probably competent to do this job there should be no problems since the modification instructions are clearly described in the DDS-2A manual and even the necessary bits are provided. If advice is needed. AOR UK are on hand to answer any queries, but as far as I was concerned with my KWM-2 the job couldn't have been easier.

The "Eyes" Have It

Having connected the 12V d.c. supply, the DDS-2A switches itself on automatically when the main receiver or transceiver is switched on, and the display is one of those excellent back lit l.c.d. units which show black characters on a glowing amber background. Tired eyes caused by age, drink, or both, really appreciate this type of display... I can vouch for that. Never mind the Tony Blair advert - I really do look like that! Frequency readout uses eight digits to give 10Hz resolution, and the DDS v.f.o. moves in 10Hz steps at the smallest tuning rate. An underline cursor is located under the frequency digits, and this can be moved left or right using two buttons located alongside the tuning knob. The tuning rate is determined by the position of this cursor, so if it sits under the 1MHz digit, the tuning steps are 1 Mhz, whilst under the 10Hz digit the tuning steps are 10Hz. This means that if you need to move swiftly from 4 to 28MHz it can be achieved in seconds, then revert to finer tuning steps for actual tuning of signals. I made comment in a previous review about my own disquiet with automatic "speed-up" on synthesizer tuning controls, but in the DDS-2A I have found my ideal. For me, the concept of selecting which digit to change and then whizzing the tuning knob without encountering any "speed-up" s perfect, but I appreciate that it is only a personal preference and should not be taken as a definitive statement on which method is best.

The main display also shows memory channel number; which v.f.o., A or B, is in use; a "remote" legend to show that the DDS-2A is under computer control; and other information on the various operational functions. 100 frequency memories are provided with easy entry and recall, the memory channel being selected either by the main tuning knob or the two up/down buttons alongside it. When the v.f.o. is in use the memory channel number disappears from the display so that you know at a glance whether you are using the v.f.o. or a memory. The twin v.f.o. system gives the facility of being able to switch between two independently set frequencies, and for the transceiver user the further facility of split transmit/receive frequencies, although because of the exciter/preselector tuning control the two frequencies must be in the same band. Split band (cross band) transceive is not possible.

The functions of the control buttons on the panel are obvious from the photograph, and represent the normal range of facilities provided on modern transceivers. One thing which may puzzle the non-Collins person is the arc of five l.e.d.s designated A to E alongside the control buttons. The letters refer to the five frequency ranges of the preselector control on the KWM-2 or 75S, and remind the user to select the correct band switch position and peak the preselector for the frequency range in use. This may seem an antiquated thing to have to do in these days of broadband receivers, but anyone who has read my previous articles will know that the presence of a preselector improves the receiver performance no end, particularly in rejection of second order intermodulation products, and that the lack of preselection has resulted in observable deficiencies in some very recent, and expensive, amateur radio transceivers - but as a Collins owner you know all that, don't you?

What's On The Back?

Well, there are the expected things such as the power input connector; you need 12V d.c. at 600mA; two coaxial outputs for the v.f.o. and h.f.o. drive to the KWM-2/75S; a 9-pin connector for the inter-unit control (this is the lead which plugs into the "VFO Power" socket inside the transceiver); and the inevitable RS-232C port for computer control. Any PC terminal emulation software should work with the DDS-2A and the control codes are all explained in the DDS-2A manual, basically being straightforward commands duplicating the front panel controls. The DDS-2A responds to computer instructions by returning frequency, memory channel number and so on, and frequencies can be sent from the computer keyboard and used as v.f.o. or written to a selected memory. All very comprehensive and easy to follow from the manual.

However, there are also two slide switches on the rear panel labelled "TX-RX" and "ON-OFF" which require a mention. In any s.s.b. receiver or transceiver, the exact frequency you are receiving depends on the frequency of all conversion oscillators used in the signal path, including the carrier reinsertion oscillator (b.f.o.). Because the DDS-2A offers not only readout to 10Hz but also outstanding stability from the use of a TCXO master oscillator, it is essential to include the receiver carrier insertion oscillator in the readout calculation if the display is to be correct, and because this oscillator is often aligned to suit the particular mechanical filter within the transceiver it is not possible to use a standard offset value in the DDS-2A. AOR therefore have provided these two little switches on the rear panel which enable the DDS-2A to be adjusted by the user to suit any particular carrier oscillator setting thus ensuring that what the readout says is correct. All explained in simple terms in the manual, and done in a few seconds - very neat. The TCXO incidentally gives frequency stability of 5 p.p.m. (0.005%) which is a distinct improvement on the "100Hz after warmup" specification for the original equipment (and you don't have to wait for warm-up).

Other Technical Niceties

Collins instruction manuals include a note that the second harmonics of the v.f.o. and variable i.f. fall within the range 5 to 6.5MHz and spurious responses will occur. The v.f.o. tuning range is 2.695 to 2.495MHz which is below the normal operating frequency range of the transceiver, but the second harmonic is 5.390 to 4.990MHz whilst the third (although not mentioned by Collins) tunes 8.085 to 7.485MHz. The reason for quoting the frequencies tuning high to low is that due to the system configuration in the KWM-2 or 75S, the v.f.o. tunes "backwards". Fig.1 shows how the second harmonic of the v.f.o. coincides with the receiver frequency in two places within the 5 to 5.4MHz bands, and at these points a spurious signal will be heard. Fig. 2 shows the effect of the third harmonic in the bands between 7.5 and 8.1MHz. In the original Collins design no attempt was made to do anything about the v.f.o. harmonics and consequently they are very prominent (almost paralysing), but AOR have thoughtfully provided a band pass filter in the signal feed from the DDS-2A to the KWM-2 which knocks the v.f.o. 2nd harmonic down by almost 30dB and the third by about 45dB. When tuning across the points of coincidence using the DDS-2A, the harmonics are still there and reasonably strong, but greatly improved on the original design. Someone at AOR in Tokyo certainly knows their Collins equipment.

Never Mind The Technical Detail, How Did You Like It?

Using the DDS-2A transforms the KWM-2, and also presumably the 75S, into an up to the minute piece of equipment. The sudden availability of the complete frequency range from 3 to 30MHz made me wander off tuning all sorts of frequencies I'd not heard before on the transceiver, and I revelled in the "Collins sound" coming from aircraft and utility channels. The stability of the DDS-2A made ECSS reception of broadcast stations really easy, and although I would now like someone (are you listening John Thorpe?) to make a synchronous a.m. detector for me, I'm pleased enough with the performance as it is. Of course, for the owner of a 75S-3 series receiver, the addition of the DDS-2A makes a real difference, because the 75S already has a.m., s.s.b. and c.w. modes provided together with a tunable i.f. notch and even a tunable b.f.o., so what a combination to have. For the radio amateur, the scene is even better because having the split v.f.o., 100 memories and everything else including a transmitted signal with low third order intermod products (due to the Collins p.a. and driver design, not the DDS-2A), brings the station right up to date without having to spend œ3000 plus on a solid state replacement transceiver with possibly inferior receiver second order intermodulation performance. As far as r.f. performance is concerned, the DDS-2A produced the same sensitivity and dynamic range as the original Collins oscillators, together with identical power output on transmit. The output from the DDS-2A v.f.o. was cleaner than the Collins PTO, probably due to the use of a DDS system, but the HFO (132 frequencies) was noisier than the Collins crystal oscillator in the KWM-2. This is not in the least bit surprising because there is still little to beat a good crystal oscillator for noise performance; in fact most engineers and reviewers (including me) use crystal oscillators as clean signal sources. That being said, the increase in reciprocal mixing noise using the DDS-2A makes little practical difference in use, and the combination is still on a par with most modern solid state equipment.

Conclusions

For anyone owning Collins equipment the DDS-2A is manna from heaven, and AOR are to be congratulated for taking the time and effort to produce this unit. Sales will obviously be limited by the relatively small numbers of Collins owners, so for UK customers the DDS-2A will only be sold direct from AOR UK in Belper, Derbyshire. As I have mentioned, adding the DDS-2A to a 75S series receiver will make a wonderful general coverage unit. whilst the radio amateur can revitalize his Collins station without difficulty. The unit is very well designed and constructed and a great deal of thought and knowledge has gone into its production. The slight degradation of reciprocal mixing performance is more than offset by the enormous benefits given by having full general coverage operation together with up to the minute memory and remote control facilities, and it's really refreshing to find genuine enthusiasm from a small (ish) manufacturer of communications equipment. Units such as the DDS-2A could never come from any of the "Big Three" because they would never be profitable, so it's a big "Thank you" to AOR from the hobby in general and Collins owners in particular.

Reprinted with permission of:
Dick Ganderton - G8VFH
Editor - Short Wave Magazine

"The Wilson Review" AOR DDS-2A
Vol 54 Issue 10 - October 1996

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