Using the DFS1201 11.2 to 12GHz Oscillator

These oscillators have been appearing recently on the surplus market covering the range 11.2 to 12GHz. This document records what I have found out about using them.

Looking at the Elcom web site (1) they showed a DFSS and a DFSL unit but not a DFS unit. However they did have a datasheet showing how the serial interface worked (2).The device needed SPI signals, as used in previous projects (3) (4) (5) where a PIC was used to generate the signals. According to the serial interface datasheet, the data conveyed by these SPI signals that should be sent to the unit are eight 4 bit words representing the frequency in MHz in BCD. The bits have to be sent with the LSB of the least significant word first through to the most significant bit of the most significant word last

As 8 words this is 11880000. Converting this to binary we get 0001 0001 0100 0100 0000 0000 0000 0000. Remembering the order of the bits, this has to be sent as 0000 0000 0000 0000 0010 0010 1000 which in hex is 0000228.

Initially I was told that the units were pin compatible with the Verticom units previously used (5), so I built the same PCB. With no PIC installed, the unit was powered up with 12 and 8V and the output observed. After 3 seconds of switch on the unit locked to 11.6GHz. It was noted that the LED was on at power up then would go out briefly after 3 seconds before turning back on and staying on when locked.It looked as internal circuitry was starting up the unit on 11.6GHz.

Next a PIC was programmed with the code for 11.880GHz from the calculations above and the unit powered up. The unit again produced 11.6GHz after 4 seconds with the LED showing the same sequence. It appeared to be ignoring the PIC.

The PIC was then reprogrammed to delay sending anything to the unit until 6 seconds had elapsed from switch on. This time the unit produced 11.6GHz after 4 seconds, then when the PIC commanded it with 11.88GHz it went out of lock; the unit produced some arbitrary drifting frequency. It was noted that the LED was again on at startup, then after 3 seconds went off briefly, then at 6 seconds went off and stayed off showing unlock.

An afternoon was then spent sending different (random) 32 bit sequences to the unit and seeing it would lock. On a couple of occasions it did lock, but the output frequency bore no resemblance to calculations on the data sent! At this point the project was put on one side for another day.

After a week it was decided the only option was to call Elcom and ask them about this device. Apparently the DFS201 was a platform that was customized as the buyer wanted it with regard to step size, power up frequency etc. They emailed me the generic data sheet on the device. The pinouts were the same as the Verticom internal oscillator with the exception that Pin 1 had 6Volts applied, not 8V as was required by the Verticom.

Fortunately it seemed to have survived 8V. They had no record of what the unit I had intended to do, but confirmed that the serial data method I was using was the correct one. So I sent some more hours sending data to the device with no success. I phoned Elcom back the following day and was put through to an engineer. “Ah, they didn’t tell you? Those devices have a multiplier on board. You have to send them three times the frequency”! With this information we can recalculate:-

Multiplying by 11880000 by 3 we get 35640000. Converting this to binary we get 0011 0101 0011 0100 0000 0000 0000 0000. Remembering the order of the bits this has to be sent as 0000 0000 0000 0000 0010 1100 1010 1100 which in hex is 00002CAC

This sequence was programmed into the dual frequency PIC along with the sequence for 11774MHz (00044CAC hex) and the unit powered up. The unit locked up at 11880MHz. Success! Unfortunately commanding the unit to 11.774GHz produced an output of 11.773333GHz. Further investigations by programming different frequencies between 11770 and 11780MHz revealed that the units can only programmed in 10/3MHz steps (one of the steps mentioned in the data sheet). This will restrict its versatility, as the only integer frequencies produce able will be multiples of 10MHz.

From the above findings it appears that this unit can be programmed to a frequency between 11.2 and 12GHz in 10/3MHz steps. This does restrict the number of useful (to hams) frequencies available. It was decided to incorporate the facility for the PIC to move between two frequencies depending on the state of pin 4 of the PIC. This would allow incorporation of an offset for receiver local oscillators. The final assembler listing is available at (6).

The final circuit diagram of the controller is shown in Figure 1. The component listing is shown in Table 1. A PCB was designed; the layout is shown in Figure 2 with the component layout in Figure 3. Note the 7812 and 7806 regulators, LED and C4, C5 and C6 are not mounted on the board. The regulators need mounting on a heatsink to prevent overheating. Figure 4 shows the PCB mounted on the unit.

Component	Value	Component	Value
U1	PIC12F675	R3	1k
U2	78L05	C1,C7	0.1u 25V Tant
U3	7812 (with heatsink)	C2,5,6	1uF 25V Tant
U4	7806 (with heatsink)	C3,4	0.1u Ceramic
R1	100k	LED	As requires
R2	2k2	JPI	2 Pin 0.1" Jumper

To test the stability of the unit readings of frequency were taken at 1 minute intervals using an HPIB controlled EIP548A counter with a rubidium 10MHz reference oscillator. The programme was written in Visual Basic 6. Communication between laptop and counter was done with a National PCMCIA GPIB card. The results are shown next:-

It was noted that the long term stability looked good but that unit took 90 minutes to really stabilize which could be an issue for roving operation. The phase noise was measured using KE5FX’s software (7) using an HP8563E spectrum analyser with a HP Z3801 GPS locked reference 10MHz timebase. Communication between laptop and analyser was again done with a National PCMCIA GPIB card. The results were as follows:-

This article has described how to deploy the Elcom DFS1201 surplus oscillator. Having now done the hard part, of working out how they operate, the author awaits the publication of some articles using it with interest!