Reference Locking the IC-910.pdf

Table of Contents

An Easy Way to Lock your Radio to a Rubidium Reference...............................................................2

Current Solutions..................................................................................................................................2

An Easier Solution................................................................................................................................2

Specification Comparison....................................................................................................................4

Figuring out the FE-5680A..................................................................................................................4

Programming the FE-5680A................................................................................................................5

Building a Low Noise Tripler...............................................................................................................5

Additional Amplification......................................................................................................................7

Completing the Unit.............................................................................................................................8

Modifying the IC-910.........................................................................................................................10

Receiver Phase noise measurements..................................................................................................11

Conclusion..........................................................................................................................................11

Appendix A – IC-910 Modification, reproduced with permission of VK7MO..................................12

Appendix B – Trippler schematic.......................................................................................................13

Appendix C – Construction Notes & Errata.......................................................................................14

Illustration Index

Illustration 1: Typical FE-5680A .........................................................................................................3

Illustration 2: Prototypes......................................................................................................................6

Illustration 3: Assembled Tripler..........................................................................................................6

Illustration 4: MAR-6 amplifier...........................................................................................................7

Illustration 5: interior with everything in place....................................................................................8

Illustration 6: Fully assembled, ready for operation.............................................................................9

Illustration 7: Schematic Circuit Modification for the IC-910...........................................................10

Illustration 8: RX Phase noise at 1296.100 Mhz...............................................................................11

Illustration 9: Modification of the IC-910..........................................................................................12

Illustration 10: Trippler Schematic.....................................................................................................13

Illustration 11: Parts List....................................................................................................................13

An Easy Way to Lock your Radio to a Rubidium Reference.

For many of us playing around with microwave transverters the inherent stability and phase noise of

our base radios leaves something to be desired. Some manufacturers offer "High Stability" options

for their radios but they still fall short of of the ultra stable and clean oscillators required for use at

SHF frequencies.

Current Solutions

GPS based 10MHz standards have become ubiquitous with many types available via eBay or other

online sales mechanisms. Most are ex Telco equipment and have excellent performance

characteristics, many of these have formed the basis of GPS locked oscillators for various radios.

The problem with most GPS based systems is that they're bulky and required voltages other than

12v to operate. If you want to go mobile with your reference you have the additional problems of a

second antenna to worry about and most GPS references require themselves to "self survey" once

moved, which means they wont have a disciplined output for 30 minutes or more !

Another option is to drive your oscillator with a 10MHz rubidium standard which usually takes 5

minutes or less to produce a disciplined output, but once again the more popular rubidium standards

require +24v to operate making portable use harder than it should be.

An Easier Solution.

In researching the different ways I could create to lock my radio ( in this case an Icom IC-910 ) to a

reference oscillator, and maintain portability, I stumbled across another ex-telco rubidium reference

call the FE-5680A, originally used in Motorola cell sites.

What set this unit apart from all the others was that it would operate from 12v even though its

specifications are 15 - 18v, was tiny, and it had a programmable output thereby simplifying the

additional circuit requirements to attain the required operating frequency of the radio's oscillator.

A side benefit is that its cost is comparable with purchasing the ICOM CR293 high stability option,

currently ~$169.00. This Rubidium standard cost about the same including the box and circuits to

complete the build.

Given the programmable nature of this unit, it can easily be used to lock nearly any radio with a

minimum additional circuitry and in some cases could be used directly. David, G4HUP, has

compiled a list of radios and their LO frequencies. That list can be found here...

http://g4hup.com/XCVR%20MO%20list.pdf

FE-5680A's can be found on eBay from a few sellers at reasonable prices and others at not so

reasonable prices, so make sure you do a good search for them. Some have an SMA for the 10MHz

output, some have it on pin 7 of the D connector and some require modification to bring the RF out

to the front panel. In any case its straight forward to get the RF out of the unit.

More detail on the FE-5680A can be found here....

http://www.freqelec.com/rb_osc_fe5680a.html

Illustration 1: Typical FE-5680A

Specification Comparison

The specifications of the FE-5680 are comparable to the popular GPS reference, the Trimble

Thunderbolt, as this table of the main parameters show...

Parameter

FE-5680A

Trimble Thunderbolt

Frequency

10MHz

Type

Sinusoidal

Amplitude

+7dBm

+12.5dBm

Harmonic Level

-30 dBc

-40 dBc

Spurious

-60 dBc

-70 dBc

Phase Noise @ 10Hz

-100 dBc/Hz

-120 dBc/Hz

@ 100Hz

-125 dBc/Hz

-135 dBc/Hz

@ 1000Hz

-145 dBc/Hz

-135 dBc/Hz

10MHz Accuracy

2 x 10 -11 /day

1.16 x 10 -12 /day

DC Requirements

12V to 18V

+12V, -12V, +5V

Input power

11 watts steady, 27 watts peak 10 watts steady, 15 watts peak

Size

25 x 88 x 125 mm

51 x 102 x 127 mm

It is expected that the accuracy of the Trimble to be higher than the RB unit since its constantly

comparing and updating against the standards orbiting above. Also the phase noise of the FE5680 is

slightly worse due to its output being that of a DDS rather than directly from a VCXO. The FE5680

does contain a VCXO but that is part of the loop in front of the DDS.

That being said, the specifications of both units are excellent and more than enough for amateur

radio applications.

Figuring out the FE-5680A

While the RB is programmable for any frequency between 1Hz and 20MHz, the factory builds a

band pass filter for the resultant frequency prior to shipping. Thus if you program a 10MHz unit to

some other frequency there is significant reduction in the output of the unit, so much so as to make

it unusable.

My first idea was to reprogram the RB to 15.1MHz and simply double it to 30.2MHz to suite the

IC-910, but the output was to far down to be usable. My second idea was to program it to

10.06666666 MHz and tripple that to get close to 30.2MHz. In this I was successful.

Programming the FE-5680A

Data on programming the RB can be found here...

http://www.fetaudio.com/wp-content/uploads/2009/10/fei-5680A-pinout-jap.pdf

Alternatively a program has been written to reprogram the FE-5680 and that can be found here..

http://www.redrok.com/FE5680A.zip

Building a Low Noise Tripler

Having looked at various tripler circuits on the internet I stumbled across an excellent article by

Charles Wenzel of Wenzel Associates Inc, titled...

"New Topology Multiplier Generates Odd Harmonics"

The article can be found at this link...

http://www.wenzel.com/pdffiles1/pdfs/RFDesign2.pdf

This multiplier is also low noise and contributes little to the overall noise in the system.

Using this circuit I added an additional 7 pole chebyshev, low pass filter and amplifier stage to

reduce any residual harmonics and amplify the signal to a suitable level. I also added an LED to

show when power is applied.

I've used PCB mounted SMA connectors for the RF and the board is designed so that the SMA's can

be mounted from either side of the PCB.

The complete circuit is reproduced in Appendix B.

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