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10MHz Rubidium Standard

I have a working Rb 10 MHz oscillator, so why not give it a nice home?

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I wanted to give my Rubidium 10 MHz oscillator a nice home. Some addons would be nice, also - particularly a distribution amplifier for the analog 10 MHz reference outputs. A set of CMOS outputs would also be helpful. Finally, I really like my homebuilt comb generator, so why not build that in and have an accurate comb source for RF work? I re-purposed a broken Harrison Labs power supply chassis, and a new front panel will complete the project.

This whole project was inspired by the many different distribution amplifier designs that I've seen put into service for 10 MHz reference oscillators. I finally ended up going my own way, but it was really interesting to see the many approaches. Aside from the dist. amplifier, this unit also has 5V CMOS outputs (easily scalable using R-dividers to 3.3V, etc.) and a built-in comb generator with selectable rate.

I think the best way to understand the design is to look over the schematic page-by-page. I'll start with the power supply (p4). The 24V input is applied to U11 an Infineon BTS5090-1EJA ProFET smart switch, and the input of U9 a Sanken DC-DC converter module. The ProFET is the solid-state-relay that ultimately routes the 24V to the Rb oscillator. The DC-DC converter acts as a 10V pre-regulator. The 10V, in turn, is regulated down to 5V for the logic and CMOS circuitry, and to a switchable 9V (really more like 8V) supply for the analog distribution amplifier. An Infineon TLE4470 dual LDO U10 performs both of these functions.

Next, the RF distribution circuits on p5 are worth a look. The Efratom LPRO-101 oscillator connects via J14. The LPRO-101 has about +8dBm of 10 MHz output power. This is applied to a passive splitter X6, to result in two branches each about +5dBm. The first branch is slightly attenuated to +2dBm, then amplified by U13 to about +20dBm output. This is applied to a passive 4-port splitter X5 to yield four ports each at about +13dBm. Each of these four outputs is then made available as a SINE output on the front panel.

The second branch from X6 is applied to a Sine-to-Square converter (see p.3). This is simply a 74ACT08 quad AND gate. Section U8A is configured as a sine-to-square converter. The other three sections U8B,U8C,U8D are each responsible for providing a CMOS logic high drive (+/-24mA) output to the front panel.

The output of U8A is also applied to a comb generator circuit, which I have completely copied from an old issue of Ham Radio Magazine (see schematic p.1 for credits). This is a circuit that I've used for a long time, so I figured I'd built it into my Rb timebase! My contribution was to add the 3-8 decoder U4 that will illuminate one LED depending on the comb rate seleced.

Finally, the microcontroller on p.2 ties everything together. It is primarily responsible for monitoring voltage and the BITE signal from the Rb oscillator, and shutting down if anything goes wrong. Control switch conditioning, a Neopixel status LED, and some ADC input buffer circuits round out the rest of this page.

  • 1 × Efratom LPRO-101 Rb Oscillator
  • 1 × BTS5090-1EJA Infineon solid-state-relay
  • 1 × PIC18F45K22 Microprocessors, Microcontrollers, DSPs / ARM, RISC-Based Microcontrollers
  • 1 × 74AC08 quad AND gate with +/-24mA drive
  • 1 × TLE4470G Power Management ICs / Linear Voltage Regulators and LDOs

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  • Buttoned Up

    tomcircuit06/28/2015 at 20:36 0 comments

    Well, actually, this has been buttoned up for a while, but life (and sadly also death) intervened in the past two months. I've been enjoying a nice stable 10 MHz reference source for my experiments. I used it recently to calibrate OCXO replacement project for my HP 5328 frequency counter. The last bits I finished up this weekend were to fix some software bugs - the most annoying was the too short a waiting time to allow a cold LPRO-101 to achieve warmup. I also added the LPRO case temperature sensor and worked out the ADC scaling to report in degrees C.

    I still have the runt pulses in the comb generator to work out, but I'll probably debug that using one a spare PCB and, once I find out what the issue is, apply the fixes to the completed unit. So, not quite "complete" yet, but really close enough for the summertime!

  • Front Panel Arrives

    tomcircuit04/27/2015 at 03:24 1 comment

    I used Front Panels Express to fabricate the front panel for my standard. I received the panel on Friday. Wow! Very nice indeed, much better than my normal acrylic panel attempts.

    It took a couple hours to install all the switches and LED and connectors to the panel, but when it was done, it's all working.

    ...well, sort of...

    The comb generator, which I hadn't tested much because it's sort of the "low priority" item, has an issue with some of the comb rates. I suspect it might be in need of a bit more bulk supply bypassing in the 74HCT151 mux chip, but that's just a guess. There are some "runt pulses" into the 74AC74 pulse generator FF that cause it to double-trigger. weird.

  • Too slow!

    tomcircuit04/09/2015 at 03:14 0 comments

    I started on the firmware for the PIC that I chose for this project - the PIC16F887. I had several of these laying around, so it seemed appropriate to use one of these for the job. I also decided to take a try at the Microchip XC8 free C compiler, rather than Sourceboost C I had been using for many years now.

    I used a state machine approach for the control code, because that's normally how I think. In this case, it's really simple, so the state machine is a breeze.

    One of the goals for this project was also to use a WS2811 based T-1 3/4 LED for the status indicator. These have a pretty annoying PWM-type interface, that runs a bit fast. Unfortunately, so fast that the PIC 16F877 can't generate the proper bit timing when running from the internal 8 MHz clock. I even tried bodging in a 20 MHz xtal oscillator clock, but it was still too slow (why? not sure. wouldn't have expected that).

    So, I'm swapping out the PIC 16F887 for a PIC 18F45K22, which has an internal 16 MHz oscillator and a 4x PLL, so I can boost the clock up to 64 MHz. That will most certainly be able to generate the WS2811 bitstream.

    So, yes, pretty dumb that I ended up going to a whole new micro just to accommodate the status LED but, hey, that's the way things go sometimes. I'm also eager to work with the PIC 18 series again - it's been a long while since I've done so.

  • Stuffing and Testing

    tomcircuit03/22/2015 at 19:33 0 comments

    I've been incrementally stuffing and testing the controller board. First was the power supply section; it works fine. The 10V intermediate rail came right up, and from that the 5V rail was trivial. The 8V rail required that I force the 8V rail enable transistor (doh - should have had a test point there) Q5 on. I trimmed that rail to 8V without issue. Not tested is the 24V high-side switch; need to find the chip in my messy shop.

    Next, I stuffed the 10 MHz distribution circuitry. This is a 2-way splitter right from the 10 MHz Rb oscillator. One branch (the "analog branch") gets amplified up and then fed into a 4-way splitter. The other branch (the "digital branch") gets Z matched to about 1.5K ohms and fed into a 74AC08 gate to convert from sine to square. I used my HP 8660 sweeper to feed in a 10 MHz test signal at -20dBm, and used my HP 8557 spectrum analyzer to check levels along the analog path. Then increased the power to +13 dBm (about the same as the Rb oscillator) and saw that I was getting the expected levels at all four outputs of the final passive splitter. The digital branch is working, also, but I think I'd like to play with the DC level to get the duty cycle on the 10 MHz square output closer to 50%.

    It's hard to test the comb generator without the microcontroller up and running, so I'll focus on the micro next. The comb generator needs an 8:1 multiplexor forced to select the comb rate (100kHz ... 10 MHz) so that's the holdup there. So, next steps are getting the control program written to allow switches to turn on/off outputs, etc.

  • Got the PCB's

    tomcircuit03/17/2015 at 04:02 0 comments

    I received the PCB's today. They look pretty good - this is my second project to use the "dirty PCB" service, and I'm satisfied. I'll start the build in a week or so - this week there's too many other things going on to get the uninterrupted time that I'd like.

  • DirtyPCBs shipping!

    tomcircuit03/07/2015 at 17:58 0 comments

    Waiting on boards...I submitted this and the DerbyLink order to DirtyPCB service, so looking forward to seeing what comes back!

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