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ClockTamerFAQ

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= ClockTamer Frequently Asked Questions

1 Hardware and usage

1.1 USRP doesn't work with ClockTamer

Check that you have modified the USRP exactly as described in USRP re-clocking modification.

1.2 What is the tuning range of ClockTamer?

It depends upon the oscillator and VCO+PLL chips you have installed. The default configuration is to have 10MHz or 20MHz TCXO and LMX2531LQ2080E VCO+PLL installed. This gives you 2.84-65.83MHz continous tuning range and a few ranges in higher frequencies. If you have LMX2531LQ1515E VCO+PLL installed, then you should have following set of tuning ranges: 1.42-32.92MHz, 32.95-35.91MHz, 36.25-39.50MHz, 40.28-43.89MHz, 45.31-49,38MHz, 51.79-56.43MHz, 60.42-65.83MHz, 72.50-79.00MHz, 90.63-98.75MHz, 120.83-131.67MHz.

1.3 Board with GPS does not seem to work

ClockTamers with GPS won't work with stock USRP power, since they need more current than it can supply via the fan connector. External power is required, or to replace R**7 current-limiting resistor on the USRP with a 0-Ohm resistor. R**7 is located nearthe fan connector on the USRP motherboard.

1.4 What VID/PID does the ClockTamer use for USB connection

VID=0x03EB (ATMEL) PID=0x2FFA (ATUSB162)

1.5 ClockTamer is not recognized as a serial device on my system

Try the following:

  1. Disconnect the USB cable from ClockTamer.
  2. Power it off.
  3. Power it on.
  4. If the LED is illuminated, then ClockTamer booted to flashing mode. Check that it does not contact with any conductive areas and that you don't touch ClockTamer pins with your hand. In this case try to turn power off and on again with care.
  5. If LED is not lighting, then ClockTamer has booted correctly or is not working at all. Try connecting USB cable to computer and then to the ClockTamer. If the ClockTamer is working the LED should flash once and then light constantly.

1.6 How much current does ClockTamer draw

It depends on the ClockTamer version and any hardware modification you have made. Common modifications for ClockTamer-v1.0 include:

Modification Current
LMX=2080 LMK=1010 OSC=20 with 1 output 0.13-0.14 A
LMX=2080 LMK=1010 OSC=20 with 2 outputs 0.15-0.16 A
LMX=2080 LMK=1010 OSC=26 VCTXCO GPS with 1 output 0.31 A

1.7 What is the difference between the two power connectors on ClockTamer?

They are connected tpgether and thus equivalent. It makes no different which one you connect power and to which one you connect the USRP fan to.

1.8 CMOS output gives me 5V peak-to-peak output. Isn't this out of spec for USRP?

It's intended to be a 3.3V output, but due to the reflection / ringing of the signal at those rise-times, the signal voltage almost doubles as overshoot.

However, if you terminate the USRP's clock input with a 50 Ohm resistor you will get a nice 2V peak-to-peak signal. See the USRP re-clocking modification and and Features and technical specification.

1.9 Wavelength of 64 MHz is pretty long. Do I really need to terminate it with 50 Ohm?

Note that the signal is a square wave, not sine. So the rising times are short ( ~ 15 ns [[FeaturesAndTechnicalSpecification#Output_waveform_of_ClockTamer_-1.2|as measured] ]), which means that the highest frequency component of the signal is much more than 64 MHz.

2 Software

2.1 What is a difference between SET,,OSC and SET,,OUT commands?

(Below it is assumed you have ClockTamer with a 20MHz TCXO installed)

OSC and OUT are just different values. OSC is a frequency of hardware oscillator on ClockTamer, while OUT is a desired output frequency. If OSC is set to an incorrect value, then the ClockTamer will generate the wrong frequency at output (or fail to generate anything at all, as you saw previously). So "calibration" should be done by changing OSC — then changing OUT will always give you a good (calibrated) output. The only problem with the OSC value is that it has 1Hz granularity at 20MHz, i.e. at output you will have 52/20*1Hz=2.6Hz granularity of calibratio,n and at 900MHz you will have 900/20*1Hz=45Hz granularity of calibration. After calibration is done you can tune thr output with slightly better granularity (1Hz at 52MHz instead of 2.6Hz at 52MHz) by changing the OUT value.

So, just to be clear clear: by changing the OSC value you preserve calibration among different output frequencies. Changing OUT value you naturally can't preserve calibration among different output frequencies, but if you're interested in only one frequency to be generated, it's fine to use OUT for fine tuning.