On 04/20/2012 03:49 PM, ewkeh...@aol.com wrote:
It is a circuit that they for instance use in the 2110 where they take the reference input in case of 10 MHz divide by 2 and also divide the 5 MHz down to 500 Hz use an exor and out comes 5.000500 MHz filtered and divided by 5.
A similar approach is being used in many rubidiums to generate the 5,3125 MHz signal, taking the 5 MHz, divide it by 16 and then XOR them together and then toss it through a LC tank to pick out the right frequency component. The rubidium is then tweaked using the C-field such that the locked 5 MHz lines up with SI second. That's just one aspect of why rubidiums have been relatively cheap from the start. The isotopic match of D lines allowing fairly easy filtering and selective pumping is another.
The result is 1.000100 MHz which is mixed with the unknown divided to 1 MHz. The result is 100 Hz counted with a 100 MHz period counter and you have 1 E 12 in a second. My counter which is part of the system and thanks to Richard MCC is a PIC, has 0.1, 1, 10 and 100 seconds gate time. The 100 or 200 MHz are generated from the reference channel. All logic is in a MAX3000A G/A. The output is RS 232 and can also be stored on a USB memory stick, no PC needed. Austron uses a Xtal as a filter and I use 2 because I do not have access to their Xtal but it works. A nicer solution would be to use an AD 9833 DDS but it would require an additional PIC to do the math since the DDS can not produce an exact 1.000100 MHz. If some one is willing to do that chip please contact me off list.
If you do quadrature signal multiplication, you can avoid the mirror frequency without high-Q filters. Look up the Tayloe detector for some inspiration.
In this case you can generate an I and Q signal by adding a DFF. By producing a 2 kHz and 500 Hz, you let the 500 Hz be the I signal and then let the additional DFF have that as D input and clocked by the 2 KHz it will produce the 90 degree shifted Q signal on the Q output.
By quadrature separation of the 5 MHz you can then use the 5 MHz I and Q, mix and then analogue sum prior some mild filtering such as a LC-tank.
Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
