Frequency counter to monitor LO

Discussion in 'Wireless & RF Design' started by kuch128, Jul 25, 2014.

  1. kuch128

    Thread Starter Member

    Mar 7, 2011
    20
    1
    Hello everyone here is the issue that I am having. I have built a receiver around the SA602 mixer with a local oscillator being provided by a MAX2606 VCO. What I want to do is to be able to control the Vtune voltage on the MAX2606 digitally through the use of a DAC or digipot. The MAX2606 provides two LO outputs one of which is being used to feed the SA602. What I want to do is take the other output and be able to monitor the frequency and adjust the Vtune voltage to lock on to the desired channel.

    My problem is finding a means of counting this frequency and producing a value compatible with the ATMega16 already incorporated in the project. A couple things I have considered is some sort of frequency to voltage converter to be monitored by the ADC on the Mega16 and adjust the digipot/DAC accordingly. Another idea I had which I quickly found out wasn't doable was externally clocking the counters with the LO frequency. But because the external clock sources are synchronized with the uC clock (16MHz) this wouldn't work. Adding to this last point I was thinking of maybe using a frequency divider to get the LO frequency down to a frequency that could be handled by the uC, I know I would lose resolution on the count but I was thinking it still may work.

    The frequencies that I am receiving are in the lower portion of the US FM band specifically 87.7 - 88.9 MHz.

    I didn't want to start my first post to long out of fear of losing your interest so if I have perked your interest and require more information I will be happy to provide it. I have attached the datasheets for the SA602 and MAX2608 below.

    SA602 -http://www.nxp.com/documents/data_sheet/SA602A.pdf

    MAX2606 - http://datasheets.maximintegrated.com/en/ds/MAX2605-MAX2609.pdf
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,363
    To put things into a nutshell, you want to measure frequency around 100MHz using an ATmega16 mcu.

    Because the internal counter of the ATmega16 clocked at 16MHz is not fast enough to count at 100MHz, one solution is to prescale the signal with external counters. Or you can choose to do the entire counting with external counters.

    Resolution of 1Hz will require a 27-bit counter. However a resolution of 1:10^8 is beyond your typical crystal oscillator.

    Resolution of 100Hz will require a 20-bit counter. You will need a time-base stable to 1ppm for which you may need a special temperature controlled crystal oscillator.
     
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