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What is affected by the lower Master Clock Frequency of the 6300?

Alan - KA4B
Alan - KA4B Member ✭✭
edited April 2019 in FLEX-6000 Signature Series
What all is affected by the Master Clock Frequency of the 6300 being 1/8 that of the 6500 and the 6700?  I am thinking there may be some of the benefits of the 6500 over the 6300 that I am overlooking.

Answers

  • Alan - KA4B
    Alan - KA4B Member ✭✭
    edited February 2015
    This is something Steve will probably need to address.
  • Alan - KA4B
    Alan - KA4B Member ✭✭
    edited February 2015
    This is a question I am still very interested in.  Obviously having only two slices instead of four reduces the need for clock cycles by half. And reducing the maximum viewable band from 14 MHz to 7 divides the need for clock speed in two again.  It seems like the reduction from 4 to 2 DAX channels and the reduced width from 192 to 96 KHz would be part of the same 4 to 1 reduction in clock speed.  Does limiting the wide band frequency coverage from 72 MHz down to 54 MHz further reduces the need for clock speed by 25%?  What am I missing? I always enjoy your technical answers, Steve.  Can you throw any light on this?
  • Gerald-K5SDR
    Gerald-K5SDR FlexRadio Employee ✭✭
    edited April 2019
    While Steve can answer on the software side, I can shed some light as well.  To understand this fully, one needs to understand what is called the Nyquist frequency.  That subject requires more space than is available here.  Simply put, the Nyquist frequency is 1/2 of the sampling rate.  Below Nyquist you are "oversampling" and above Nyquist you are under sampling.

    In order to prevent images from signals above Nyquist from "folding" back into below Nyquist frequencies, a anti aliasing filter must be employed on the input of the ADC.  To make the math simple lets use an example sampling rate of 100 Msps.  This would mean that the Nyquist frequency is 50 MHz.  Let's say we want to be able to receive a maximum frequency of 45 MHz.  The alias or image frequency will be at 55 MHz, which would fold back to 45 MHz and be heard.  The stop band cut off of the anti aliasing filter at the image frequency determines the image rejection at the folding frequency.  

    So to compare the 6500/6700 sampling rate of 245.76 Msps with the 122.88 Msps of the 6300, the Nyquist frequency is doubled.  Nyquist on the 6500/6700 is 122.88 MHz compared to 61.44 MHz on the 6300.  The image frequency on the 6500/6700 receiving at 72 MHz is 173.76 MHz - a spread of over 100 MHz.  That makes it very easy to get excellent stop band rejection in the anti aliasing filter.  On the 6300, the image frequency is 68.88 MHz, which limits the maxumum input frequency as well as limits the anti-aliasing filter stop band filter rejection.  We do have FM broadcast band notch filters on all of the FLEX-6000 series radios to add additional rejection in the 88-108 MHz range where there can be very strong local signals.

    Hope that helps.

    Gerald
  • Alan - KA4B
    Alan - KA4B Member ✭✭
    edited February 2015
    Thank you, Gerald and Steve.  I am wondering if these differences will effect the quality of the NR or NB when they are improved.  That is, will the performance of these features be as good on the 6300 as on the more expensive units?
  • Gerald-K5SDR
    Gerald-K5SDR FlexRadio Employee ✭✭
    edited April 2019
    ADC sampling rate has no effect on NR and NB.
  • AE0MW
    AE0MW Member ✭✭
    edited January 2017
    This was a great thread, thanks for the in depth responses guys!

    I have one question though, after reading Steve's response a few times to understand it better, what is the limitation that prevents 2m from working on the 6500 series? I understand it's quite limited even on the 6700, I'm just curious as it appears from the datasheet the only difference between 6500/6700 is the additional SCU, no changes in clock frequency or FPGAs.
  • Gerald-K5SDR
    Gerald-K5SDR FlexRadio Employee ✭✭
    edited April 2019
    To reduce the cost of the FLEX-6500, we do not have the additional filtering and added low noise preamp that is needed for 2m.  The receiver performance is not limited at all on 2m on the 6700.  The exciter is limited to +10 dBm as a transverter IF but can drive a DEMI amplifier as a number of customers are doing.
  • AE0MW
    AE0MW Member ✭✭
    edited January 2017
    Thanks for the explanation Gerald!


  • Alan - KA4B
    Alan - KA4B Member ✭✭
    edited February 2015
    Gerald and Steve, I have not experienced any other amateur radio company whose leaders interact with their customers and prospects at the level that you and some of the others at Flex do.  Your willingness to answer questions in such detail, address concerns and discuss new features so freely is truly a breath of fresh air. This forum was a brilliant idea. Thank you again!
  • Gerald-K5SDR
    Gerald-K5SDR FlexRadio Employee ✭✭
    edited April 2019
    Thanks for the feedback.  I just wish there were more hours in the day so that I could stay in closer touch with our customers than I do.  ;>)  

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