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AFE7900EVM: Query regarding the AGC mechanism inside AFE79XXEVM

Maitry Khambhati
Intellectual 370 points
Part Number: AFE7900EVM
Other Parts Discussed in Thread: AFE7900, , AFE7950EVM, AFE7950

Hello Support Team,

Below are the queries regarding AGC mechanism for AFE79XXEVM. Please guide.

1. Analog , digital peak detectors are inbuilt in the AFE7900 and when we use internal AGC controller, it is used for below as per my understanding. 

  •   Peak the main RF input power level through internal Analog RF peak detector.
  •   Sets the specific internal DSA settings as per ADC input range requirement. 

As my input dynamic range is high, so I also need to use external VGA to support weak signals to fall into the ADC range, So, Is it possible to control the external VGA through internal AGC controller via GPIO pins? in that way, the internal DSA as well external VGA/DSA can be automated irrespective of the input RF signal range?

2. digital gain compensation function is used to compensate all the attenuation settings , gain provided in between the RF input to JESD output, right? So, it also compensate all the DDC mixer losses affected the input signal's amplitude? 

3. If input power is unknown and I want to measure the actual power input in the digital domain, So, I think It is possible to correlate the analog peak detector output to AGC and the total gain compensation made throughout the chain, Can you please suggest ?

  • 0
    TI__Genius 10505 points

    Hi Maitry,

    1.  The internal AGC can be configured to control an external amplifier bypass input. It's a one bit control per Rx chain. This can be used to extend the dynamic range of the analog input.

    2. Digital Gain Compensation is used to compensate for the the DSA attenuation value and to keep a constant gain from the analog input to the JESD output. The DGC does not compensate for the DDC mixer losses, this compensation is done in the DDC block and is independent of whether DGC is enabled or not.

    3. Peak detectors trigger based on input signal level going beyond a programmed threshold level and not based on input power level. The block diagram of peak detector is shown in Figure 8-6 in datasheet.

    Regards,

    Vijay

  • 0 in reply to Vijayendra Varma Siddamsetty
    Intellectual 370 points

    Hi Vijay,

    Thank you for your accurate response. I got your point that internal DDC mixers losses will be compensated.

    1. For the external VGA control , It is only possible to bypass or enable the VGA through internal AGC , right? that means , it is easier to choose fixed gain external amplifier with bypass/enable option. that bypass or enable can be controlled by internal AGC. As my main purpose for the usage of external VGA is to amplify the weak input signals ( -36 dBm approx.) . So, please guide if my understanding is correct?

    2 If I want to measure the amplitude of the RF input signal,(which is unknown to me ), The original input RF signal's amplitude will be vary by external VGA, internal DSA to fall into ADC's Full scale power range. Once the signal falls into the full scale power range of ADC, then after , the signal's amplitude will be affected by below.

    • DDC mixer loss( which will be compensated internally)
    • Decimation filter loss 

    So, What about the decimation filters losses on signal's amplitude ?

    Also, please let me know, is there any other losses affect the amplitude?

    3. As the Input RF signal goes through External VGA/Amplifier, internal DSA, DDC, decimation blocks etc. and always provide a same amplitude at JESD output as internal AGC will maintain constant gain between RF input signal and JESD output, then it will be complex to measure actual signal's amplitude at JESD point.

    So, Is there any way to verify the actual gain compensation done throughout the chain from RF input to JESD output to add that compensation to measure the signal's amplitude at JESD output?

    Kindly request you to guide and further support. Thanks in advance.

  • 0 in reply to Maitry Khambhati
    Intellectual 370 points

    Hi,

    Is there any update on the above queries? Awaited your response.

  • 0 in reply to Maitry Khambhati
    TI__Genius 10505 points

    Hi Maitry,

    1. Your understanding is correct. DSA can be used for fine attenuation when fixed gain external LNA is enabled. So fine external gain control (with a VGA) is generally not needed. If the signal level can be very low, you can use one fixed gain LNA that cannot be bypassed followed by another LNA with bypass control. 

    2. All loses in digital (mixer, DDC etc.) are compensated internally. The full scale power number given in datasheet is measured in a DDC mode. So all these loses are taken into account. You can initially configure the AFE with AGC disabled and manually select the DSA value. In this mode, the full scale power at ADC input is specified in datasheet. Based on digital output amplitude w.r.t full scale, input signal amplitude can be found, 

    3. When AGC is enabled, current DSA value and current state of LNA bypass can be read through SPI. Using these values, actual gain through AFE can be calculated. 

    Regards,

    Vijay

  • 0 in reply to Vijayendra Varma Siddamsetty
    Intellectual 370 points

    Hello ,

    Thank you for your accurate response. I will plan to find suitable fixed gain LNA followed by LNA with bypass control. 

  • 0 in reply to Maitry Khambhati
    Intellectual 370 points

    Hello,

    1) I have query regarding the absolute maximum input that the AFE7900EVM Rx input pin can handle with 0 dB DSA setting( with o dB attenuation) . As for the max full scale power range of the ADC is given up to 17.8 dBm for 30 MHz but here , DSA setting is not provided. So, Is 17.8 dBm is with 0 dB DSA setting ? If yes, then , With max. attenuation setting of 25 dB of DSA, Is it possible to give around 37 dBm input and with the help of DSA , RF-ADC input will fall into the full scale power range.

    So, my Question is that the given 17.8 dBm max full scale input power is with DSA=0 or DSA= max(25 dB) ? i.e, if 17.8 dBm is the max input for RF-ADC , what is the max input for inbuilt DSA ? 

    2) Also, I found one more observation from the AFE7900EVM schematic is that for all RX input (RXA_IN, RXB_IN, RXC_IN and RXD_IN) , different frequency range baluns are placed( X Band , very low band, L/S WB , L/S Band respectively ) , But as our requirement is the low frequency band for all four inputs , So is it possible to have AFE7900EVM Evalutaion board placed with the Lower band baluns ( TCM2-33WX+ ) at all 4 RXA, RXB, RXC , and RXD inputs ) ? or do we need to purchase extra baluns as per our requirement and replace the baluns at our place?

  • 0 in reply to Maitry Khambhati
    Intellectual 370 points

    Hi Vijay,

    Request  you to provide response and guidance on the above case.

  • 0 in reply to Maitry Khambhati
    TI__Genius 11990 points

    Hi Maitry,

    The max full scale input power given, 17.8dBm for 30MHz, is the max that should be applied at the ADC pins at any time. This means that the input power should not go over 17.8dBm even while using the max DSA setting.

    In regards to the Baluns, like you mentioned each channel has a different balun and if you are interested in only lower frequencies you will have to change the baluns and matching circuits. However, the balun that is installed on RxB does not fit into all the other balun footprints. This means that if you want to replace the baluns you would need to find a balun that matches the footprint for each channel.

    Regards,

    David Chaparro

  • 0 in reply to David Chaparro
    Intellectual 370 points

    Hi David,

    Thanks for your response. 

    As first we will be using Evalutaion board. If I need to replace the baluns in evaluation board for which footprint should be matched. and also I need to tune and change the matching circuit. these all task would be easy for developed board but to make changes in Evalutaion board is bit complex.

    But, Instead of making these kind of changes in evaluation board, Is it possible from TI end to provide the evaluation board with lower frequency band in all  4 channels so that we do not change anything in the Evalutaion board. Then it would be helpful. 

  • 0 in reply to Maitry Khambhati
    Intellectual 370 points

    Hi David,

    I have gone through different types of transformers. But as our requirement is to have lower frequency band . For lower frequency band , the transformer technology used in core & wire. 

    While for higher band in evaluation board, LTCC package (NCR2-123+ and NCS2-33+ ) are being used. and as the given NCR2-123+ and NCS2-33+ footprints are not standardized. So, It is not possible to get lower frequency band baluns with exact same footprint.

    So, Is it possible from TI End to have Evalutaion board with TCM2-33WX+ Baluns are used in all 4 input channels ( each channel: lower frequency band) ? As it will not be possible to have same footprint balun as I mentioned above. And my requirement is to use Evalutaion board's all 4 channels simultaneously with lower frequency band.

    Request you to provide support.

  • 0 in reply to Maitry Khambhati
    TI__Genius 10505 points

    Hi Maitry,

    Due to many engineers attending a Microwave conference this week, please expect delay in response.

    Regards,

    Vijay 

  • 0 in reply to Maitry Khambhati
    TI__Genius 10505 points

    Hi Maitry,

    We don't yet have AFE7950EVM that supports lower bands. We are in the process of developing an EVM. I will email you offline to understand your frequency band requirements and try to support with a custom EVM.  

    Regards,

    Vijay

  • 0 in reply to Vijayendra Varma Siddamsetty
    Intellectual 370 points

    Hi Vijay,

    Yes, I have shared you the details. please check . and one more thing , our selected board is AFE7900EVM , not AFE7950. please note. As AFE7950 min freq is 600 MHz , which is not suitable to us. 

    The only suitable AFE is AFE7900EVM. 

  • +1 in reply to Maitry Khambhati
    TI__Genius 10505 points

    Thanks Maitry. I see that you are using AFE7900EVM and not AFE7950. I apologize for the confusion. We can continue this discussion over email.

    Regards,

    Vijay