Post Go back to editing

AD8302 and log-amps for phase detection

Hi, I was reading with interest some phase detection circuits' datasheet proposed by Analog. In particular, I was reading of the AD8302 so a conceptual question has popped out into my mind.

I am new to log-amps theory and I would be glad to go deep in detail, but I'm questioning how the output of a logamp can drive a XOR-like phase detector like the one in the AD8302 and let it measure the phase difference also of modulated signals (indeed in the bandwidth of the device).

So, if anybody has links, book chapters, white papers or something that would me help going deep in this topic, I am glad to welcome your precious help!

  • Hi there, 

    There's a book titled "Logarithmic Amplification" by Richard Smith Hughes. The book is dated but still can be quite educational. 

    Also there's an old IC that used to be popular, the SA614. The data sheet for that part can be found online and is also educational. This IC has just one limiter chain, not two like AD8302. In this IC, the quadrature demod part of the design is analogous to the AD8302 phase detector.  -Bruce H. 

  • Hi Bruce, thanks for your precious feedback. For what concerns the SA614, I found a datasheet but it seems quite "new" in its design. So, for disambiguation purposes: are you talking about the IC by NXP Semiconductor?

    Another question: do you think the "Log Amp Theory" section (p. 9 and following) of the AD8307 may be an helpful crash summary of the log-amps theory? I don't need to go deep in the details of their design, I just want to understand if and how they can be employed as a "modulation purger" for high frequency signals. My aims is to employ them for carrier detection.

    Thanks again and regards!

  • Hi AF95,

    Yes, you found the correct IC. If I recall correct, SA614 was a Signetics design. Signetics was bought by Philips, who were later bought by NXP. 

    AD8307 datasheet has good description of the log amp.

    See also the following Analog Devices publications:

    The limiting action of the cascadable gain stages removes amplitude modulation and offers quick overdrive recovery, while preserving phase and frequency information. Devices like SA614 were popular in FM receivers back in the 1980's and 1990's. You could think of the SA614 RSSI output as analogous to the log output of AD8307 and AD8310 products, along with the many newer log amp products from Analog Devices.  -Bruce H.