SLIM-LD-8306
Logrithmic Detector

5-Updated 5-15-08 Added more information
SLIM-LD-8306, Log Detector, size-A
Use your mouse's "right click" and "Save Link" to download:
a.   SKSLIM-LD-8306, Schematic, in ExpressPCB software.
b.   PWB-LD-8306, Base artwork for PWB, in ExpressPCB software.  Use this drawing to order the pwb from Express, or to locate the parts on the Board.
c.   PLSLIM-LD-8306, Parts List for SLIM Board, in .txt format.  Open with Exel or Lotus, etc.

    The SLIM-LD-8306 Log Detector is a dual function module.  It is used as a detector to convert RF voltage to DC voltage (RSSI).  And, it is used as a high gain, RF limited amplifier.
    The module has an input impedance of 50 ohms (J1) and a bandwidth of  3 MHz to 160 MHz.  The RSSI dynamic range is -90 dBm to +10 dBm, with a DC output of +0.4 volts to +2.4 volts.  This is J2, "MAGVOLTS".  The Limited I.F. Output (J3) is a 50 ohm source with 50 mv peak to peak output.  The limiter input dynamic range is from -77 dBm to +10 dBm.
    It should be noted that this module is extremely sensitive to outside noise influence.  It is important that this module be well shielded, with a perimeter fence and a cover.

SK-PWB-LD-8306, Schematic of SLIM-LD-8306, Log Detector
slim/skslim_ld_8306.gif
    R3 is added to match the 1K ohm input impedance of the I.C. to the output impedance of the transformer.  The Limited I.F. Output, J3, is biased by R6 for a minimal output.  This will help prevent excessive feedback within the module, that could induce self-oscillation.  The output is 50 mv peak to peak.  It can drive a high impedance or 50 ohm load.  If the Limited IF Output is not used, omit R4, R5, R6, and C12.
    The bandwidth of this module is determined by the bandwidth of the input transformer.  The AD8306 has a much wider operating bandwidth.

PWB-LD-8306, Artwork for PWB and Layout for Log Detector, SLIM-LD-8306
slim/pwb_ld_8306.gif  slim/logdet.JPG
    The input transformer could be replaced with other transformers, for different input impedances.  It could also be omitted and the transformer area used for an impedance matching network, for narrow band applications.

    The input impedance to this module is 50 ohms.  The input impedance to the AD8306 is 1000 ohms.  That is why the transformer is included.  It is a 1:4 turns ratio transformer.  The voltage transformation ratio is 1:4, or 1 volt to 4 volts.  The impedance transformation ratio is 1:16, or 50 ohms to 800 ohms.  The power transformation is, of course, 1 to 1.  The 4.02 K (R3) shunt resistor is to decrease the total load resistance, on the secondary of the transformer, from 1000 ohms (the chip) to 800 ohms, for proper impedance matching.  The transformer is the factor for bandwidth limitation of 3 to 160 MHz.

    Since the AD8306 has a high input impedance (1K ohm), Analog Devices uses voltage measurements (dBv) in their specification sheets, and not power measurements.  I am using power measurements (dBm) in the previous paragraphs, because I am specifying the inputs and outputs of the module itself, not the actual AD8306.  Once we move from the primary to the secondary, we are no longer in a 50 ohm system.  It is more accurate to use voltage measurements.
    As a reference point, let us assume the input voltage to the module (primary of transformer) is 1 volt (rms).  This is 0 dBv.  The output of the transformer is (1:4) 4 volts.   This is +12.04 dBv.  The transformer looks like it has a gain of 12.04 dB.  For voltage only, this is true.  But for total power, there is no gain.  Analog Devices specifies the dynamic range of the chip to be from -91 dBv to +9 dBv.  Since this is the secondary of the transformer, the dynamic range on the primary side, is -103 dBv to -3 dBv.  This equates to a 50 ohm power input of -90 dBm to +10 dBm.
    The Limiter amplifier of the AD8306 begins limiting at -78 dBv and remains stable to +9 dBv.  This equates to an input voltage on the primary side of the transformer of -90 dBv to -3 dBv.  As a 50 ohm input power to the module, this is -77 dBm to +10 dBm.  If the Limiter Output of this module is never going to be used, it is advisable to omit components, R4, R5, R6, and C12.  This will lower power consumption and eliminate any possibility of limiter feedback oscillations.