I have a question about the function of HMC601 Log Detector.
Actually, I am verifying the function of the detector by checking the following:
1) In detector mode, the Vset pin is connected to the Vlogout pin where the obtained voltage values (0.6 to 2 V) for different Input signal levels (-75 to +5 dBm) are accurate and look like the values in the datasheet.
2) In controller mode, the Vset receives a voltage of 0.9V through a voltage divider. But for different input signals ranging from -75 to 5 dBm, Vlogout ranges from 4.8 to 4.9 V only.
Are the results in controller mode accurate? Since I expected a value for Vlogout equal to that applied to Vset pin.
Thanks for your valuable answer.
Hi Bassil, please pardon the delayed response to your question.
In controller mode as you describe with VSET= 0.9V, we would expect to see LOGOUT slew from rail to rail as the input amplitude goes above or below about -50dBm (exact amplitude depends on frequency and individual IC). Make sure your controller mode connections are correct. Note 5 on last page of datasheet gives detailed instructions. You might need to verify with a voltmeter that the voltage on VSET pin is indeed 0.9V as it should be for this test case.
Dear Bruce_H, thanks a lot for your valuable response.
The voltage on VSET pin was verified by the DMM to be 0.9V. This was achieved by connecting a series 135 Kohm resistor between the +5V DC voltage and VSET pin in order to obtain the voltage divider effect with the 30 Kohm internal resistance.
The problem was solved and VLOGOUT started to slew from rail to rail just when the 135 Kohm resistance is replaced by 110 Kohm. A suitable voltage divider is inserted between VLOGOUT pin and GAIN of the VGA. The operating frequency is 100 MHz.
Connecting the 110 Kohm resistor resulted in 1.03V at VSET pin which is correspondent to about -47 dBm.
My question now is: what if I want a signal level less than -47 dBm? Increasing the series resistance at VSET pin affected the function of the detector. How can I obtain a lower voltage at VSET (down to 0.6 V)? Is there any limitation concerning the value of the resistance?
Thanks in advance.
The device should function in controller mode, down to the lower power levels as shown in datasheet.
The problem sounds like one of 2 possibilities:
1) If the detector is functioning as part of a closed-loop feedback control system, that control system could be oscillating. Use an oscilloscope to confirm that LOGOUT and other measured voltages are indeed static.
2) Because you're using large resistor value to derive VSET, there could be some unknown leakage path that might be pulling (or pushing) the actual VSET pin voltage up or down, interfering with the measurements. Just for the sake of experiment, try a stable, low-noise voltage source on VSET, derived from a good lab-grade power supply, to see if the problem goes away.
Hope that helps! -BruceH