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  1. #1
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    MCU-free higher resolution RF power signal meter

    Hi, I want to build an RF power meter for QRP powers (<10W) on HF (<30MHz).
    I have seen this circuit https://www.qsl.net/va3iul/Homebrew_...eter_W1GHZ.gif (the top side).
    He uses the AD8307 log amplifier and a linear scale LM3914.
    The use of the AD8307 is optimum but this costs a lot.

    So I think instead to use this circuit http://www.hamradio.in/circuits/imag...sm/nsh_fsm.gif Which uses a "linear" amplifier but a log display LM3915.

    Since I am going to calibrate the display later on, do you think that the second solution is ok?

    Either of the two circuits to use, I will use this display for higher resolution https://www.qsl.net/n6bg/thunt/ledsmtr.html Do you think that this can be used as it is with the LM3915?
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    Re: MCU-free higher resolution RF power signal meter

    You only get 10 LEDs to show the power and it won't be very accurate. Have you considered an AD8307 module from that well known auction site? I just got one ready assembled including input socket for almost nothing. It has a 0 - 2.5V output range and works up to 500MHZ.

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    Re: MCU-free higher resolution RF power signal meter

    Quote Originally Posted by betwixt View Post
    You only get 10 LEDs to show the power and it won't be very accurate. Have you considered an AD8307 module from that well known auction site? I just got one ready assembled including input socket for almost nothing. It has a 0 - 2.5V output range and works up to 500MHZ.

    Brian.
    What is the good with the module in comparison to the bare chip? Cost? I really prefer something more discrete, as a way for the circuit to be able to be built after many years, when the AD chip won't be available possibly.
    So one way is to go with a log amp and a linear display, and the other way to go for a "linear" amp and a log display (I guess).

    How about the discrete way I mentioned above? It only has to work on HF.
    With this circuit https://www.qsl.net/n6bg/thunt/ledsmtr.html I get a much higher resolution. But can it be used as it is with the LM3915 instead?
    Last edited by neazoi; 6th January 2019 at 13:53.
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    Re: MCU-free higher resolution RF power signal meter

    I am thinking of using this circuit https://www.qsl.net/va3iul/Homebrew_...eter_W1GHZ.gif (the AD8307 detector) and this circuit as a display https://www.qsl.net/n6bg/thunt/ledsmtr.html but with the LM3915 instead.

    So I will have a LOG detector combined with a LOG display. This should increase the resolution of the lower level signals (oscillators etc). If I want to increase the resolution of the higher level signals, I will include a switched RF attenuator at the input.

    Is this LOG detector LOG display thought a good one?
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    Re: MCU-free higher resolution RF power signal meter

    Quote Originally Posted by neazoi View Post
    The use of the AD8307 is optimum but this costs a lot.
    Here you can get for 20 cent/piece:
    https://www.aliexpress.com/item/AD83...834723514.html

    I got from them while ago and they work just fine, as Analog Devices mention in their datasheet.


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    Re: MCU-free higher resolution RF power signal meter

    Quote Originally Posted by vfone View Post
    Here you can get for 20 cent/piece:
    https://www.aliexpress.com/item/AD83...834723514.html

    I got from them while ago and they work just fine, as Analog Devices mention in their datasheet.
    Do you know if my idea will work? I.e. to use this log amplifier in combination with the log display, for even greater low level signal resolution than using a log amp and a linear display.
    Professional engineering is the top, but amateur engineering is more fun.
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    Re: MCU-free higher resolution RF power signal meter

    In my opinion it will work but you will be disappointed with the results. If LM3915s are still available (they went out of production around 1995) they will give you 30dB per device range but only in 3dB steps (10 LEDS x 3dB = 30dB) so it will not be particularly accurate. Cascading them will not improve that but it will make them more prone to noise. If you consider the worst case scenario of all LEDs on, the display alone will consume around 420mA so you would have to be extremely careful with power distribution and regulation to achieve stable results.

    There is a huge difference between the 'hamradio' design and the AD8307 design, The 'hamradio' is suitable as a local field strength meter or for monitoring the output of a power amp, it needs volts out of the detector, but the AD8307 is sensitive to much lower signal levels.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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