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Wednesday, December 11, 2024

An Evening Bandscan on 40 Meters using the High-School Direct-Conversion Receiver


This video shows how useful this receiver really is.  Build one of these!
at 8:38 PM
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2 comments:

  1. WN2ADecember 12, 2024 at 12:41 AM

    Hi Bill,
    I watched the full video, and it sounds spectacular, with great DCR presence! It also proves that in the busy evening 40 meter RF environment, the lack of single-signal capability didn't impair usefulness one iota.
    Two questions:
    1) Was the audio output to the HB speaker directly from the output transformer of the Third Audio Stage? Sounds like ample volume.
    2) Could you make a measurement of the LO leakage for me? Set the RF attenuation for minimum (max RF gain), tune the double-tuned filter in band center (no stagger tune) if you can.
    Take spectrum analyzer data across the band. That will give us an idea what LO rejection you are getting with no problems.
    Mucho TNX in Advance! 73de WN2A

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  2. AnonymousDecember 12, 2024 at 2:45 AM

    Hi Bill

    Congrats on your basic DC receiver operating from a 9v supply.

    RE: your post mixer network comment 1 video back:

    The shunt cap [ usually 100 nF in series with a 51 resistor ] provides a 50 Ω termination for a somewhat narrow band of frequencies from ~~ LF into VHF . It depends on the capacitor value + the value of the series inductor [ usually 10 or more turns on a FT37-43 ferrite toroid ] since the shunt R + cap --- and the series L form a network.

    The main function of the inductor is to block RF from getting into the audio amplifier input — RFC.
    Thus, this simple post mixer network does not offers a 50 Ω termination for audio frequencies at the product detector IF port.

    We normally must AC couple the audio amp input to the post mixer network. Typically this is a via a capacitor in the order of 6.8 - 220µF. This cap’s reactance means that only audio frequencies will get terminated in the input Z of the post mixer preamp.

    The audio amplifier itself provides some 50 Ω termination if it has a wide band 50 Ω input impedance. If the 50 Ω input Z audio preamp offers a strong input return loss -- say >=20 dB from ~ 60 Hz to up to maybe 10 KHz, you may get a reasonable 50 Ω return loss at AF at the product detector “IF” port.

    The lead to the popularity of the common base as a post product detector amplifier. Certainly, you might design a 50 Ω input common emitter type as well. I made a 2-part video about this particular amp only using discrete components.

    I did some post network experiments here:

    https://qrp-popcorn.blogspot.com/2024/01/audio-frequency-return-loss-bridge-50.html

    I added another shunt capacitor after the RFC to better filter RF + extend the network bandwidth. I chose a 4.7 nF instead of a 100 nF cap for 50 Ω RF termination. This gave a peak IF port return loss at 10 MHz, and a decent return loss up to almost 100 MHz , but, sadly only down to 5 MHz . You can only do so much with such a bog-simple network extending over a massive bandwidth that includes AF.

    Best to you!
    Todd--VE7BPO--

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