Tayloe detector success

Discussion in 'Homebrew and Kit Projects' started by N5HXR, Jul 14, 2020.

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  1. N5HXR

    N5HXR Ham Member QRZ Page

    Yeah, and hence I've been burying myself in amplifier texts for the last few weeks. I've never made a good amplifier so far, so here's hoping that all the studying pays off!

    And yeah, I figure any FSKish mode should be possible. So I'll start with hellschreiber, and should have a good foundation for trying more complicated ones.
     
  2. VK4HAT

    VK4HAT Ham Member QRZ Page

    I have made a couple that im happy with. Design on paper, simulate in ltspice and hope like hell it works lol. My main tip, keep the gain in each stage low as practical. If 2 stages will do, use 3 and dtop the gain needed in the first 2.
     
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  3. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    A good source of info are books from the 1970s on operational amplifiers and many other electronic topics, ranging from engineering level to cookbooks. Frequently they can be had for less than $10 including shipping (US). AbeBooks is a used bookseller I’ve had good experience with.

    For op amps and other analog integrated circuits the Burr-Brown series are excellent. McGraw Hill series are also good, somewhat more aimed at class work.
     
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  4. VK2CLF

    VK2CLF QRZ Member

    Good job, but there are a couple of issues with the circuit that could be improved.

    One issue with Tayloe mixers is charge injection from the switches. The FST3253 has nice low on-resistance, but the charge injection of 1.5pC gets dumped into those 470nF capacitors, meaning you have 3uV of the local oscillator frequency on the output of the mixer (the SoftRock is 10x as bad, BTW!). You could reduce this by adding an extra R/C stage here before the op-amp. A second passive filter pole here could improve things quite a bit.

    The op-amp is an audio type, GBW of 55MHz. At a gain of 100, its output goes high impedance around 500KHz. That means the injected LO signal blasts straight past the op-amp - the low pass filtering action of C5/C6 actually assists that, it doesn't filter it. You need a much faster op-amp if you want this filter to be effective at reducing LO feed-through.

    The op-amp Vnoise/Inoise (best input impedance for low noise operation) is 2.7nV/1.6pA or 1700 ohms. You are operating it at 100 ohms. You could get better low-noise performance by increasing R3-6, R7/7 to 470R/47k or 1K/100k, and reducing C5/6 by the same factor.

    Clifford Heath VK2CLF
     
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  5. N5HXR

    N5HXR Ham Member QRZ Page

    I don't completely understand why there would be any LO getting through the opamp. When I probe the opamp pins with my scope, or when I examine the IQ outputs, I don't see anything obvious (maybe I don't know what to look for). How do I test for this? What deleterious effect would I be looking for? I don't currently have C5 and C6 installed, would that make a difference?

    This makes more sense to me than your other comments -- I figured I'd start with the values I saw in other circuits I used for reference. I noticed that several I reviewed used the same 100R values, despite using very different opamps. I believe I'm using the same resistor values and opamp as the QCX (since I've built one of those, I figured I knew it worked...) I admit I don't have much intuition on some of these topics. I have two more boards -- I could definitely whip one up with the replacement values you suggested.

    Thanks for the feedback -- this kind of insight is exactly what I am hoping for when I post here.
     
  6. VK2CLF

    VK2CLF QRZ Member

    If you're not seeing LO feedthrough, it's because C5&C6 are not installed. The LO would bypass the op-amp through these capacitors and appear directly on the output. The amplifier will try to fight this bogus signal but it doesn't have enough gain at this frequency to be successful. If the op-amps are stable without these capacitors, you might be better off without them. They're placed to reduce gain to high frequency in case the amplifiers are unstable.

    LO feedthrough is a problem if you pass the audio to anything that resamples it, like a sound card. The sampling frequency will beat with the LO and cause birdies. This is a big problem in the SoftRock receivers, worse there because C1-4 are only 47nF.

    It's best avoided with a filter on the output of the mixer, say an C-L-C pi network, followed by the input resistors.

    It's possible that the QCX copied the SoftRock mixers, or both copied something earlier. There always seems to be more cargo-cult copying than analysis in the ham community.

    Clifford Heath VK2CLF.
     
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  7. N5HXR

    N5HXR Ham Member QRZ Page

    OK, that helps, and ruminating over your comment also helped make some of it gel for me. I saw in another article I found (when trying to understand these bypass capacitors) that another strategy is to add a parallel capacitor on the opamp output to shunt high frequencies to ground. The note said it's more effective than the feedback caps, but has the downside of affecting output impedance. Your suggestion about the C-L-C filter is different, so I'll have to read up on that and maybe work that into my experimentation.

    Haha, I do kind of get that impression. I'm just trying to make my baby steps, learning this stuff. I'm an RF toddler, so I don't really know any better. I think I'll take stock of what parts I have around in 0805 sizes, and put in an order with digikey and try your suggested values. I have only a very rudimentary ability to inject a test signal (and very few options at the moment for attenuation), so I don't know if I'd be able to test for the difference. Thinking about it enough will probably put some more test gear on my wishlist.

    Again, I really appreciate your feedback -- you've dropped some really useful tidbits, and it'll inform my study while I wait for funds and PCBs for the other pieces I'm playing with. Thanks!
     
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  8. KE4LH

    KE4LH Ham Member QRZ Page

    I've been looking into this design for a portable transceiver too. Congrats on your success. I recently came across a document written by the inventor and it's quite an eye opener, at least for me. You might want to get it and read up on the operation as it's explained quite well there. BTW, I'm thinking you are a surface mount person and I could use some expertise in that area if you would like to talk to me directly. I'm old school like 40+ years and know analog fairly well but designing this new project has proved to be tough getting discreet components so going surface is the future. I might be able to help you with analog too.
    You can get the document here: http://norcalqrp.org/files/Tayloe_mixer_x3a.pdf
    Hope that helps.
    Tom - Ke4LH
     
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  9. N5HXR

    N5HXR Ham Member QRZ Page

    Thanks, that is a great document!

    I've gotten pretty comfortable with most SMD stuff. I think talking to other people who were afraid of it poisoned the well, and I was nervous getting into it. But when I finally jumped in, I realized it's really not that bad, in most cases. In some ways, I think the SMD stuff can actually be easier than through hole. And in one of the most important respects, namely board area, it's superior -- ordering PCBs is a lot cheaper when the design is inherently smaller :).

    I'll drop you a message and we can chat more!
     
  10. KJ7AGP

    KJ7AGP Ham Member QRZ Page

    Dang this is a really neat project. I'm currently working on a multi-purpose VFO board, among other things, but I'm looking forward to building a receiver. Besides the hodgepodge class D amplifier I have sitting on my desk right now, I have nothing else really to hook up to my VFO prototypes, which make developing on it rather difficult.

    I think I have some of those FST chips, might slip in a reciever board into my next oshpark order... :D
     

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