Fun with sdr's and vintage receivers...

Discussion in 'Amplitude Modulation' started by N2DTS, Oct 31, 2019.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    Frank and Perry were on 85 talking about vintage receivers and their IF adjustments, bandwidth, and filter shape.

    One very useful tool when fooling with the old gear is a cheap SDR receiver.
    I have a few, I really like the sdr-iq from RF space, no longer made but around, very well made, metal case, good design.

    You can hook it up to the IF on a vintage receiver and SEE the filter shape.
    In the old days they had sweep generators or wobulators which were rare and hams used more guesswork then anything else.

    I have an IF tap on the homebrew receivers off the last IF, just a 3pf cap.
    That gives a nice scope output, plus allows me to see the IF shape.

    That is homebrew RX number 2, just band noise.
    Quite a good filter shape from the Kiwa filter after the mixer.

    That is the narrow filter switched in, its only 1Kc less.


    That is homebrew number 1, I can see it needs an adjustment of the IF cans to correct the filter tilt.

    Adjusting things like trimmers and the IF cans, you can SEE the filter shape and also see more or less signal strength as things are peaked.

    You can stagger the IF cans to broaden the response, or peak things up to be narrow.

    This is very fun to play with.
    KA0HCP and W8KHK like this.
  2. W2VW

    W2VW XML Subscriber QRZ Page

    Sorta reminds me of this:

    Attached Files:

    AG5CK likes this.
  3. K4CCW

    K4CCW Ham Member QRZ Page

    I’ve been wanting to experiment with this use of an SDR for some time, as restoring boatanchors is my main interest in the hobby lately.

    I finally have one on its way so I’d be glad to hear about any further experimenting you may do along these lines and may need to pick your brain along the way.
  4. N2DTS

    N2DTS Ham Member QRZ Page

    Not much to it, just pick up the if by getting a probe close to the last IF transformer or use something like a 3pf cap.
    You want the signal level mid scale, like S9 to +20 over but its not critical.
    Just use band noise from a quiet spot to show the IF response.
    You can use an SDR-play and SDRuno, the sdr-play is about $100.00.
    Find an empty spot on the band:

    As Frank told me (W2SDR) it would be best to have a noise generator but you do not need it.
    Elecraft may sell a noise generator.
    If you look at the output of a modern radio, you can see the phase noise they output.
    The little Nouveau 75 is very clean:

    I had a Ten Tec Argonaut 5 that was VERY bad in that respect.

    The sdr-play stuff is not a great receiver as its only 12 bits, but it makes a fair spectrum analyzer for $100.00.
    The sdr-iq is a 14 bit unit and much better, but only goes to 30 MHz.
    The sdr-iq goes down to 100 Hz, (yes Hz) and I have a hard time thinking of 100 Hz as rf.
    Some receivers have a 50 KHz IF, some 2 MHz, some 9, and so on...

    Hard to really know what is going on by just peaking the cans with the S meter.....
    W8KHK likes this.
  5. K4CCW

    K4CCW Ham Member QRZ Page

    Thanks for the response.

    The SDR Play RSP-1 is what’s on its way.
  6. W2NBC

    W2NBC Ham Member QRZ Page

    Yes, “looking” at waveforms is interesting!

    However, with all that eye candy methinks we’re losing a bit of that intrinsic ability to HEAR signals almost supernaturally..

    Let me explain..

    I remember long ago my parents walking into my bedroom (shack) and asking, “who are you talking to?” Apparently, what THEY were hearing was static, squeals, and indiscernible radio noises.. What I was HEARING was that little voice in the RF wilderness poking holes in the Ether with that magical “contact”..

    So, consider this :

    “Can you hear me now?”


    “How do I look to YOU?”

    Maybe I’m a throwback to the days of shortwave signals wafting through a tube receiver inside a dimly lit room.. but “hearing” and “copying” signals actually meant something then..

    Oh, and by the way, you LOOK marvelous
    W1TRY, WZ5Q, W8KHK and 1 other person like this.
  7. N2DTS

    N2DTS Ham Member QRZ Page

    In this case, its not listening to anything, its looking at the response and bandwidth of the IF chain in a vintage receiver.

    As far as the use of sdr's go, I find it very useful to see where signals are instead of just where I am tuned.
    You can tell some things about a signal by looking, how strong and wide they are, and the distribution of their audio,
    and where they are in frequency, but nothing about how they sound or who they are.

    In the Flex 5000, I can see a signal I can not hear....
    I was working Brian (N3TBW) last night and the band went away mid transmission.
    I could not tell he was transmitting at all except the small bump on the sdr display that was just above the noise level.
  8. W2NBC

    W2NBC Ham Member QRZ Page

    “I could not tell he was transmitting at all except the small bump on the sdr display that was just above the noise level.“

    Exactly ..

    I use SDR’s too! I’m just trying to identify an interesting issue regarding operating methods and changes in general ..

    And Bret you always look marvelous..
    W8KHK likes this.
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    FWIW, you can do much the same thing without an SDR or an IF tap. Just route the receiver's audio output into a PC's sound card and run one of the free audio spectrum analyzer programs. You do get the addition of any audio chain audio response tailoring in addition to the actual IF filtering but it's a quick way to visualize how different filters impact receive selectivity.

    Here's a screenshot of the audio output of a TS-830s with the narrow CW filter selected as viewed on the Speclab app.

    And here's a Collins 75S-3 with the narrow CW filter enabled:

    You can definitely see some of the audio chain artifacts like 60Hz hum and its harmonics but it's still a pretty quick and inexpensive way to measure and visualize receive chain response and filtering.
    Last edited: Nov 3, 2019
    W8KHK likes this.
  10. N2DTS

    N2DTS Ham Member QRZ Page

    Looking at the IF allows you to see each stage if you wish, but the audio looks useful as well.

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