Notes on a Homebrew CATV tuner based Spectrum Analyzer

Subject: Old working wide band catv tuner.
Date: Mon, 13 Nov 2000 19:43:35 -0800
From: "Rein A. Smit" 
Newsgroups: rec.radio.amateur.homebrew

  Hello All,

  I am looking for a 50 - 800 Mhz catv tuner that can be tuned and ramped
  with a dc voltage.
  Perhaps someone can tell me the reason why I can't find this stuff on the net
  ( Is it the 900 MHz Fed policy? )

  73 Rein W6/PZ0ZN

Subject: Re: Old working wide band catv tuner.
Date: Mon, 13 Nov 2000 20:57:32 -0800
From: John Miles 
Organization: MDI
Newsgroups: rec.radio.amateur.homebrew

Rein A. Smit wrote:
>
>   Hello All,
>
>   I am looking for a 50 - 800 Mhz catv tuner that can be tuned and ramped
>   with a dc voltage.
>   Perhaps someone can tell me the reason why I can't find this stuff on the net
>   ( Is it the 900 MHz Fed policy? )
>
>   73 Rein W6/PZ0ZN

    Do you really need a tuned front end?  If not, you can think of
    http://www.minicircuits.com as your own personal tuner-construction
    menu.  They sell inexpensive VCOs, mixers, filters, and MMIC amplifiers
    that can be used to construct just about any broadband 50-ohm RF system
    with no more difficulty than hooking up a kitchen sink.

    -- jm

and catv tuner.
Date: Mon, 13 Nov 2000 21:23:08 -0800
From: "Rein A. Smit" 
Organization: MindSpring Enterprises
Newsgroups: rec.radio.amateur.homebrew

    Hi John,

    Thanks for response. Lots of interesting stuff on you WEB site!

    I like to build a spectrum analyzer around one of those tuners.. Sort of
    faced with a 50-60 Mhz. IF so I one needs some form of preselection I think.

    Will look into the minicirciuits approach, Never thought about that,
    quite frankly.

    I can't understand why I can't find surplus tuners on the WEB. There
    must be plenty around and I am also confused how those LO.s are driven
    in VCR's, I guess it must be digitally, at least in the newer ones.

    Rein

Subject: Re: Old working wide band catv tuner.
Date: Tue, 14 Nov 2000 01:24:51 -0500
From: Michael Black 
Newsgroups: rec.radio.amateur.homebrew

On Mon, 13 Nov 2000, Rein A. Smit wrote:

>
>     Hi John,
>
>     Thanks for response. Lots of interesting stuff on you WEB site!
>
>     I like to build a spectrum analyzer around one of those tuners.. Sort of
>     faced with a 50-60 Mhz IF so I one needs preselection I think.
>
>     Will look into the minicirciuits approach, Never thought about that,
>     quite frankly.
>
>     I can't understand why I can't find surplus tuners on the WEB. There
>     must be plenty around and I am also confused how those LO.s are driven
>     in VCR's, I guess it must be digitally, at least in the newer ones.
>
>     Rein
>
      I'm certain they are all voltage controlled.  Though some might be a
      better choice than others for your purpose, because greater integration
      might make it more difficult to use the tuner as a non-stepped device.

      It should be very easy to find such tuners, and if your first find
      isn't suitable, another one should come along soon.

      Places to look are tv sets, something recent enough that it isn't the
      old turret type.  TV sets have gone through some iteration since the
      turret tuners were dropped, so some sets may provide a more useable
      tuner.  There was a time when TV sets were made where you'd have a fixed
      number of channels, but little ports behind a panel to tune those channels
      to your local selection.  Then there are the sets where you could tune
      all the channels, but they are all digitally tuned.  I can't think right
      now, but I'm sure there were some other schemes in between that all
      used voltage tuned tuners.  Keep an eye out on garbage day, and just
      strip out the tuner.  TV repair places are often a good source o9f junked
      tv sets, and you might even ask them directly if they had any junkers.

      External cable converters should also be a source.  I see these at
      garage sales in plentiful quantities, selling for about five dollars or
      so.

      VCRs have such tuners too.  You may be able to find or buy for a minimal
      amount a VCR that is mechanically broken, but the tuner works fine.

      In the tv sets, the tuners tend to cover segments, and need switching
      as you move up in frequency (though the tuning within the segments will
      be voltage controlled).  So the VHF channels will be broken down into
      2 or three segments, plus another one for UHF.

      All the cable converters I've looked at tune in one band.  I don't know
      how consistent this is, but from an article in Radio Electronics about
      building such a "spectrum analyzer", they showed a block diagram of
      a cable converter, and it tuned everything up to a fixed IF, and then
      in another mixer down to channel 3 or 4.  I imagine a scheme like
      this is used in some tv sets.

      VCRs probably use different schemes.  Some of the earlier ones had
      those little tuning knobs behind a panel, and I thought they required
      that you set a tiny switch to VHF-lo, VHF-hi or UHF.  But lat4er ones
      probably act work like the cable converters.

      Often, the tuner modules in all three instances are distinct modules,
      and have the pins marked on the metal case, though the markings may
      be a little cryptic.  A little guessing, a little experimenting, and
      you should be able to figure out the pinouts.

      The frequency coverage probably varies depending on whether the
      unit covers just the traditional over the air channels, or the cable
      channels too, since the latter puts channels into non-tv frequencies.
      Something that covers channels 2 to 13 probably won't go all the way
      up to 800 MHz, though perhaps the tuner would have some extra coverage
      on both sides.  UHF would be covered by a separate tuner.  SOmething that
      covers the cable channels would have wider tuning range, though I don't
      know the exact range.

      I remember in one article about using cable tuners for a "spectrum
      analuyzer" that some tuners may be a better choice than others, but I
      think it was because some ran off a negative voltage.

      Tuners that work with some digital scheme will have a prescaler on
      the VCO (though sometimes, the prescaler is separate), though often
      they have a high division rate.  But it provides a means of measuring
      the VCO frequency;  tuners that were merely voltage controlled may
      not have the prescaler, or any VCO output so you'd have to add
      that if you needed access to the VCO output.

      It should be really easy to find something to get started, and if
      it doesn't work fully or not at all, a little more patience should
      turn up something locally for a few dollars.

   Michael  VE2BVW

Subject: Re: Old working wide band catv tuner.
Date: Thu, 16 Nov 2000 03:24:47 -0800
From: "Rein A. Smit" 
Newsgroups: rec.radio.amateur.homebrew

     Hello Michael and Others,

     You provided quite some useful info. It is clear now to me now, that
     they ( the tuners ) all use VCO's and that some are driven directly
     with a DC voltage and that others are driven by the voltage from a
     comparator/freq synthesizer, the new designs that use remotes etc

     My question is how difficult is is to separate that driving module from
     the actual vco voltage drive point.

     Another qustion I have is what is the freq range of these VCR tuners if
     they say 181 chaneels. From 50 to approx 800 MHz?

     I found a web page with a nice clean design, but I do not like to
     draw attention to it. If you contact me direct I will send you the URL
     The reason? Well I am sure by now, that this tuner business is a hot issue
     as it relates to the scanner laws in a way. And that is the reason that so
     many links have gone dead.

     73 Rein





     Actually I've never run across a general-purpose commercial spectrum
     analyzer that uses a preselected front end at HF/VHF/UHF frequencies. 
     The usual practice is a dual- or triple- (or even quadruple-) conversion
     technique, first upconverting the RF region of interest to a very high
     first IF.  That way, a single fixed filter can be used to reduce image
     response (probably doing a better job than the front ends of the cheap
     CATV tuners).

     For example, you could choose a POS-2120W ($21.95) to sweep from 1 to 2
     GHz, driving something like an ADE-5 mixer ($3.45, hard to believe
     not a misprint).  The IF would be around 1 GHz, with RF response
     from HF all the way to 1 GHz.

     Then follow the mixer with a bandpass filter of some sort.  Maybe a
     BLP-1000, followed by a MMIC buffer, followed by a PHP-1000; total cost
     about $30.  You could probably omit the highpass (PHP) part if you
     wanted, with some additional risk of spurious signals coming through the
     whole chain.  Better yet, buy one of the nice commercial-grade cavity
     filters that (for example) kmec@aol.com is always selling on eBay.  The
     cell phone industry seems to dump nice UHF bandpass filters on the
     surplus market like they're going out of style or something.  The
     choices are endless, because your image response is going to be many
     hundreds of MHz away from the desired sideband, and it won't take much
     filtering to get rid of it.

     Once you have your 1 GHz first IF signal, you can mix it with a 950 MHz
     signal to get your desired 50-60 MHz final IF.  Again, images should be
     no problem as long as you have even a crude lowpass filter that cuts off
     somewhere between 50 MHz and 2 GHz after the 2nd mixer.

     You can get the ~950 MHz 2nd LO from another POS-2120W -- it would
     probably be quite stable with a simple fixed bias network, maybe a
     thermistor for temperature compensation, or a simple fixed-modulus PLL
     if you feel more ambitious.  Most of the homebrew spectrum analyzer
     schemes out there just use a 10-turn pot or something similar to tune
     the CATV tuner's LO, so using another VCO with a fixed voltage divider
     as your 2nd LO wouldn't be a terrible thing to do.

     If you want digital tuning, you'll have to drive the first LO with a PLL
     of some sort, or at least a high-precision DAC.  An obvious solution
     would be to multiply the output of a DDS VFO by 256 or so.  A 74HCT4046
     from Radio Shack could do the job, with a frequency divider made up of
     some of the Motorola ECLips chips.  I wouldn't try this at first -- it's
     a fair bit more work, and it wouldn't make sense unless you phase-locked
     the 2nd LO as well to keep it from drifting.  Just tune the VCO with a
     precision potentiometer, and you have a workable DC-1 GHz receiver front
     end for less than $100.  With all the cheap 50-ohm system components out
     there, it's all just plumbing.

     One VERY nifty advantage to a 1.0 GHz 1st IF would be to use a cheap 2
     GHz handheld counter to monitor your 1st LO with a piece of black tape
     over the first '1' digit.  Voila -- your receiver or spectrum analyzer
     now has a direct-reading digital frequency display.

     Also, be sure to have a look at Wes Hayward's analyzer project page
     (http://www.teleport.com/~w7zoi/SA.html) for further ideas and
     construction hints.

      -- jm

     http://www.qsl.net/ke5fx


Subject: Re: Wideband RF homebrewing (was Re: Old working wide band catv tuner.)
Date: Thu, 16 Nov 2000 03:47:44 -0800
From: "Rein A. Smit" 
Newsgroups: rec.radio.amateur.homebrew

     Hello John,

     Again thanks. I have not had the time to look up the minicircuits
     catalog. The approach is certainly attractive. But at this point
     I like to go the tuner route.
     There is a lot of tuner specs. info available from Asian sources
     and what they publisch is not too bad as far as image and NF data
     are concerned.
     My intention is just to have a device that I can use it to tune LO's
     perhaps filters, in short as a simple tool to help with a number of
     vhf / uhf projects that I have planned to do.
     Your modular approach is certainly something I will look for if I ever
     make it up to 10Ghz.

     73 Rein

Subject: Re: Old working wide band catv tuner.
Date: Thu, 16 Nov 2000 01:21:51 GMT
From: dr.dongle@juno.com
Organization: Deja.com - Before you buy.
Newsgroups: rec.radio.amateur.homebrew

In article <3A10B4E7.BD48DD66@ix.netcom.com>,
  "Rein A. Smit"  wrote:
>
>
>   Hello All,
>
>   I am looking for a 50 - 800 Mhz catv tuner that can be tuned and ramped
>   with a dc voltage.
>   Perhaps someone can tell me the reason why I can't find this stuff on the net
>   ( Is it the 900 MHz Fed policy? )
>
>   73 Rein W6/PZ0ZN
>
    There a number of "generic" CATV tuners avilabe from commercial
    outlets like MCM Electonics and Parts Express. I am currently working
    on a RCVR based one I bought for that purpose it has a 70 Mhz IF using
    a xtal filter fron a cell phone, the reciever is a TDA 7000 IC and LM
    380 audio amp. Tuning is with a 100K 10 turn pot.

    dr.d

    Sent via Deja.com http://www.deja.com/


Subject: Re: Old working wide band catv tuner.
Date: Thu, 16 Nov 2000 03:34:31 -0800
From: "Rein A. Smit" 
Newsgroups: rec.radio.amateur.homebrew

    Hi Dr D,

    Tnx for message and info. Yes, I had seen that page earlier and had
    a little problem watching it as it in Postscript. But I got it printed!
    As said in a previous message, I think we should not draw attention to it
    because whoever has it up there, may violate some Fed. law. The reason
    I write this is that after long searches with search engines I have
    gotten convinced that there is something going on about these tuners
    and their docs. TOO many dead and not available web pages.

    Regards Rein

Return