Poor Man's Spectrum Analyzer

[Matt Osborn]

The Poor Man's Spectrum Analyzer I'm building uses a varacter driven
cable tuner as its front end.  The cable tuner has an input impedance
of 75 ohms. Most instruments have 50 ohm input impedance and my usage
will be primarily with 50 ohm devices.

Maybe it isn't worth worrying about, but I'm looking for a convenient
way to convert from 75 ohm to 50 ohm input impedance.  I've seen some
writeups on building a 50 ohm to 75 ohm broadband unun, but the
articles were incomplete (or my knowledge level is too low to
recognize a complete article).  The analyzer covers 5MHzto 500MHz.

Does anybody have any advice?  Leave it alone, build an unun, buy an
unun, any other options?
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[k6mr]

If you don't mind degrading the input specs a bit, a minimum loss pad is
about as simple as you can get.  Two resistors in the input line.  I
think you lose something like 4 dB when going from 50 to 75 ohms for a
voltage measurement (you can calculate it exactly).  This trick is used
in the CATV industry since most test equipment is 50 ohm and they are
75.  There are commercial versions available with the proper connectors
on each end, but unless you can find them surplus they would probably be
pretty expensive.

Ken  K6MR

Matt Osborn wrote:

>The Poor Man's Spectrum Analyzer I'm building uses a varacter driven
>cable tuner as its front end.  The cable tuner has an input impedance
>of 75 ohms. Most instruments have 50 ohm input impedance and my usage
>will be primarily with 50 ohm devices.
>
>Maybe it isn't worth worrying about, but I'm looking for a convenient
>way to convert from 75 ohm to 50 ohm input impedance.  I've seen some
>writeups on building a 50 ohm to 75 ohm broadband unun, but the
>articles were incomplete (or my knowledge level is too low to
>recognize a complete article).  The analyzer covers 5MHzto 500MHz.
>
>Does anybody have any advice?  Leave it alone, build an unun, buy an
>unun, any other options?
>_______________________________________________
>Elecraft mailing list
>Post to: [hidden email]
>You must be a subscriber to post to the list.
>Subscriber Info (Addr. Change, sub, unsub etc.):
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>
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>Elecraft web page: http://www.elecraft.com
>
>
>  
>

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[Bob - W5BIG]

Hi Matt,

You can use two resistors to make the signal source think it is terminated
in 50 ohms and make the SA think it is being driven from a 75 ohm source.

I posted a diagram of the circuit here:
http://w5big.home.comcast.net/50ohm_to_75ohm.gif

The first resistor is 43.3 ohms in series with the input to the SA.
The second resistor is 86.6 ohms to ground at the signal input.
(87 ohms isn't a standard 5% value, but you could put two 43 ohms, 5%
resistors
in series).  1% metal film resistors are even better if you are getting into
the UHF region.
The leads should be as short as possible. With this arrangement,
you have an easy-to-build wideband matching circuit without transformers.
The loss is 4db which can be accounted for when you calibrate the SA.

73/ Bob - W5BIG


----- Original Message -----
From: "Matt Osborn" <[hidden email]>
To: <[hidden email]>
Sent: Sunday, October 09, 2005 9:42 PM
Subject: [Elecraft] Poor Man's Spectrum Analyzer


The Poor Man's Spectrum Analyzer I'm building uses a varacter driven
cable tuner as its front end.  The cable tuner has an input impedance
of 75 ohms. Most instruments have 50 ohm input impedance and my usage
will be primarily with 50 ohm devices.

Maybe it isn't worth worrying about, but I'm looking for a convenient
way to convert from 75 ohm to 50 ohm input impedance.  I've seen some
writeups on building a 50 ohm to 75 ohm broadband unun, but the
articles were incomplete (or my knowledge level is too low to
recognize a complete article).  The analyzer covers 5MHzto 500MHz.


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[Gregory P. Daly]

Matt:

     If you can't tolerate the loss of the minimum loss pad,
Mini-Circuits makes a little transformer for just such emergencies

http://www.minicircuits.com/

search for the ADT1.5-1

 they're about $3 each and work from about 500 KHz to 650 MHz. I've used
them measuring noise figure in CATV line amps. and a few other spots
that can't stand the pad

     Most of the time, the pad is the way to go, simple and broad band,
but once in a while the 4 dB loss is too much.

     Mini-Circuits also has nicely packaged Minimum loss pads, with
"official" 75 ohm connectors, and your choice of which end gets what sex
connector. If you do get "real" 75 ohm N or BNC female connectors, be
careful not to damage them with 50 ohm males, the 75 ohm pin is smaller,
and it only takes once (don't ask how I know this :-)

     Mini-Circuits is reasonably nice to hams, look around in their web
site, there's all kinds of RF goodies to keep a ham entertained. I don't
know their minimum, I've bought as little as $25 from them and they
haven't gouged me too bad for it.

73 AND GOOD DX DE WB7RSG

Greg



On Sun, 2005-10-09 at 21:42 -0500, Matt Osborn wrote:

> The Poor Man's Spectrum Analyzer I'm building uses a varacter driven
> cable tuner as its front end.  The cable tuner has an input impedance
> of 75 ohms. Most instruments have 50 ohm input impedance and my usage
> will be primarily with 50 ohm devices.
>
> Maybe it isn't worth worrying about, but I'm looking for a convenient
> way to convert from 75 ohm to 50 ohm input impedance.  I've seen some
> writeups on building a 50 ohm to 75 ohm broadband unun, but the
> articles were incomplete (or my knowledge level is too low to
> recognize a complete article).  The analyzer covers 5MHzto 500MHz.
>
> Does anybody have any advice?  Leave it alone, build an unun, buy an
> unun, any other options?
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft   
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

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[n6wg]

Matt
You could consider making a 5 dB pad that also
matchs from 50 to 75 ohms.  5 dB would not
sacrifice too much sensitivity, and it would be
very broadband, being nothing but three resisters.
Good luck and 73
Bob N6WG

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]]On Behalf Of Matt Osborn
Sent: Sunday, October 09, 2005 7:43 PM
To: [hidden email]
Subject: [Elecraft] Poor Man's Spectrum Analyzer


The Poor Man's Spectrum Analyzer I'm building uses a varacter driven
cable tuner as its front end.  The cable tuner has an input impedance
of 75 ohms. Most instruments have 50 ohm input impedance and my usage
will be primarily with 50 ohm devices.

Maybe it isn't worth worrying about, but I'm looking for a convenient
way to convert from 75 ohm to 50 ohm input impedance.  I've seen some
writeups on building a 50 ohm to 75 ohm broadband unun, but the
articles were incomplete (or my knowledge level is too low to
recognize a complete article).  The analyzer covers 5MHzto 500MHz.

Does anybody have any advice?  Leave it alone, build an unun, buy an
unun, any other options?
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[Matt Osborn]

On Sun, 09 Oct 2005 21:42:52 -0500, Matt Osborn <[hidden email]>
wrote:

>The Poor Man's Spectrum Analyzer I'm building uses a varacter driven
>cable tuner as its front end.  The cable tuner has an input impedance
>of 75 ohms. Most instruments have 50 ohm input impedance and my usage
>will be primarily with 50 ohm devices.


Thanks to all for the fine ideas and proposed solutions.  Greg's
suggestion of the Mini-Circuits ADT1.5-1 seems to be the exactly what
I need.

This is a _really_ great group!


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[Matt Osborn]

No trouble at all. I ordered the kit from Bruce Barlowe at The Science
Workshop.

http://www.science-workshop.com/

It's definitely not an Elecraft kit, you'll have to pick buy your own
enclosure, power transformers, switches and front panel controls.

The schematics are all hand drawn with no component values marked, but
the parts list is complete. The assembly instructions are sparse,
(install 16 resisters, install 14 capacitors, install 4 diodes, etc)
but it will get the unit assembled.  There is very little 'theory of
operation' of the actual circuits, but the optional book offers lots
of information on using the analyzer.

On Tue, 11 Oct 2005 19:53:53 -0500, "james" <[hidden email]> wrote:

>Matt  just wounder have u had a hard time getting the parts for the spectrum
>analyzer was thinking about building one  Sincerely harold n5tog

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