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Spectrum analyzer vs scope for radio experimentation

J

Jamie

Jan 1, 1970
0
M. Hamed said:
I am getting ready to start working on my first radio receiver (not counting the crystal radio). I plan to start my experimentation first on the AM and FM broadcast bands and then move to the amateur bands once I get my license, so I initially won't go much higher than about 100 MHz.

Can I get by without a spectrum analyzer? I have a 100 MHz scope at home, and intermittent access to a 1 GHz scope at work. Would that be enough for circuit verification?

If not what would be the max frequency I should be looking for in my spectrum analyzer (presumably from ebay)? I had read somewhere that even with low frequency stuff my oscillators or mixers could be giving out spurious oscillations at several GHz that I should be aware of.

I have also searched the group and found some leads to signal hound analyzers and I wonder if those would be adequate for this kind of experimenting.

Aside from crystal radios and a few low frequency oscillators that I built, I have very little experience with radio and so I expect there could be plenty of things going wrong spectrum-wise.

At your entry level what you need to do is not design receivers or
transmitters, accept maybe a little tiny bug type, is to get a radio kit
receiver and follow the instructions. There are many of them out there
and they work just fine for people like you.

Read fully the operation details and theory of the design to get a full
understanding of it.

As for getting a Ham license, you must remember that HAM operators and
electronic design isn't quite the same. Although there are many that
actually started with design work and then at some point got a license.

I was in design work for years before I even became a ticketed
operator. I had/have friends that I assisted in getting their advanced &
extra license for the theory part but never had a ticket myself. I just
didn't seem to have the time to sit and just gab on the radio! I was
more interested in talking about the movement of electrons.

P.S.

One day we had some clients visit our place and when they got to my
mid evil work center, they were amazed at all the instrumentation I had
lit up on the bench.. One of them asked me exactly what job I did, and I
thought it was kind of obvious with all that was taking place on the
bench but evidently it wasn't. So I told them I was an Electronologist.

Jamie
 
J

Jamie

Jan 1, 1970
0
Phil said:
An excellent goal. The more people on a project who understand more
than just their specialty, the less falls through the cracks between
specialties, and the better things go.

Spectrum analyzers are pretty cool gizmos, and if you don't mind boat
anchors, you can get amazingly good ones for reasonably cheap. I paid
$1400 for my HP 8568B three or four years ago, and they've come down
since then. That's quite a few restaurant lunches, but it's about the
same as you'd pay for a good used oscilloscope in the 500 MHz or 1 GHz
class, and you can do stuff with a SA that are very difficult otherwise.

There's a lot of overlap between radio and DSP. If you explore the
connections between continuous-time and discrete-time Fourier analysis,
you'll find that the notions of signal-to-noise ratio and all the useful
theorems are either identical or closely analogous in the two domains.

This is a really amazing time for buying second-hand equipment. Since
starting my consulting business about 4 years ago, I've furnished an
entire electrooptical laboratory for about $30k, which is about three or
four cents on the dollar. (I paid about 6% of new for a Tek TDS 694c
last year, and that dragged up the average a bit--previously it was just
about exactly three cents.) You can see the list at
http://electrooptical.net/www/EOILab/LabEquipment.html .

Cheers

Phil Hobbs

(PS: Don't take Phil A. too seriously--he's an audio guy who knows his
stuff, but he's also mentally unstable and becomes nasty when he's off
his meds, which apparently he is at the moment. You can learn a lot
from him about audio, but nothing about how to behave, whether on Usenet
or anywhere else.)
I have a 1Ghz service monitor that works very nicely at home base. It
has all the usual things needed for a service monitor, along with a
microphone to allow you to modulate on that ever freq you wish. Of
course the output level isn't must to speak of and I didn't get it for
that anyway..

I do have a small problem with it that I have only looked at once and
didn't find anything obviously wrong at first glance. This think is
hard to work on because you can't get at the boards very easy to do live
testing, you have to clip leads on specific locations and put the board
back in. I am sure the service center for these units had break out
extension boards to solve that. The problem being is the horizontal
sweep isn't as wide as it should be. This just happen one day when I
turned it on. It all works and all, I just have to make up for the
differences in calculations. THe testing of the HV and sweep board
didn't yield any obvious problem. This thing is full of RCA op-am can chips.

Jamie
 
T

Tauno Voipio

Jan 1, 1970
0
I am getting ready to start working on my first radio receiver (not counting the crystal radio). I plan to start my experimentation first on the AM and FM broadcast bands and then move to the amateur bands once I get my license, so I initially won't go much higher than about 100 MHz.

Can I get by without a spectrum analyzer? I have a 100 MHz scope at home, and intermittent access to a 1 GHz scope at work. Would that be enough for circuit verification?

If not what would be the max frequency I should be looking for in my spectrum analyzer (presumably from ebay)? I had read somewhere that even with low frequency stuff my oscillators or mixers could be giving out spurious oscillations at several GHz that I should be aware of.

I have also searched the group and found some leads to signal hound analyzers and I wonder if those would be adequate for this kind of experimenting.

Aside from crystal radios and a few low frequency oscillators that I built, I have very little experience with radio and so I expect there could be plenty of things going wrong spectrum-wise.

It seems that you're in need for basic information about building
radios. Please get and read the ARRL handbook. It answers to most of
your questions, and many more not arrived yet.
 
M

M. Hamed

Jan 1, 1970
0
** You do realise that 99% of radio hams for the last 30+ years have simply
bought commercial ham radio equipment.

I thought I got some encouragement from you early in the thread but now that's a complete let down :)
The HF bands have been abandoned and VHF is too hard to roll you own for all
but engineers with well equipped workshops.

Yes, but I'm not interested much in getting on-Air (at least for now) and talking to other hams with zero drift and 120 dBc phase noise (Ok I'm makingup stuff from memory here) as much as I'm interested in learning theory and having fun building things and tweaking them to become better.
 
M

M. Hamed

Jan 1, 1970
0
Thanks for all very informative answers. After reading through all the suggestions, I am leaning towards continuing with my goal and also buying a spectrum analyzer off ebay or some local dealer. It seems it will save me sometroubleshooting time and I can learn more this way.

The used equipment may probably have some problems, but I'll take the risk and hopefully some reduction in performance wouldn't be noticeable to my newbie self.

You read too much and build too little, or read the wrong things.

That very much describes me.

Masochist.

Watch me tell people their names in Dah-Dit's now. Probably looking crazy to anyone passing by.
 
M

mike

Jan 1, 1970
0
I do have a small problem with it that I have only looked at once and
didn't find anything obviously wrong at first glance. This think is
hard to work on because you can't get at the boards very easy to do live
testing, you have to clip leads on specific locations and put the board
back in. I am sure the service center for these units had break out
extension boards to solve that. The problem being is the horizontal
sweep isn't as wide as it should be. This just happen one day when I
turned it on. It all works and all, I just have to make up for the
differences in calculations. THe testing of the HV and sweep board
didn't yield any obvious problem. This thing is full of RCA op-am can
chips.

Jamie
You've probably already tried this, but make sure the horizontal width
doesn't change with intensity.
And that it's not also short vertically.
You can chase your tail if the HV bleeder goes open and the HV
increases.
 
J

John S

Jan 1, 1970
0
I am getting ready to start working on my first radio receiver (not counting the crystal radio). I plan to start my experimentation first on the AM and FM broadcast bands and then move to the amateur bands once I get my license, so I initially won't go much higher than about 100 MHz.

Can I get by without a spectrum analyzer? I have a 100 MHz scope at home, and intermittent access to a 1 GHz scope at work. Would that be enough for circuit verification?

If not what would be the max frequency I should be looking for in my spectrum analyzer (presumably from ebay)? I had read somewhere that even with low frequency stuff my oscillators or mixers could be giving out spurious oscillations at several GHz that I should be aware of.

I have also searched the group and found some leads to signal hound analyzers and I wonder if those would be adequate for this kind of experimenting.

Aside from crystal radios and a few low frequency oscillators that I built, I have very little experience with radio and so I expect there could be plenty of things going wrong spectrum-wise.

I have read most of the replies suggesting reading material. Do yourself
a favor and get:

http://www.arrl.org/shop/Experimental-Methods-in-RF-Design/

In my opinion, you will not find a better book to start your quest.

There are many more I could recommend, but I think you should absorb
that one first.

Cheers and enjoy,
John
 
J

John S

Jan 1, 1970
0
I have read most of the replies suggesting reading material. Do yourself
a favor and get:

http://www.arrl.org/shop/Experimental-Methods-in-RF-Design/

In my opinion, you will not find a better book to start your quest.

There are many more I could recommend, but I think you should absorb
that one first.

Cheers and enjoy,
John

By the way, if you feel that the material in that book is too simple for
you, then get:

http://www.amazon.com/Introduction-Radio-Frequency-Design-Hayward/dp/0134940210

John
 
J

Jamie

Jan 1, 1970
0
mike said:
You've probably already tried this, but make sure the horizontal width
doesn't change with intensity.
And that it's not also short vertically.
You can chase your tail if the HV bleeder goes open and the HV
increases.
Yes I've done that, the intensity and focus stays put. It's a weird
thing that happen. Instead of getting the full width of the screen I
only get 75% of it.


Jamie
 
M

Mark

Jan 1, 1970
0
to the OP, if you have a SA at work, then you know how valuable it can
be in troubleshooting..

suggestion, since you have a scope..

design or look up how to build a workable 50 MHz to 1GHz SA from an
old Cable TV converter box, that uses your scope as a display..

with the wide range log/ RSSi chips they have today, its rather
easy...


Mark
 
T

Tim Williams

Jan 1, 1970
0
John Larkin said:
WWII radars had ranges within about 2:1 of theoretical limits. A good
tube
communications receiver or SSB transceiver was as good as anything you
can buy
today, except for frequency drift. Some specs may well have been better.

Something you see often in their writings is how low-noise the things
were, despite using obviously noisy components. Early schottky ("hot
carrier") diodes, planar triodes, plasma tubes to switch the waveguides
and protect receivers, etc. For all the noise that those components add,
it's only a few dB total, and there isn't really any other source of
noise. The big difference is basically no atmospheric noise in the GHz,
so either you're seeing signal or your amplifier sucks and should be
fixed. And the engineers of the day were already quite knowledgable about
building low noise receivers.

Frequency drift wasn't even that bad; there are plenty of things today
which still use resonators, which would've been one of the primary
references available. All that takes is a good silver plated, say, invar
cavity, machined to exacting specifications.

Things we take for granted were invented back then; PLLs would've been
easier if they had dividers, since reflex klystrons are just big, hot
VCOs. They might've done harmonic lock to crystal oscillators using
harmonic multipliers alone; it's a lot of stages between 10.000 MHz and a
few gigs, but the last few stages can be passive diode multipliers, and
there are buckets of tubes that'll do VHF no problem. The last stage need
only be a diode mixer to generate the DC offset to feed back into the
error amplifier and klystron.

Yeah, that's not even terribly hard: start with a 10MHz reference
oscillator, two 5x multipliers (not very good efficiency on each, but a
tight filter on each will keep them harmonically clean), then amplify,
filter and buffer the 250MHz a little bit, then send it into a pair of
diode doublers, first 500MHz then 1GHz. You could even push 2 or 4 gigs
with another stage or two, but as the remaining power drops exponentially
with subsequent stages, you're asking a lot for more. All this was easily
possible with '40s technology: I've seen 6BQ5s used in ARRL Handbook
exciters and up-converters as high as, I think ~300MHz with deratings.

Tim
 
J

Jamie

Jan 1, 1970
0
Jan said:
A increasing HV wil usually leave focus and intensity pretty much normal.
It is a common fault, measure the HV.
I even had it with a new scope, just from the factory.
Fixed it and reported it back, they were happy...

Problem is that a changed HV also changes your vertical sensitivity as Mike already pointed out.
Needs recalibrating.
Of course it could be anything else in the H amp, or some other supply.

I did some checking with it last night, the service manual is 670 pages
and it is hard to look at on the screen when the schematic you need is
towards the end and windows is dogging.

The vertical level is as close as I need it to be. I don't think it
could be any better than it is. But what I did find was the H-blanking
signal isn't correct and in that section, where it passes the H-scan
gain circuit. There Is a PNP transistor there and I have my suspicions
that it may have lost some beta, because it acts like it has very low
gain in the circuit.

I also noticed the menus on the CRT are showing H retrace lines so
this still leads me back to the sweep card.

This is a IFR 1500 service monitor. I'll check the HV, I'll have to
scroll a few hundred pages to find ot what it should be so I can probe
it. I am not really that concerned about it since it isn't worth much
these days but It does make a good boat anchor in case you are in a
hurricane!

Jamie
 
P

Phil Hobbs

Jan 1, 1970
0
I finally twigged: that 1.13 is the ratio rather than the dB value--2/
sqrt(pi) ~= 1.1284 .
The current version of the app note is now 120 pages long:
"Spectrum Analyzer Basics"
<http://cp.literature.agilent.com/litweb/pdf/5952-0292.pdf>
The noise RMS/average correction factor is mentioned on Pg 66 and is
now 1.05dB, which coincides with your calculations.

Drivel:  I printed a copy which I suspect will make useful reading
next week, should I be selected for jury duty.  Sigh.

Don't worry, they never pick engineers for juries.

Cheers

Phil Hobbs
 
J

Jeroen

Jan 1, 1970
0
Don't worry, they never pick engineers for juries.
Phil Hobbs

That's true. I've sat on one jury[1], but was dropped for cause from
three others. I think I've found the trick on how to do it. The
attorneys always ask prospective jurors a series of questions, usually
read from a script. I noticed that the attorneys were not paying
attention or making notes until the last question, which was "what are
your hobbies". I kept score and found that if a prospective juror
answered something that takes minimal brain power, such as sports,
couch potato, hunting, gardening, racing, etc. they were usually
selected. However, if the answer was something more cerebral, such as
ham radio, building anything, writing science fiction, computing, home
nuclear physics, kitchen chemistry, etc., they were dropped for cause.
The courts really doesn't want anyone that thinks, especially that
thinks independently.


[1] I think that everyone should serve on a jury at least once, so
that they experience how strange our legal system has become.


Makes sense, doesn't it? They want a jury that can be manipulated,
influenced, scared, into whatever opinion they need. Independent
thinkers will see through such schemes and might come up with
original viewpoints, leaving the attorney speachless. Everyone
will agree that a speachless attorney has a serious problem...

Jeroen Belleman
 
F

Fred Abse

Jan 1, 1970
0
It was as crude as the equipment they developed it with.

I saw a W.W. II vintage Spectrum Analyzer put together by Crosley out
of a modified communications receiver, scope and several panels of custom
electronics, including several block down converters. It was a .5 to 30
MHz S.A. that filled an entire six foot relay rack. It was crude, it
drifted and took a lot of work to set up & use, but it was all they had
when they developed the shipboard RADAR for the US Navy at their Glen dale
Milford Road, Cincinnati Ohio defense plant. It, and the 60 foot RADAR
dish were still in place in the mid '70s to repair the last of the early
shipboard RADAR systems.

Remember Panadaptors for the AR88?
 
J

josephkk

Jan 1, 1970
0
I appreciate all the help and answers. Aside from quoting you, I tried this time not to address you directly as to make my follow up questions directed to the group. Can't say I'm experienced with USENET but I see question/follow up to the question a standard practice here. I did not expect you would have to answer my followup question in any way, I was merely expressing need for further explanation. I do not know how I wouldhave done it differently. In any case, the group here has a number of very helpful people and very knowledgeable and I do not wish to alienate anyone. My apologies.

Don't mind P. Allison's very bad attitude. He does have some knowledge.
Many here have filtered him out.

?-)
 
J

josephkk

Jan 1, 1970
0
I finally twigged: that 1.13 is the ratio rather than the dB value--2/
sqrt(pi) ~= 1.1284 .


Don't worry, they never pick engineers for juries.

Cheers

Phil Hobbs

Really? I have been picked more than once.

?-)
 
M

M. Hamed

Jan 1, 1970
0
By all means, if you can afford a spectrum analyzer, buy one.
But find someone experienced to help you decide.
Demand far exceeds supply.
Basic rule of thumb is that if a spectrum analyzer is affordable,
it's seriously busted and the parts to fix it are unavailable.

I'm (almost) out shopping for a used SA. What would be the price range on eBay for let's say a few kHz to a few GHz analyzer at which I wouldn't suspect the thing is busted. "Affordable" here is kind of vague. For example I see many many analyzers on eBay may be 200 kHz to 22 GHz that go from $1000 to about $7000 or may be more. Are the $1000-$2000 ones guaranteed to be busted?
Start with copying other's designs and go from there. Realistically,
you won't be able to do any better for a long time.
People here can tell you how to get the last microdb of noise figure,
but it won't make any difference to you if you're not trying
to do mars bounce.

I looked into some of those. They seem a bit advanced for me to assemble for the moment.
 
M

mike

Jan 1, 1970
0
I'm (almost) out shopping for a used SA. What would be the price range on eBay for let's say a few kHz to a few GHz analyzer at which I wouldn't suspect the thing is busted. "Affordable" here is kind of vague. For example I see many many analyzers on eBay may be 200 kHz to 22 GHz that go from $1000 to about $7000 or may be more. Are the $1000-$2000 ones guaranteed to be busted?

I don't have any current info. About 25 years ago, I built a TEK
492-ish out of junk
parts. Had to fix most of the boards, but the price was right.
Just calibrating it was a nightmare.

First thing to worry about is the first mixer. It's sticking right at
the front
panel just waiting to be busted...and it doesn't take much at all to
seriously
injure the balance. And you may not realize it until you get down
to some serious measurements. And the part may not be available...at
any price.

A common problem with service monitors is some of the PLL's coming
unlocked. Same with SA. If you don't have the test equipment
to see what's going on, or parts to substitute, or manuals, you're SOL.

Spectrum analyzers often have modular construction. If you buy a pallet
of 'em at govt surplus, a common trick is to swap parts around to make
a few excellent ones and put the marginal stuff in the cheaper ones...as is.

I've heard a lot of stories about stuff just missing from the instrument.

I was looking for some sampling gear. I found what I wanted about half
the usual price on ebay...powers on, no way to test it.
I had a few exchanges with the seller about the condition.
He accidentally replied from his corporate email. He was at THE
major test equipment refurbisher in the USA. Can't test it my ass...
 
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