View Full Version : Spectrum Analyzer suitability for CB radios
okrobie
12-10-2021, 08:00 PM
Hello all, I have recently purchased a spectrum analyzer. It is an Avcom PSA-37D. After I bought it (for a good price) I realized that it is primarily intended for use with satellite communications. It does have 10 Mhz. as its low end range but as I'm setting it up, I'm not seeing output from transmitters in the CB frequency range.Is anyone out there familiar with this model? Is it suitable for us as a setup instrument for CB radios?Thanks, Jim
P.S. I have been involved with 2 way radios since 1956 when I got my first ham radio license (W3FAW) I haven't always worked directly with radios, but I have been in some form of electronics or computing since then.
okrobie
12-10-2021, 09:34 PM
I'm putting together a set of service equipment: BK Precision E-2000 RF Signal Generator, BK Precision E-3108 Sine / Square Generator, BK Precision 1850 Frequency Counter, Tektronix 2445 Oscilloscope and the recent Avcom PSA-37D. Still need a Dummy Load and Power meter.
Bobcat
12-13-2021, 03:19 PM
okrobie, you sound like my kind of guy!!!! I went to DeVry for electronics from 75-77, came out and worked at NASA JSC in the Shuttle Mission Simulator for about 11 years. BUT.... 8 years in.... I switched from hardware... to "operating system internals" and have been software ever since. NOw that I am getting closer to retirement... I want to get back to my "electronic roots" and start messing with electronics... so I TOO am building a test bench for both radios AND tube guitar amps.
Please don't think that I am on the attack.... but one of the first things I want to talk about..... is how you connect a radio TO a spectrum analyzer. You didn't mention how you have connected it...and you can FLAT blow the front end on one of those easier than you can imagine...because they are SUPER SUPER sensitive.
I am hoping that you have not direct cabled the output of your transceiver straight to the spectrum analyzer. Very often the input limit on an SA is 10 dBm (100 mWatts into 50 ohms) so you cant pump a radio straight into it or it will burn out the front end.
The way to connect them "directly" is either through power attenuators that attenuate the signal and bring it down to a SAFE level WHILE ALSO dissipating the wattage that the radio puts out. If you choose the right attenuators ... what comes out will be safe for the SA. YOU CAN ALSO use an RF sampler of some kind. Typically these connect between your radio and a dummy load...and have an attenuated "tap" that you cable over to your SA.
To use it INDIRECTLY... you can just plug in a lead of some sort....and have your radio tied to ITS OWN antenna...and just pick up the RF by that open lead. Some people make a loop of wire to act as a mini-pickup for the SA.... so that there is NO direct connection.
I am going to shut up here and look for a response to see what you have been doing.
Again, don't take this as a reflection of doubt on your skills.... I simply did not see you mention it... so I brought it up to express concern for safety to your SA. SA's can be damaged REALLY easily...and when they work.... they show you things you won't see ANY other way. Let's talk some more
By the way, mine is an older one, a Hameg 5006... good for 500 MHz or so...and has a tracking generator.
Like I say, let's talk some more!
I rejoice in your achievement! (35 years!)
Bob
Bobcat
12-13-2021, 04:08 PM
Oh .... and on this end... so far I have collected:
Tektronix 465M 100 Mhz scope
Hameg 5006 500 Mhz Spectrum Analyzer with tracking generator
Sinaddr (sinad meter)
Boonton 102F RF Generator
Beckman Industries FC130 - 1.3 Ghz Frequency Counter
Heathkit IM-11 VTVM
Eico 147A Signal Tracer
Eico 950B Resistance-Capacitance Bridge tester
Wavetek Audio Function generator
okrobie
12-13-2021, 09:28 PM
2502Hi Bobcat, yes indeed. It looks like we have a lot in common.
Never fear. I did have enough sense to attempt the input through several "alternate" means. Basically, I made an antenna out of a coat hanger (not literally) and connected it through the BNC connector. I am seeing a signal of sorts but not a pretty spectrum picture. I guess I'm picking up enough RF to make it to the display but it's not pretty. At this point I'm becoming concerned that the former owner may have fried the front end. Let mepost a picture of what I'm seeing.
Bobcat
12-14-2021, 10:33 AM
Okay. Good! And I hope you don't think I was "hammering on you" about your knowledge... I just don't know what you do or don't know. I will tell you flat out... I knew NOTHING about SA's when I got mine. I knew theoretically what it was FOR......but had never touched one in my life! SO I had a lot to learn.
So let me ask... have you used an SA before ... and are you familiar with your unit and the controls on it?
With that, I can tell you.....
MY SA is set up on my bench cabled to a transformer (I wound it!) based RF sampler that is pretty close to -40 db in attentuation. I am using an old antenna switch box...and using one of the positions as RCV and the other as XMT. SO I have the cable that screws onto my radio... going to the switch box.....going to the RF sampler....going to the MFJ dummy load. Then the TAP off of the RF sampler (-40 dB.... so that knocks the radio WAY down around 0 dbm) goes to the SA. IN my world....at the moment...it STAYS that way.
The fun part is...when I transmit on a radio (I have not only CB but am a general class amateur as well so I have an Icom 7300 for HF work).... if I have the SA set up right..... it picks it up right off of the antenna. So I can hit the transmit button and look over at my SA and see the display. IN my case... it usually looks pretty normal... like one would expect an SA display to look.
That said, I have found it VERY handy to have the SA on my bench as it shows me when "nasty harmonics" are at play in my setup or in a radio under test.
okrobie
12-14-2021, 12:15 PM
OK Bobcat, were in the same boat. I know nothing about SA's either and you are my hero for plowing ahead. I'm trying to hook up my RF sig gen to get a clean CW signal into it. I like your idea of a transformer. How many turns and what did you use for a core. I'm also using an antenna switch as a sampler. No direct connection. Just the proximity of the switch contacts inside the switch. Even though I don't have photos yet, I already see that it is not going to be a nice clean CW spike. Unless you can help me get it right, I can still return the SA to the seller.
I' m also an active ham but at the moment just 2 meters. I'm keeping my eyes peeled for an HF rig even if it's only 10 meters. I'll let you know haw the photos of the sig gen signal turn out. 73
Bobcat
12-14-2021, 02:26 PM
My phone was not showing me the picture that you sent earlier... so I just got on the laptop and got a look at it.
One thing I see that is somewhat strange (to me) .... I see the narrow signal spikes....but what stands out to me... to the LEFT of the signal activity is LOWER than to the RIGHT of the signal activity. I would expect the noise floor to be the same on both sides.
When you have all signal off...... and the SA has "nothing to look at"... is the trace level and even from left to right? I am not talking about "rotated"... but both ends of the sweep should see the same level... I am going to try to attach a couple of photos I just took.... playing around here......
The first one is my SA with nothing going on. What it shows is a flat line all the way across the bottom... indicating no frequency being detected at all.
Second shot is the screen of my 7300 set up in the 20 meter band in AM for a quick test transmission. Note that the radio goes straight out to the end fed halfwave antenna outside the house... there is no direct connection to the SA. Also.. I have not plugged anything in to the SA to act as an "antenna". Just the regular RG58/U BNC cables that are always hooked up.... (which makes me re-think buying cheap no name brand BNC cables!!!!!)
Third shot is when I key up the 7300. This shot of the SA shows a single hump in the middle. I have the scan width stretched out to where it looks fairly wide....but all you see is the carrier peak.
That is what I see when I just key up a transmitter in the area.
Bob
Bobcat
12-14-2021, 02:46 PM
Let me ask this about your picture........
What RF activity did you have going on? Was that a signal generator?
Did you have a transmitter keyed up?
Or was that just whatever RF noise happened to be in the room at that time (and I am assuming the gain is way up if it is adjustable)
Hi Bobcat, yes indeed. It looks like we have a lot in common.
Never fear. I did have enough sense to attempt the input through several "alternate" means. Basically, I made an antenna out of a coat hanger (not literally) and connected it through the BNC connector. I am seeing a signal of sorts but not a pretty spectrum picture. I guess I'm picking up enough RF to make it to the display but it's not pretty. At this point I'm becoming concerned that the former owner may have fried the front end. Let mepost a picture of what I'm seeing.
Bobcat
12-14-2021, 03:33 PM
Also you asked about the RF sampler I built. I am going to try to attach the PDF of the project that I used as a springboard...and a few pics of "gut shots" of mine. Would be happy to answer any questions that I can.
(note... the PDF exceed s the max file size allowed for a PDF on the forum... I would have to find some other way to send it!)
okrobie
12-14-2021, 08:24 PM
sorry, I didn't tell you what the picture is. It was using the sampler I described through the antenna coax switch into a BNC scope probe, with the mic button pushed and no modulation or tones.
Bobcat
12-15-2021, 01:35 PM
Okay, I have one more question about your attempt to get a display......
If you look at the 2nd analyzer display that I sent you.... the one with the "bump" showing...
If you look above and to the right you see a little display window with "14.5" showing in it.
THAT was my Center setting for the analyzer. That setting tells the SA what frequency I want to "center" on
so IT makes the center-most vertical line on the screen be (in my case) 14.5 Mhz. I was actually somewhere in the 14.2 Mhz area... so I was a little to the left of it...but it DID show up on the screen.
Now to YOUR picture....
I see a box in the upper right that appears to say Center Frequency but all I can see in it is a "-" and a "0".
Did you indeed have your "center frequency" set on the SA?
For instance, if you were going to try on Channel 19... you would want your center frequency set for as close
to 27.185 Mhz as you can get it. That way the screen will "zero in" on the content in that area of the spectrum.
If that is not set right.... all you may see is harmonic "hash" of some kind...and that is what that display looks like.
SO I am double checking.... did you have "center frequency" set somewhere NEAR the channel you were using to test?
Thanks!
Bob
okrobie
12-16-2021, 05:26 PM
Hey Bob, sorry I haven’t been able to text for a minute. I decided that my Avcom wasn’t working so I exercised my opinion to return it to the seller. But by coincidence there was another SA on Facebook Marketplace here locally. It is a Tektronix 491. I took a freq meter and an RF sig gen with me to make sure it was working. It wasn’t. I couldn’t get the signal to show on the screen so I made the guy a $50 as-is offer and he took it. There is much more technical support out there for this model than the Avcom so I should be able to fix it. (I hope). I’ll get it on the bench soon and keep you updated as I progress. 73, Jim
okrobie
12-16-2021, 05:51 PM
BTW, I’m taking the gamble that it has a blown front end and that I’ll be able to fix it.
Bobcat
12-16-2021, 08:19 PM
Okrobie, you can tell me to shut up ANY time you want!!!!!!
I just went out and found a Maintenance Manual on the Tek 491 made by Tektronix for the US Army.
I tried to attach it... but the limit is just a hair over 9Mb for a PDF file upload.
If you need it.... maybe I can email it to you or something....
okrobie
12-16-2021, 09:02 PM
Hey Bob, I’ll never tell you to shut up. We’re brothers of a different mother.😎
Where did you find this manual? Maybe I could download it directly from the same site you did.
It sure sounds like what I need.
73
Bobcat
12-16-2021, 09:13 PM
Here is the link I got it from . Up at the top is a GREEN Download button.... start there....
I am not sure how full it is...but there seems to be a lot in it. I hope it does you some good.
Bob
https://www.manualslib.com/manual/484429/Tektronix-491.html
okrobie
12-16-2021, 10:18 PM
Thanks Bob, that's quite a manual. If I can't fix it with that, it can't be fixed, Thanks, Jim
Bobcat
12-17-2021, 01:32 PM
You are welcome! I hope that it helps you get to a fully functional Spectrum Analyzer!
okrobie
12-18-2021, 09:29 PM
Manual or no manual. The first thing I’m going to do is open it up and look for smoke. I’m as sure as I can be that someone has hooked this thing up to an 80 meter 5KW flame thrower and I’ll see a smoky mass on the other side of the BNC input connector. Wish me well.😎
Bobcat
12-20-2021, 10:11 AM
You know... to speak the plain truth... I feel that I was lucky with mine.
I bought it at a hamfest here in the Houston area....but I bought it in the "tailgate" area outside the main building.
I didn't know a SINGLE thing about spectrum analyzers.... other than..... it didn't seem like I could beat it.... for the price of $130.
It was after I got home and started studying that I discovered how easy they are to damage AND HOW EASILY it could have BEEN damaged when I bought it.
So either luck was with me, or the many was decent.... I'd kind of like to think BOTH!!!! As it worked fine when I got around to trying it out.
I didn't know about building a little "coily pickup" non-contact probe until somewhat later.... and it took me a while to understand EXACTLY how the "care and feeding" of an SA worked. I had to get comfortable with the concept of dB and dBm and just HOW both attenuation AND proper power dissipation had to work together....to provide a 0 dBm (and NO MORE than +10 dBm!) signal .....and THEN built the -40 dBm sampler that I built.
I definitely have enjoyed having it on the bench though. Perhaps I am foolish....but I tend to lean towards being a "clean air / clean modulation" nut and have already taken the CB that I had on my desk "out of service" after seeing IT on the SA. HORRIBLE .... HORRIBLE modulation harmonics present in the signal!!!!
Again, as I eventually "close in" on retirement... I have a stack of radios of my own here... that I want to evaluate, repair, align and HOPEFULLY make them work as well or better than they ever did.
Again too, I wish you luck in getting yours going. Do keep me in mind and let me know how it goes for you.
okrobie
12-22-2021, 04:57 AM
Not as easy as I thought. First of all, the problem doesn't seem to be in the input section. The input is traceable for quite a ways. No smoke, no charcoal. The seller knew he had a dog so that's why he let it go so cheap.
I am in my retirement and for some time now I have enjoyed fixing machinery such as air compressors. generators and misc. small engine projects.
Problem is, my wife is sick of seeing our back yard looking like Sanford and Sons, and as we get older she keeps talking about "downsizing". The future is that my hobbies must fit comfortably into a spare bedroom. Part of that will be computing and the other part will be radio operations and repair. Thus my quest to build a radio repair bench. She will likely be retiring within the year so the heat is on.
The future also holds prospect for camping and RVing. In my spare time I'm converting a Ford E-350 van into an RV/Radio shack with solar power and a battery bank suitable for boondocking.
I currently have a 5HP, 2 stage, 80 gallon compressor and a Grizzly table saw with dust collector for sale, and the proceeds will help fund these projects.
Other than that, I don't have much to do.
Best wishes for a Merry Christmas and Happy New Year to you and yours. 73, Jim
Bobcat
12-22-2021, 01:22 PM
A sort of "reverse" situation here regarding retirement. My wife retired from the work force last in September 2020. I am still working for a couple of years... but I love my job so it really isn't a load on me. I have a room in the house... sort of to myself. This is a room the previous owner built up out of the original back porch. Did a really nice job... the only issue is... the way they did it... there is no DOOR to the dining room area... it is wide open all the time. This means.... guests that walk through the dining in to the kitchen... if they turn their heads.....are looking right in here... so it is incumbent on me to keep it as neat as I can....but she really is not too fussy about it.
As I go I will be making improvements to the room, the workbench, the shelving unit for equipment, the radio desk, a cabinet or two for storage...and try to keep it as presentable as I can.
Here is a couple of older pics of what I have now. I have realized that this kind of shelving is not going to work well so it will be replaces ultimately. Not showing are the 1.3 Ghz Beckman Freq counter and the Eico 950B Capacitor/Resistance bridge/tester... and the Wavetek audio function generator.
Bobcat
12-22-2021, 01:24 PM
Wishing you and all of yours a very Merry Christmas and Happy New Year.
Our flock will start descending on us tomorrow mid-day. I hope you have a great Holiday Season.
de KG5JAI
okrobie
12-24-2021, 02:46 PM
Thanks for your Christmas wishes. Same back atcha.
I'm seeing a little glimmer of hope with my SA. I'm seeing some RF getting through to the screen. I wasn't seeing any signal in the low band at 30 Mhz. So I tried band 2 at 300 Mhz. The signal is getting through and displaying on the screen as a sine wave. Not displaying any frequency sweep for the spectrum display. At least it shows that the front end is healthy on that band.
okrobie
12-31-2021, 12:50 PM
Hi Bob, just a little FYI. I’ve been in the hospital with COVID for a few days. I’m getting better now but I’ll be in quarantine for nine more days. Please keep me in your thoughts and prayers as I recover. Thanks, Jim
Bobcat
12-31-2021, 02:33 PM
I regret hearing that and hope that you make a climb out of that.
Both my wife and I had it in late May of this year. We had both been Phizered in Jan/Feb so we were one of the breakthrough cases but I can say that from the vax... Vax... symptoms only lasted a week.. no hospital. My wife was positive but never had symptoms at all.
We followed up with a booster in September. We just found out on Christmas day that... on Christmas Eve.. we were in direct contact with a family member who was tested THAT NIGHT and found positive. We are now 7 days down the road and no symptoms or effects at all.
I already have offered a prayer that you recover fully .... and that you get that SA back to working... so that we can get on to the fun stuff!
Take care of yourself my friend.
Bob
Sent from my SM-G991U using Tapatalk
okrobie
12-31-2021, 03:52 PM
Thank you for your prayer Bob
I have had the J&J and my wife had both Pfizer but we hadn't yet taken boosters. I think we caught Omicron because our symptoms were like a heavy case of flu. Never felt threatened.
What is this Tap-A-Talk? I have been a member of many forums throughout the years and in fact hosted a few, but I never needed a third party app to participate
Bobcat
12-31-2021, 06:33 PM
I think of it as a phone app that can handle connection to forums.... IF the forum knows how to deal with it.
This way.... I get in the app.. and log in to this forum.... And NOW it just acts like an app. A little easier than dealing with a forum on a web browser! At least that is the way that I use it...
B
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