Spectrum Analyzer suitability for CB radios

[okrobie]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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!)