Fry's cheapie: Airlink AWL5025 $15

Jeff Liebermann hath wroth:

Not even a guess? Oh well. This is a good example of why you can't directly use a spectrum analyzer in place of a power meter or bolometer with wideband (SS, video, UWB, etc) signals.

The indicated power level on the display is 0dBm. However, the power is splattered over a bandwidth of 25Mhz. We want the equivalent carrier power, or the power of the de-spread signal. So we add: 10 * log(25MHz/0.5MHz) = 10 * log(50) = 17dB So, the real power is: 0dBm + 17dB = +17dBm

Even this is not exact because the power spectra is not equal power across the 25Mhz bandwidth and the IF filter is not a brick wall at

500KHz. There's also quite a bit of power outside of the 25MHz window. The rms (heating) power is also different because of the transmit duty cycle.

This depends on the "intelligence" of your spectrum analyzer. For the audio spectrum analyzer I have, you just push a button and it computes the energy in the bandwidth since it has all the information available to do such a computation. I don't know if there are RF spectrum analyzers with such intelligence, but I don't see why not.

In datacom (again, back to voice band communications), some modems used guard tones to indicate the line contained data, not voice. The CCITT specification is based on energy, so making such measurements (energy in a bandwidth) is a desirable feature. That is, the tone had to be a certain value relative to the data energy, which was spread out.

There are plenty that have enough intelligence to grind out true power. I can't afford any of these. I can't even afford one with obsolete IEEE488 bus.

Sorta. *ALL* modems 1200 baud and up include a scrambler. What the scrambler does is evenly distribute the energy across the entire bandwidth, thus reducing the peak power, and thus making it easier to transmit. The required dynamic range is also reduced. OFDM and DMT DSL modems do the same thing by splitting the data up into many discrete carriers. The total energy in the bandpass is the same whether scrambled or de-scrambled, but the peak power is quite different.

However, this has nothing to do with what I was mumbling about. I was questioning the original comment that one could just read the tx power from a spectrum analyzer. With spread spectrum, it's not that easy.

Without the scrambler, it is possible to create data sequences that you can't "clock recover."

Hey, at least you have an analyzer that goes out to wifi bandwidth. ;-) RF gear really holds it's value, more than audio test equipment.

Well, yeah. It does depend on the coding protocol and whether the clock signal and synchronization is encoded into the data. It's kinda easy with 802.11, where the preamble takes care of any sychronization issues.

Wrong. My pile of test equipment mostly quits at 1GHz. I use down converters (mixers) to deal with anything on 2.4 and 5.7Ghz. I am borrowing a Wiltron 610C with 6215D 1 to 4GHz plugin and VSWR bridge which is VERY useful for antennas. As I recall, I recommended that you look into one of these for antenna testing. However, I have to return it to the owner next week. Grumble. Mostly, I borrow what I need.

RF test iron does hold it's value better than audio, but only because it requires more expensive calibration and repair exercises. If it's working, it's quite valueable. If it's non-functional, it total garbage. Also, lots of analog audio test equipment is being obsoleted by DSP based digital processing test equipment.

At this time, I have more test equipment to repair, than is functional. I also have 2 or more of everything as I have the bad habit of buying two or more junk units, and conglomerating them into one that works. It's often cheaper to buy a complete unit for parts, than to try to find the individual parts.

whether the

pile of test equipment mostly quits at 1GHz. I use down

My RF generator (HP8660C) reaches wifi, but the last time I fired it up the mainframe wasn't working. Well, the plug ins are the hard thing to get. I use this shop in Morgan Hill to fix it. If you have attended Foothill, you probably met the guy. Damn, I don't see his card in the pile I have handy, but the guy is good.

Yes, the VSWR bridge is still on the list.

Oh yeah, I know the deal with busted gear for parts. I was at the Livermore swap meet years ago and saw a busted version of the analog scope I use [HP1725A]. There was already a somewhat heated discussion between a buyer and the seller. It was pretty entertaining as the buyer had this thick Russian accent. The buyer really beat the guy down, then didn't have the money. I think it was like $30, so I bought it, even with the word "crap" written with a grease pencil on the screen. I stashed it and maybe a year later my boss at the time buys some surplus gear, and you guess it, it had a broken HP1725A. I let him pick at my spare since he did me plenty of favors, and he got his scope going. Eventually I bought that scope from him for a song, probably because he felt guilty about picking out custom hybrids from my spare.

The analog gear I have is mostly from the 80's, so they do have DSP in them.