help to identify

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Ur5by9
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help to identify

Post by Ur5by9 » 12 May 2017, 21:29

I have inherited some equipment from my late father who was a radio ham.

I have a Tektronix 492 Spectrum Analyser amongst many other bits of test gear.
Can anyone tell me what the purpose of this bit of equipment is used for and how this may server a purpose to me
Also being into radio and still like to tinker and if I can learn to use this then Id like to keep it.

I appreciate your reply's.

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26TM5890
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Re: help to identify

Post by 26TM5890 » 12 May 2017, 21:37

have a look at this video;

https://youtu.be/Stb4UvwqSy8
President Grant II
26tm5890 OP: BT. QTH: Redhill, Surrey.
Why illegal CB if you have legal SSB on 40 Mid {bnghd}

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Ur5by9
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Re: help to identify

Post by Ur5by9 » 12 May 2017, 21:53

Actual I looked at this and was still not much further forward? A glorified radio receiver?

In terms of radio what purpose can this serve in terms of servicing or such like?

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Re: help to identify

Post by paulears » 13 May 2017, 08:01

A spectrum analyser allows you to see visually the output waveform of a transmitter, or audio systems. Convention puts the low frequencies on the left of the screen and higher on the right. Spectrum analysers (proper ones) let you measure amplitude too. If you are tuning a transmitter, they're very handy. Lets say you have built a 2m amplifier, capable of 100W output. You stick it into your VSWR meter and get a nice low VSWR and a power output of 95W on the meter, but when you use it, you don't seem to get very far, and the local UHF repeater seems to cut out on the handheld you have on the shelf when you press the mic. If the display on the analyser is set to have say 100MHz on the left and 500MHz on the right, when you transmit you should see a lovely strong spike at 145.000, or whatever you have picked. A straight line going across the screen, then a spike, then a straight line. A perfect, clean output waveform. However, you might discover the spike at 145MHz is a tiny blip, but there's a really big one at 300MHz, and another one at 450MHz. There is 100W nearly of power, but it isn't where you expect. You got the tuning wrong, and the power is going into harmonics of the 145MHz signal. The one at 450 enough to desensitise your UHF handheld. RF is often produced at lower frequencies in the radio, and then maths used to double, and double the frequency to get the desired band. On the other hand, you could plug in your defective amp and discover that there is no firm spike at all, just loads of little ones running the entire width of the display. It's unstable, and no use at all - you have a 100W wideband noise source. Without an analyser, you'd have no idea really.

Other uses are for examining much smaller signals coming in from the antenna. In built up areas, there will be hundreds of radios on the go. Where I live, the 153MHz band is full of powerful and nasty data transmissions from a nearby tower. Some are always on, some do a few seconds in every 30, and all variations. If you set the analyser to cover 150 to 350 or so, then you can see that for every 153 spike there is a very small one at 306, and one of my local ones I think is slightly faulty because there is a significant spike on the first harmonic at double the operating frequency. If you open the range right up you can see all the popular bands working. You see the broadcast radio between 88 and 108, the airband, then hams, marine, business, then you can see all the DAB carriers - and they all look different because of the signals they carry. If you zoom in you can see that a carrier with no information on it is a very, very thin spike. If it has information present, it's wider, and digital again gives a kind of fingerprint that you can see. Radio 2 is much wider than the coastguard - you can measure it if you like to get the bandwidth. People who use radio mics, like me, have software they plug into the system that has an analyser display which clearly shows all the signals from the actors, and also can indicate unused channels - or more usefully where there should NOT be nothing - nothing indicating a fault. If you go to a clients premises with the intention of installing a system, an analyser is very handy for determining strong local signals that could cause grief, or for fixing faults with co-channel interference. Maybe the user one channel down has some old radios that are too wide for 12.5KHz channel spacing. In rural areas older 25KHz radios still work fine, but OFCOM might licence the channel next door and the old radios interfere like mad.

Other uses are when you are tuning cavities - the idea being to filter out frequencies you do not want - it's fairly easy with an analyser because you can look at the screen and see the effect adjusting a dimmer actually has - often adjusting one, means another needs a tweak - they all interact, and without an analyser it takes much, much longer and is often, trial and error.

This is RF signal analysers - audio frequency analysers can go up to maybe 192KHz now (stupid in my view) and are very useful for analysis of spaces. In this case pink noise is played in a space, and a flat response microphone has a listen and produces a display on the analyser. Ideally, it will be a perfect flat line - every frequency making the journey to the mic at the same level. This doesn't happen, and the display has peaks and troughs where some frequencies are attenuated and other emphasised. This could mean it sounds muffled, or it could make microphones very prone to feedback. If you can see what is happening, you can make sensible adjustments.

Even a small portable analyser can be damn useful for scanner users, as they can show you where the activity is. If you have come across the SDR systems people use nowadays, the main screen is an analyser and does many of the usual analyser functions. The trouble with using them for serious purposes is that the calibration is poor, and many have RF components costing maybe ten quid - which means that sometimes they show spikes that are actually generated inside the unit, making them a little unreliable for serious measurements. That said, an analyser for a few quid that lets you listen too - which most analysers don't do, is pretty good. Back in the 80s I was using my analyser of the time, mains powered, in my van, using a generator, in the middle of the night in the countryside trying to track down the source of some interference to a farmer client. I live in Suffolk, but he was across the border in Norfolk. Gradually I got closer to the interference. Driving down a farm track at 1am, the signal was very strong, and clearly a harmonic as it appeared to be just over 300MHz. Sadly, it wasn't. It was RAF Neatishead's fighter control and in those pre-internet days it was missing from the Ordnance Survey maps because it was secret. The RAF regiment have rifles and dogs - I guess at 1am with a generator running I wasn't hard to find. The good bit was that they actually were emitting a sizeable output on the farmers VHF frequency, which got fixed quickly. Damn scary though.

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Re: help to identify

Post by moj » 13 May 2017, 10:50

Excellent description Paul , i've been also looking at buying one of the handheld chinese rf explorers to find the elusive frequency's on dmr so hopefully it will live up to there name and do the searching with more accuracy than an sdr sweep can etc ;)

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Re: help to identify

Post by Ur5by9 » 13 May 2017, 14:11

Fair bit of reading and a nice comprehensive list for uses, thank you kindly.

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Re: help to identify

Post by Andy » 15 May 2017, 08:42

Just imagine it as a 'precision' version of the panadaptor that you might find on a modern HF transceiver or an SDR. The difference is that the input levels are carefully calibrated in units like dBm etc and you can use it for real, absolute level measurements. Just be careful - they don't like DC on the input and it's not advisable to shove anything more than about 0dBm into them or you'll pop the mixer. IIRC, the 492 was fairly well respected even though it didn't wear the 'magic' HP badge! Not sure if it has a 'listen' facility, but my Rigol did and so does my Signal Hound SA44b, but they are far more modern than the Tek. My old HP 8568 couldn't listen, despite its superb spec.
It really would pay dividends to learn how to use it if you like home-constructing or aligning stuff. You can learn a lot from this type of thing.
If it ain't broke, it soon will be!
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Re: help to identify

Post by Admiral » 15 May 2017, 09:12

Andy stole my thunder, no, they are not a 'glorified receiver', most of the common elements of a spectrum analyser like harmonics etc can be done with an oscilloscope in FFT mode, but an SA is more sensitive and more accurate, if you are a semi-professional radio tech or can get one for a steal then go for it, otherwise the investment isn't worth it for the end result, so as it was effectively a freebie and you want to learn more about radio then hang on to it and experiment, but as Andy said, be careful what you pump into it, consider inline attenuators and check the ohmage.
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