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Author Topic: RF frequency sweep  (Read 9292 times)

Lyle Williams

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« Last Edit: November 18, 2014, 04:12:11 am by Lyle Williams »
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Keith Broughton

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Re: RF frequency sweep
« Reply #11 on: November 18, 2014, 08:25:52 am »

Anyone tried this...
http://nutsaboutnets.com/rfviewer/
Seems like it covers the basics for $80.
The Clear Waves seems pretty good for under $400 as well.
http://nutsaboutnets.com/white-space-finder/
« Last Edit: November 18, 2014, 08:28:44 am by Keith Broughton »
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Lyle Williams

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Re: RF frequency sweep
« Reply #12 on: November 18, 2014, 03:04:09 pm »

The problem with cheap DTV-dongle based specans is resolution bandwidth.  The $79 option will let you see if a signal is present, and the relative strength of that signal.  That is sufficient for choosing a mic channel.

If you want to see signals resolved down to the last hertz, tie into an external lab frequency source, or demodulate/debug complex received waveforms, you will need something more expensive.
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Lyle Williams

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Re: RF frequency sweep
« Reply #13 on: November 18, 2014, 03:30:25 pm »

I have played with freeware dtv dongle systems a while back, and this one looks slicker in terms of overall behaviour.  I've ordered one to play with, but it will be a few weeks before it arrives down here in Australia.

I'll let people know how it goes.

I have a couple of units in the same $1000 price class as the Signalhound, and a 6-figure priced Agilent at work to compare it too.  It won't come close to touching either of those, but I'm pretty confident it will be fit for purpose for checking wireless mic spectrum.

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Pete Erskine

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Re: RF frequency sweep
« Reply #14 on: November 18, 2014, 10:40:28 pm »

.... find best RF band for an installed wireless mic system at a sports field.

Here is a collections of RF scans around the world which can be loaded into Workbench and IAS

http://www.bestaudio.com/spectrum-analyzer-setups
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Lyle Williams

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Re: RF frequency sweep
« Reply #15 on: November 19, 2014, 02:42:02 am »

I'm a bit of a radio geek, and I have all sorts of radio toys.  One more soon!

Despite that, for the handful of mics I have I would normally find frequencies by looking at which tv channels were in use and looking in the spectrum regulator's online database.  That would give me a view of where unused spectrum would exist.  I use the manufacturer's recommended intermod-free channel sets and observe if there is received RF signal on each receiver before the corresponding transmitter is turned on.

Lots of mics in a rf-dense environment certainly needs planning and monitoring.  Many lesser applications do not. 

In my case things are more likely to go wrong because I am looking at too many screens rather than there being RF problems.
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Lyle Williams

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Re: RF frequency sweep
« Reply #16 on: December 03, 2014, 10:52:11 pm »

Sadly the dvb-t dongle/software combo I got seems to perform poorly.  Even at its price point.
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David Sturzenbecher

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Re: RF frequency sweep
« Reply #17 on: December 04, 2014, 07:59:20 am »

I have been playing with this and touchstone and getting pretty poor results as well. It shows a strong DTV signature well but seems to be picking up a lot of random spikes, sometimes spaced 10MHz apart sometimes not.


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Jason Glass

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Re: RF frequency sweep
« Reply #18 on: December 04, 2014, 10:34:36 am »

I have been playing with this and touchstone and getting pretty poor results as well. It shows a strong DTV signature well but seems to be picking up a lot of random spikes, sometimes spaced 10MHz apart sometimes not.


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Hi David,

Evenly spaced spikes are often caused by LO leakage out of the RF input being reflected back into the instrument from the antenna and combining with a strong signal that may be outside of the displayed span but within the instrument's tuning bandwidth.  They combine to generate images at intervals equal to the LO frequency.  This is a very common thing with inexpensive wideband equipment.  It can sometimes be mitigated by inserting an attenuator at the RF input, or even better by inserting a bandpass filter that allows only signals within the span of interest to pass into the input.

My office has a 100KW FM tower at 101.1MHz located only a few miles away, and it can mess up measurements in the UHF TV band.  I frequently use an FM bandblocking filter, or alternatively a bandpass filter like this one http://www.ebay.com/itm/K-L-MICROWAVE-4B120-600-250-O-OP-FILTER-/331395656328?pt=LH_DefaultDomain_0&hash=item4d28b86a88 to clean up UHF measurements.  At locations where the spectrum is saturated with high-power signals I often use a much narrower passband tunable filter to sweep across the span of interest.  It always works well, but the filter is big, heavy, expensive, and sweeping across the span with the SA set to Max Hold is a slow process.

Lyle Williams

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Re: RF frequency sweep
« Reply #19 on: December 04, 2014, 02:47:48 pm »

Turning on a clean signal generator attached to an antenna (maybe -6dBm output) lifts the whole noise floor the dvt stick sees by about 20dB.  I don't just see one signal from the siggen, but about 50.

Yes, I could find the local TV stations reasonably clearly, but they are only 5 km away and either 50 or 200 kW transmitters. 

It'd be much more practical to just search in the local spectrum regulator's database.
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Re: RF frequency sweep
« Reply #19 on: December 04, 2014, 02:47:48 pm »


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