# Help with high frequency measurements

1. Jul 26, 2014

### TheBestCake

Hello everyone!

So I am currently working on a project involving classroom equipment from PASCO.
(http://www.pasco.com/prodCatalog/WA/WA-9314_microwave-optics-basic-system/#overviewTab)

What I'm working on is not exactly related to the the classroom project but closely related to the measurement of the microwave frequency and I need help measuring the approximate wavelength of these microwaves. I only need to see a difference, nothing too scientific. This kit comes with a detector with output plugs and the manual says "This output can also be used for close examination of the signal using an oscilloscope." but unfortunately that's the only time an oscilloscope was mentioned. A system like this one () would be perfect, I can buy another detector if needed and I have a 200MHz oscilloscope but I don't have the funds to buy a microwave frequency counter.

I'm not entirely sure how frequency mixers work but couldn't I use one in a system where I have my microwave signal split using a beam splitter and one of those signals will go directly to a detector then to the mixer. The other signal would go through some test material, then to the detector and finally to the mixer. The mixer would output a frequency which is the difference of the two input frequencies, and since this difference would be small we could measure it easily on my 200MHz scope.

Hope this is the right forum! If not I'm sorry :(

Last edited by a moderator: Sep 25, 2014
2. Jul 26, 2014

### TheBestCake

Is there any way I can get this viewable on the Physics forum as well?

3. Jul 28, 2014

### TheBestCake

I've been searching all over the place for more information but no luck. I'll just bump this post and hope for the best.

4. Jul 28, 2014

### TheBestCake

I only just now realized that in this video () the frequency he measured there didn't go through the microwave frequency counter but was just split to it. The way he was able to measure a that frequency was because he was measuring the beat frequency. Quite obvious now.

Last edited by a moderator: Sep 25, 2014
5. Jul 28, 2014

### davenn

he was doing both, using the counter and doing an interferometer method
there are several ways to do this one of the ways is the Michelson interferometer

here's an interesting article showing other methods ....

http://www.yorku.ca/marko/PHYS2212/Lab6

just as an interesting side note. I had a giggle when I watched that video
as I have one of those HP frequency counters in my shack. It goes up to a bit over 26GHz
The highest 2 microwave bands I work on are 10 GHz and 24 GHz

That HP counter is a nice bit of kit, from memory I paid ~ AU$550 ( ~ US$500) for it
so not too expensive.

I have FM modulated a Gunn Oscillator like you see there in the video ( and I assume you have )
and communicated up to 20km straight out of the horn and over 50km when used in front of a 1 metre diameter dish.
I finally moved from Gunn Oscillators, as they are not overly stable, to synthesiser controlled transceivers putting out ~ 2.5 Watts and getting out to >150 km

At the top of this page of my www site, you can see several of my 10GHz Gunn Oscillators etc
http://www.sydneystormcity.com/10ghz.htm

all good fun

Regards
Dave

6. Jul 28, 2014

### TheBestCake

Thanks so much! You helped greatly. I want to first make sure I need to buy equipment before I do so. Any idea how they hooked up their detector here? I believe it's the same one I own.

Last edited by a moderator: Sep 25, 2014
7. Jul 28, 2014

### Baluncore

8. Jul 29, 2014

### davenn

They work great with VHF and UHF

years ago I built a cavity wavemeter that went from ~ 400 to 1500MHz
cant find a pic of it on the www unfortunately
I first saw it published in a RSGB handbook

Dave

9. Jul 31, 2014

### davenn

The detector is just a microwave mixer diode in a cavity
for 10GHz it will be a 1N34 or similar

the output of the diode can go straight to a voltmeter so you can measure peaks and troughs in the signal level as you move the detector ( receiver) cavity
When you measure from peak to peak, you are measuring 1 wavelength which at 10.5GHz is
2.86 cm. If you are careful with your measurements, you should be able to get close to that result

Dave

Last edited by a moderator: Sep 25, 2014