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7.5GHz (input) frequency tripler - RF Cafe Forums

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Martin H
Post subject: 7.5GHz (input) frequency tripler Posted: Mon Nov 19, 2007 7:17 pm

Lieutenant


Joined: Fri Dec 01, 2006 12:03 pm
Posts: 4
Hi all,

I'm wanting to design a frequency tripler: input around 7.5GHz; output 22.5GHz. That's all I know so far. It could be active or passive.

I have no experience of such circuits but have been looking round and reading literature.

Can anyone suggest any kind of circuit that would provide this frequency multiplication?

Thanks.


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nubbage
Post subject: Posted: Tue Nov 20, 2007 4:23 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Martin
In the past I have found HP/Avago Application Notes and the MA-Com design notes very useful.
A varactor diode tripler has four sections:
1) input matching section, since a varactor has an impedance magnitude around 5 ohms
2) bias circuit (optional)
3) shunt idler circuit, a series resonant circuit at 15GHz ie. 2*Fin to circulate a high current at 2*F thru the diode
4) output combined filter to select 3*F and a step-up match to the load.
The efficiency of a tripler will be about 30%
I hope that helps a little.


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Martin H
Post subject: Posted: Tue Nov 20, 2007 1:28 pm

Lieutenant


Joined: Fri Dec 01, 2006 12:03 pm
Posts: 4
Thanks for that Nubbage.

Solutions I've been looking into have included varactor diodes. Other solutions I've found in the litarature include class C amplifiers and SRDs (the latter I want to avoid for historical reasons).

A passive solution I've just come across is using anti-parallel diode-pairs. This has been used here in the past so is a much lower-risk solution. My only concern is not the conversion loss, but the absolute output power of the 3rd harmonic and whether it can drive the next stage on my circuit. I'll continue to work to enhance the output power in my simulation, whether it be tuning input/output matches or looking at different diodes. Any advice as to which diode parameters may change/enhance the output power of the 3rd harmonic is greatly welcome.

Martin,
England.


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nubbage
Post subject: Posted: Wed Nov 21, 2007 4:17 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Martin
My experience with step recovery diodes was 35 years ago. We had a lot of problems with instability on start-up, due to device characteristic drift with chip temperature (although we under-ran the devices). We could tune the matching circuit for optimum output spectrum when drive was applied, but ten minutes later the spectrum broke up into something resembling a hedge-hog. The manufacturers eventually came up with elaborate solutions requiring forward bias circuits with a critical temperature coefficient to compensate for the chip temperature drift effects.
Regarding antiparallel diode pairs, I have only read articles on these. My impression was they were mainly suited to even harmonic multipliers. But I may be wrong here.
Class C or even Class D hard-driven MESFETs with a high Q resonator on the output is a promising possible solution. By studying the input-output characteristic curves, or using a software model, it should be possible to estimate the harmonic content without a resonator, for example using Fourier Analysis. Then with a known input to a resonator of known Q it should be possible to compute the transfer efficiency for the third harmonic. Sadly I do not have practical experience with this approach however.




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