RF Buffer (VHF)

[wesmonty]

I would like to buffer [edited here: i had said isolate] the output of a 100 MHz OCXO. It's a low noise device and I would like to use automotive parts so I'm thinking the best way to do this is to come up with a design using BJTs or FETs. Could you suggest any circuit types? Ordinarily I would just use a common collector for a buffer - would this be suitable with matching networks on the input and output?

Cheers.

 

[KlausST]

Hi,

I don´t understand. Neither a BJT nor a FET provides isolation.

What´s your idea.
Maybe a sketch could help.

Klaus

 

[wesmonty]

Hi,

I don´t understand. Neither a BJT nor a FET provides isolation.

What´s your idea.
Maybe a sketch could help.

Klaus

Pardon me, I meant to say buffer and not isolate.

 

[KlausST]

Hi,

I think you have your reason for buffering.

You need to tell us.
At least:
* Waveform
* input voltage
* output votage
* output current
* supply voltage
...

Without these data it´s almost impossible to help.

Klaus

 

[wesmonty]

Thank you. The waveform is 10 dBm sine from the output of a 50 ohm oscillator and I'll be going in to 50 ohm after the buffer. The supply voltage is 8 V.

Cheers.

 

[KlausST]

Hi,

you want a 50 Ohm signal to buffer into a 50 Ohm signal? Why?

Klaus

 

[wesmonty]

The load is reasonably 50 ohm but there could be some slight variation and I'm going to be splitting the signal a number of times so want to isolate the osc output from this.

Cheers.

 

[G4BCH]

If it is a low noise oscillator you will need a low noise buffer, both low noise floor at high signal level and good 1/f noise or you will loose the performance of your oscillator.
In low noise oscillator designs I've used common base bipolar for isolation and low noise. Good devices are getting harder to find now, the best ones went obsolete a few years ago.
To re-phrase KlausST's question, what do you want from this buffer?
You answered while I was composing this reply.
I suggest using a CB amplifier for each output.

 

[KlausST]

Hi,

so you want one source but multiple buffers?

Klaus

 

[BigBoss]

In order to buffer an Oscillator needs to use such circuits;
-Cascode Buffer Amplifier
-Emitter Follower
-Differential Pair then Emitter Follower

But if your oscillator should see a 50 Ohm Load Impedance, Much Higher Input Impedance of those Circuits will impact the Oscillator.However you can use a simple Emitter Follower ( Common Collector ) will serve you well.
If you don't become happy with it, you can also try Cascode Configuration. ( more complicated to design )

 

[volker@muehlhaus]

I don´t understand. Neither a BJT nor a FET provides isolation.

LOL

This is the RF forum section, and for RF engineers the term "isolation" is clear and does not mean DC isolation. Isolation in this context refers to low S12, to reduce the effect of load variations on the source.

 

[KlausST]

Hi,

Thank you Volker.
I really didn't know that phrase is use for this. Needless to say I'm not experienced with HF design.

Klaus

 

[wesmonty]

Thanks for the responses.

My only experience of RF amplifiers before this is some power amplifier design. For the circuit configurations mentioned, assuming 50 ohm source and load, would they require matching networks like a power amplifier does?

Cheers.

 

[danadakk]

In general your concern is stability -

**broken link removed**


Regards, Dana.

 

[BigBoss]

Thanks for the responses.

My only experience of RF amplifiers before this is some power amplifier design. For the circuit configurations mentioned, assuming 50 ohm source and load, would they require matching networks like a power amplifier does?

Cheers.

Yes and No..
The Buffer which you intend to place between Load and Oscillator will already have very high Input Impedance so Matching won't eventually be realizable at all.Direct Connection and 50 Ohm Resistor will solve your problem.But a Matching Circuit may be necessary to match between Load And Buffer Output, it's relatively easy.

 

[danadakk]

Good ref material -

https://ia600405.us.archive.org/2/items/fea_QRP_Classics_-_The_Best_QRP_Projects_from_QST_and_the_ARRL_Handbook/QRP%20Classics%20-%20The%20Best%20QRP%20Projects%20from%20QST%20and%20the%20ARRL%20Handbook.pdf

Also archive.org, search on "ARRL Handbook", lots of design info in
various chapters of these handbooks.


Regards, Dana.

 

[biff44]

i would just buy one of these and use it.

https://www.nxp.com/docs/en/fact-sheet/MBC13916FACT.pdf

42 db reverse isolation.

if you need more than that, and phase noise requirement is not too tight, you could use an electro-optical device, like a Laser diode to a photodetector, with a short fiberoptic cable in between. Should be able to get 130 dB or so of reverse isolation with not too much effort.

 

[mtwieg]

The load is reasonably 50 ohm but there could be some slight variation and I'm going to be splitting the signal a number of times so want to isolate the osc output from this.

Cheers.

By "splitting" the signal, you mean with a power splitter circuit (wilkinson splitter, etc)? Or do you need buffers for each load? How much isolation do you need from one load to another? Are the loads connected by PCB traces, or cables?