RF Siggen Franklin VFO

[rohitkhanna]

While planning a simple lC based Siggen - hopefully working upto 100MHz & down to 0.5Mhz, and allowing use of inductance switching & varactors - I was examining the Franklin dual active device topology. Frequency stability by use of Huff'n'puff or similar, uC control for everything else. Oh - and an AGC system for start-up and amplitude management.

The Franklin seems to be an excellent candidate, simulations show promise, active elements can be standard ones like J310/ 2N4416/ S9018/ etc ....... but what are the potential down-sides and why is it not that popular ?

Just wanted the collectives wisdom before I got trapped into prototyping a possible dead-end.

cheers!

 

[sky_123]

You were missing a link in your post. I was planning to build an RF siggen too, and I was planning to use many hartley oscillators (if you are winding your own coils anyway, then it is not expensive), and space for a future PLL. However, you need to build a lot of oscillators to cover a reasonable spectrum (I wanted to go to 500MHz). And, lots of filters. So it becomes more than a small project : ( As an idea, you could design a base board and build your oscillators and filters on small 'plug-in' vertical mount boards, so that you can build just a few oscillators initially, in a spectrum of interest, and
gradually expand it. It would also help with tweaking each oscillator, or if you wanted to change oscillator topology per band.
In the end, I gave up, because it just sounded too large a project, but if you can manage it, I think it would be awesome.
Alternatively you could use a DDS, but depending on your use-case, you may need lots of filters for that too.

 

[rohitkhanna]

You were missing a link in your post.

which link do you mean ?!

I was planning to build an RF siggen too, and I was planning to use many hartley oscillators (if you are winding your own coils anyway, then it is not expensive), and space for a future PLL. However, you need to build a lot of oscillators to cover a reasonable spectrum (I wanted to go to 500MHz). And, lots of filters. So it becomes more than a small project : ( As an idea, you could design a base board and build your oscillators and filters on small 'plug-in' vertical mount boards, so that you can build just a few oscillators initially, in a spectrum of interest, and
gradually expand it. It would also help with tweaking each oscillator, or if you wanted to change oscillator topology per band.
In the end, I gave up, because it just sounded too large a project, but if you can manage it, I think it would be awesome.

I played with the other topologies too - but Hartley requires switching of TWO points, Colpitts needs the feedback caps to be ALSO changed, etc etc. Thats why the Franklin topology looked so good, with a parallel LC to ground, not the series LC. Switching the L with PIN diodes under uC control I could possibly get 100MHz with 4 coils. I think. Virtually no filters needed - though I think the harmonics are down only around 40db. Anyhow I wanted to make it all automatic & supposedly seemless.

Alternatively you could use a DDS, but depending on your use-case, you may need lots of filters for that too.

I'm specifically staying away from DDS on this one - just to see where it gets me. And also because there is potential to go upto 500MHz & maybe beyond, with the exact same topology though plenty of care in the design/ layout etc. The L and C start becoming really small at those frequencies !!

The DDS would be (supposedly) much much simpler with all its intrinsic capabilities of sub-hertz resolution, but where's the fun in that ? And going to more than around 100Mhz the chips start to become a bit expensive anyhow. Not to mention the complexity of the very stable & even higher frequency clock source required. And you DO need some carefully made filters - especially if you want to push it to the limits.

Though I did put together a slightly improved version of Jespers (poor mans DDS) method and got a decent 0.125Hz -- 700KHz DDS based on AtMega48 + some old DAC chips i had lying around. Usual LCD & keypad controls for freq/ amplitude/ offset.

- - - Updated - - -

My point of concern is .... why does the Franklin oscillator topology look so good to me, and why do I not see it used more often ? Whats the downside... what am i missing ?

 

[sky_123]

This Franklin

Oh, by 'this' I thought you were intending to show a link to a particular circuit. Unfortunately I've never heard of a Franklin VFO before.
It is missing in my textbooks! (But I don't have that many, so maybe it is well known to others).
40dB is extremely good without filters, however.

 

[rohitkhanna]

Oh, by 'this' I thought you were intending to show a link ....

Oh... LoL !! OK. Corrected that.

And the ~40dB is a simulation analysis... not a practical implementation. Still have to do that. Today maybe ?