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3rd February 2009 08:02
Hi Al... I tripped over your site looking for "Post Marker Injection" and how to do it. Nothing found so far. However, I just finished building a simple circuit so I could use the Marker in my old Wavetek. Since the Wavetek I have is from about 25Mhz and up, and no good for IF alignment and experimenting with filters etc. I fed a 25Mhz self contained IC into a SLB-1 and fed the Wavetek into the other port of the SLB-1. That all comes out into a 25Mhz LP filter I feed to the filter under test. It works and the marker can be an external one fed into the back of the Wavetek... So far, I can see the marker but it is too wide to work with. However, the 2Mhz filter can be sweept and looks good with this combo. regards Jeff
5th December 2008 11:40
Arv,
I haven't tried modelling that kind of VGA before, but I've seen the topology, I think either in EMRFD and/or on the popcorn site... Seems quite logical, use the MOSFET as a variable degeneration resistance.
I do have a good spice model of a very simple ALC system for narrow-band use, but it is class-C with collector modulation, so it can't operate over an octave without switched and rather aggressive low-pass filters. Basically the detected DC feeds a NPN transistor base who's collector load feeds directly into the VGA control port. The collector voltage drops with more base current from the detected signal so the VGA gain is reduced. As long as the gain and time constants are reasonable the loop works great in a transient simulation in spice, stabilising in about 50 us - it is slightly under-damped but that isn't a big deal or too hard to fix.
The same general approach would work for the VGA I've got working, I just need to draw it up in spice (or build it). The low-distortion VGA has the advantage that it doesn't mess up the waveform and generate all kinds of harmonic energy, so the filtering at the output can be less critical. I don't want to use an IC VGA myself, as you know I like to build with discrete transistors when they are sufficient - even to the point of avoiding op-amps for the ALC loop amplifier.
I toyed with the idea of using a PIN attenuator, but I've largely given up on that for now. Might come back to that later, a PIN quad IC and a few MMICs would be a clean and easy way to achieve a levelling loop at higher frequencies. Over the next few days I'll post all the spice files of the various VGA approaches I've fiddling with in silico. They are all useful in their own right, so as building blocks for other applications they would be handy to have documented. I'll have to try the MOSFET degeneration approach too.
Regards,
Alan
5th December 2008 04:34
I wonder if your variable gain amplifier and related ALC control might be just a BJT amp with the emitter bypass cap in series with a MOSFET. By providing some rectified RF as DC feedback to the MOSFET gate you might be able to get enough gain control to meet your needs. If workable, this could be a more reproducable approach than using some possibly essoteric variable gain wideband RF IC.
Arv
_._
4th December 2008 15:51
Arv,
Yes indeed. Once I finish it I'll write it up fully with circuit diagrams, etc. It is work-in-progress at the moment. The ALC loop in particular.
The usual disclaimer applies: So many projects, so little time! Although I've been getting better about finishing stuff off to completion of late. This device in particular is important for several other projects (believe it or not, but I drew a graph of the dependencies between my various projects to work out what to attack next) so there is quite a bit of incentive for me to get it working ASAP.
One thing I am kinda confused about is how to box it up. The individual modules would be handy to have as removable units, but it would also be nice to have it all in one small box for portability... I guess I can just rebuild the modules - I've made them once after all, another time is easy. So it is a big diecast box, or lots of Altoid tins inside an instrument case... Decisions, decisions.
Regards,
Alan
4th December 2008 14:33
Alan
As usual, an interesting and intriguing article. I assume that this "teaser" introduction will be followed by schematics, and such?
Arv - K7HKL
6th February 2009 13:18
Alan Yates wrote...