varicaps?

Zeners tend to have a LOT of capacitance. I only want a tiny bit.

I am finding varicaps available. I just got concerned when some of our stock parts went EOL, and some distributors had no stock on others, and coincidentally the Skyworks web site was down and I couldn't see any of their data sheets.

I was partial to a dual, but it should work with a single, which are more common. Nice thread on varicaps, a change from you-know-what.

Ah yes of course, we had fun last year disecting the AD310 parametric input op-amp which used back to back transdiode (CB-E) bjts as varactors.

piglet

I expect that negative base bias would modulate Cce, which makes a

Ahh, parametric amps... be fun to build one. Is there some easy way to make a (demonstration) para-amp from the really crappy ceramic caps?

George H.

Am 02.04.20 um 18:42 schrieb snipped-for-privacy@highlandsniptechnology.com:

This one? I have 10 pcs. here, but never found the time to play with them.

cheers, Gerhard

(Oh, yes, I use the SR620!)

But you need an isolation capacitor anyway, so the series NPO is required. The series pair ( C2 *C2/(C1 + C2) ) is going to have a low-slope characteristic regardless. Why worry, if you only need a fraction of a pF as your delta? A main issue will be stray/PCB capacitance.

Varicap diode paramps (like the AD310 we enjoyed last year) have pF scale capacitance and were driven in 00s kHz or lo-MHz but since the ceramic caps with large voltage coefficient are uF range I like to think one could get away with power line frequencies. Which opens up the delightful steampunk idea I sketched here...

Of course if the input varactor caps aren't closely matched the offset voltage could be of the order of volts but still the DC input bias current is near zero! And 60Hz pump frequency rather limits GBW and output slew rate!

piglet

I currently have a dual common-anode varicap on the test layout. One end is ground, one is the LC, and I'm applying the bias to the common anode through a resistor. That makes for a fast control loop.

Our loop bandwidth will be around 250 KHz, and the frequency about 150 MHz, so an RC between the DAC and a single-ended varicap won't be a problem.

I'm going to experiment with an active layer 2 guard to reduce the effective PCB capacitance, specifically the horrible TC of FR4.

I see two problems: The input signal makes the capacitors change in the same direction, so do not unbalance the bridge. The diodes in the demodulator should be a ring quad, of course, not a rectifier bridge.

This needs a bit more work.

Jeroen Belleman

AD310K

Thanks Jeroen, yes I thought I got the ring modulator wrong. Might have to rethink the front end! Idiot first pass solution might be to have one capacitor fixed (loses all cancellation properties of identical parts though).

piglet

Would a 1SV280 work, or am I misunderstanding your needs?

That changes about half a pF from 8 to 15 volts. Not bad, but a bit more than I'd like. Skyworks SMV1248 might be better.

Why do people make 15 volt varicaps and only give cv data up to 8 volts?

'Oh dear, piglet you're making my brain spin.' said pooh. I have trouble with all these transformer circuits, doing modulation / demod (at 1/2 freq?) / and low pass detection.. Is that six diode bridge thing doing double duty? No forget that part... let me ask about the front end.

So let's assume we have balanced the (vari)caps so at zero V-in each C is the same. Then each 60 Hz 1/2 cycle there is the some current flowing... I guess we'll want the caps to be the major impedance limiting the current... then with voltage and unbalanced C's there are different currents in each 1/2 cycle. And that looks like some 2*f (120 Hz) 'modulation' that proportional to V-in... is that close to right?

I must admit I never know exactly how to think about a transformer.

George H.

High voltage rectifier diodes

The manufacturer has a software patch for the DSP that adds support for SSB, as well.

CH

I would not want that radio in my car. I listen to AM most of the time when driving, and not just then.

Well, they don't go that far but everything is PLL or DDS nowadays. There simply isn't a need any longer for large capacitance varicaps to run the local oscillator. As for the preselector (adjustable filter in front of the mixer), that has fallen victim to the red pencil of the bean counter, which has cost performance when in the vicinity of other strong signals.

-------------------------------

Are you being tedious or the sake of it again?

There was no such possibility with broadcast AM radio.

Why are you being such an ass?

.... Phil