Electronics > RF, Microwave, Ham Radio

Calculable Amplitude Colpitts Oscillator (?) - Elecraft XG2

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quantumvolt:
This 50 (and attenuated to 1) microvolt signal generator is probably intended for calibration of S-meters. On page 8 in:

www.elecraft.com/.../E740084%20XG2%20Manual%20Rev%20E.pdf it reads:

"Output accuracy is determined by the precision voltage regulator and 1% resistors."

To me the circuit seems to be just an (emitter feedback stabilized operating point) Colpitts Crystal Oscillator with a precision supply voltage and a very low current and output amplitude (collector tapped) attenuated and presented with 50 ohm output impedance.

My question is - keeping in mind the formulas for Emitter Resistor DC-Biasing - will not the actual transistor's Vbe and Hfe influence the collector operating point current and also (?) the RF-signal amplitude?

EDIT: Can't get the link over to work. A description and the manual is linked in http://www.elecraft.com/mini_module_kits/mini_modules.htm . It also says (new to me - quite a claim ...):

"In addition to receiver testing, the XG2 can be used as a reference to calibrate other lab instruments."

Whaddayathink?

T3sl4co1l:
If it's running with a high enough feedback ratio and load impedance, the amplitude will be limited by Vce(sat), which has little tempco.

If it's running with a lower load impedance, it might be limited by Vbe instead, which would be a bit of a problem.

It doesn't strike me as a precision device, not one that can be expected to hold constant with respect to temperature.

Tim

quantumvolt:
Thanks. I've been thinking about that: If the oscillation occurs from transistor cut-off to saturation, then the math works out so so ignoring Vce-sat.

But can one expect 1-3 (or 5) % accuracy? Does anyone know of other HF-frequency oscillators that has a predictable calculable amplitude?

uncle_bob:

--- Quote from: quantumvolt on June 26, 2016, 10:22:55 pm ---Thanks. I've been thinking about that: If the oscillation occurs from transistor cut-off to saturation, then the math works out so so ignoring Vce-sat.

But can one expect 1-3 (or 5) % accuracy? Does anyone know of other HF-frequency oscillators that has a predictable calculable amplitude?

--- End quote ---

Hi

A standard generic cheap CMOS output oscillator will give you a specific output frequency. If you put it into a divide by 2 (D flip flop) the duty cycle will be pretty close to 50/50. It also should be fairly predictable from the data sheets on the divider. Run that into a buffer ('125) that runs off of a precision supply. The logic high and low should be quite close to the supply and ground. It's not (yet) 50 ohms, but you have a very predictable peak to peak output.

Next step is to run an L pad to get it to 50 ohms. Exactly what you run depends a bit on the logic you are running and the frequency. Something like a 470 ohm combined with a 56 ohm is s good start. You now have a pretty good 50 ohm source and a known p-p signal. A check of the relative amplitudes at the fundamental, second, and third harmonics will give you a pretty good idea how well the divider is doing. Since it's a square wave, you have levels that should be useful out to at least the 5th and possibly the 7th harmonic.

Bob

quantumvolt:
Thank you. Yes, a rail to rail square signal has crossed my thought. But my primary goal is to see what I can do with analog RF (only HF) circuits first.

I rigged up an LC oscillator at around 300kHz. I managed to get a decent correlation over a wide voltage range (LT 10mV to GT 700mV) between (the visual scope and) the AC Voltage function of an HP 34401A and a cheap eBay AD8307 based RF power meter that also displays mV - like the one here http://www.eham.net/ehamforum/smf/index.php?topic=103001.0.

So I will go to 1Mhz / 10MHz crystal oscillators first and check the power meter against the scope. Then I will try to build a precision rectifier front end for the 34401.

If I can get a precision oscillator working at around 0dBm / 223 mV RMS in 50 ohm, I should be able to use the instruments I have. Then I can step down towards microvolts by standard dB attenators.

RF millivoltmeters are few and IMO overpriced on eBay. So I will try this way first.

But I might go for the square wave also. I might learn more about filters and other stuff.

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