Andrew Holme wrote:
I'm designing a 40-70 MHz synthesizer using the Mini-Circuits POS-75 VCO.
The VCO output varies from 9 dBm at 40 MHz down to 7 dBm at 70 MHz. My
circuit needs to provide isolation, tolerate open and short-circuit loads,
and deliver 15 dBm into 50 ohms across the tuning range. What do folks
think of the solution below? In particular, what about C2? This "hack"
seems to compensate for the input level variation; but I don't know if it
will work for real. This is for very low volume production and C2 could be
set-on-test.


Well, you're running 2.6V/100 ohms or 26 mA through that poor
transformer, which may not like it much...and the low-frequency input
impedance of the transistor is about 1 ohm, plus whatever the extrinsic
emitter resistance of a 2N3904 is. At 70 MHz, the capacitor's reactance
is about 15 ohms, so it won't do a whole lot on its own if the input
resistance is really low.

On the other hand, since the beta of the transistor is dropping like a
rock in the low VHF, the base resistance will be making the emitter
inductive-looking, so you might be parallel-resonating the emitter
inductance.

I'd be wanting to use a better circuit for this, myself--something along
the lines of a faster transistor running common-emitter, with a lead-lag
emitter degeneration network (i.e. R in parallel with a series RC).

Cheers,

Phil Hobbs