Quote:
Originally Posted by regenfreak
... I made a variable tuned MMIC amp using a 7-gang air gang variable capacitor. It is too lossy and difficult to tune to my liking. After all, it seems not a good idea. When it comes to variable tuned MMIC cascade amplifier, it is impossible to obtain the end coupling caps matching 50ohm I/O impedance across the whole FM band. ...
Attachment 6 is my 300 ohms to 50 ohm balun. I tested with 300ohm resistor, I got SWR of 1.4 at FM band. I am working on a new version that will give SWR close to 1 at VHF.
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There is not really a problem to match Hi-Z resonant circuits to any arbitrary low impedance. Look at any application note for any VHF/UHF dual gate MOSFET. A mismatch along FM band (around 3dB or so) is not a problem also (easily solved for purpose of any measurement).
The problems with MMICs, like for any BJTs, are intermodulations and spurs. Diode mixers are not much better in this respect.
... But you might put a MMIC or two in use to boost the dynamic range of the Hi-Z probe we've talked about.
This balun doesn't look good. Low insertion loss requires tight coupling, what means good intimacy of wires involved over the whole length of winding. Think of them as of a micro-transmission line. To wind a good 300 ohms to 50 ohm (6:1) balun is not a trivial task. You would better take a 4:1 one and use ordinary resistive matching.
If you need isolated primary to secondary, 300 Ohm to 75 Ohm balun (or 200 Ohm to 50 Ohm), you would start with 3 pieces of thin enameled copper wire, ~10 cm long, as convenient. Twist them together (about 1 full turn/cm length), then wind through the core few turns (say 6), cut the ends to the required length. Now, the ends of one of the wires will be the 75 ohm side. The other two will be connected in series (take care of phases), common point serving as mid-tap, both other ends as 300 Ohm balanced ends.
You may look at the Figure 2 here:
https://www.minicircuits.com/pdfs/MPD_Transformers.pdf
This is a 4:1 balun wound on a toroidal core.
Note, how the separation of primary versus secondary terminals was solved, whereby all 3 wires pass exactly the same path around the core.
NB. For sake of core losses its volume should be kept as small as possible. High permeability material is advantageous.