Integrated circuits
What is meant by the term MMIC?
Monolithic Microwave Integrated Circuit.
A complete microwave amplifier or mixer on one chip, typically in a tiny package with 50-ohm input and output. MMICs are why microwave amateur work became approachable — a stage that once needed careful hand tuning is now a part you solder in.
Which of the following is an advantage of CMOS integrated circuits compared to TTL integrated circuits?
Low power consumption.
CMOS draws current essentially only when switching. That is why it dominates everything battery-powered, and why TTL survives only in legacy designs.
What kind of device is an integrated circuit operational amplifier?
Analog.
An op-amp is the archetypal analogue IC: a high-gain differential amplifier whose behaviour is set by external feedback components. Worth holding against MMIC and CMOS as the analogue member of the set.
Ferrites
What determines the performance of a ferrite core at different frequencies?
The composition, or “mix,” of materials used.
Ferrite is not one material. Each mix is optimised for a frequency range, and using the wrong one is a common and invisible mistake — a mix intended for 100 MHz does very little at 3.5 MHz. Mix 31 for HF common-mode chokes, mix 43 for VHF; the numbers are worth knowing because the cores are otherwise indistinguishable.
How does a ferrite bead or core reduce common-mode RF current on the shield of a coaxial cable?
By creating an impedance in the current’s path.
Impedance, not absorption. The choke makes the path unattractive to common-mode current, so less flows. It does nothing to the differential signal inside the coax, because that current’s outbound and return paths are equal and opposite and the ferrite sees no net field.
What is an advantage of using a ferrite core toroidal inductor?
All these choices are correct — large inductance with fewer turns, high Q, and self-shielding.
Self-shielding is the one people undervalue. A toroid’s magnetic field is almost entirely inside the core, so a toroidal inductor neither radiates into its neighbours nor picks up from them. That is why they are used in tight RF layouts where a solenoid coil would need a can around it.
RF connectors
The frequency limit is the specification that matters.
| Connector | Limit | Notes |
|---|---|---|
| BNC (50 ohm) | about 4 GHz | bayonet, quick, test-bench standard |
| Type N | useful to 10 GHz | moisture-resistant, threaded |
| SMA | several GHz | small, threaded |
| RCA phono | low frequency or DC | audio and control, not RF |
| PL-259 / UHF | a few hundred MHz | not constant-impedance |
Which of the following describes a type N connector?
A moisture-resistant RF connector useful to 10 GHz.
Type N is the outdoor choice — the weather resistance is designed in rather than achieved with tape.
Which of these connector types is commonly used for low frequency or dc signal connections to a transceiver?
RCA Phono
Cheap, unshielded to any real standard, and entirely adequate for audio and control lines. Not an RF connector, despite appearing on the back of radios.
LEDs, once more
How is an LED biased when emitting light?
Forward biased.
Same as the Technician answer, stated in the bias vocabulary rather than the current vocabulary.
Check yourself
- You fit a ferrite choke on a coax and the interference is unchanged. Name two likely causes.
- Which connector for an outdoor 70 cm feed line junction?
- Why does a toroidal inductor need no shield can?
Answers
- Wrong mix for the frequency, or the problem is differential rather than common-mode — a choke only addresses common-mode current.
- Type N — moisture-resistant and good to 10 GHz.
- A toroid is self-shielding: its magnetic field is contained almost entirely within the core.