Cable Tracer - Receiver

Hey, I just got a US technician class ham radio license about a week ago and even made my first transmission with a handheld radio just a couple days ago on the 2 meter band using FM modulation. I thought you might be interested to hear since you like radio stuff. What cool ideas do you have for me to do with my brand new license? I bet there’s tons of fun stuff to be done.

Again, interesting. Are you going to add circuitry for the pulse and the 4.5V?

Hey Robert. The pulse generator is actually the sender circuit, wired to the cables. The 4.5V is a voltage divider from the 9V battery.

@jason9 that's cool 😎 I remember my first FM project. I bought a low power PLL modulator from Aliexpress and turned it into 7W with a class C amp. It was fun playing around with LC matching in RF circuits. I can give you tips on playing around with the transmitter circuits, but for the licensing it really depends on the country and the frequency range.

That’s neat. How is the VCO in these PLLs made? I’ve always been interested in how VCOs work but they all seem to either not work in EC or require variable capacitance diodes which don’t exist in EC.

Yeah, they use variacs indeed for the PLL. Can't simulate this in EC. A simple VCO can be simulated with a multivibrator with its supply rail modulated. The primary effect would be Amplitude modulation, but due to the VCO nature of the multivibrator, you'll get parasitic frequency modulation as well

Do you know of any commercial PLLs that use that type of FM (like with an inverter chain since MOSFETs switch faster at higher voltages) or do they all use variacs as far as you know? If so, do you know why variacs are the preferred method?

It's not about switching with PLL. The reverse voltage on actually every diode leads to change in the capacitance of the diode. When you add a diode in the LC tank circuit and apply some DC to it, you can change the capacitance of the diode and therefore the LC resonant frequency based on the DC applied. The way the whole thing works is it takes the output frequency of the oscillator (let's say 100MHZ), divides it by a given denumerator (let's say 128 times), which ends you with around 780kHz at output of the divider. You can then compare this frequency (or more importantly phases) with a reference wave from a much more stable oscillator (such as a quartz resonator) with a phase detector and work out the difference between them. The difference is usually pulsed, so you need to run it through an RC filter to only leave the DC component of the difference. This DC component can be fed back to the diode in the LC circuit to change its capacitance accordingly so that the oscillator will match the PLL frequency. Adding the modulating signal in this DC path will frequency modulate the output of the oscillator. The benefits from PLL is that due to using a stable reference frequency, you will be able to stabilize the much higher frequency from variations.

Right. What I was asking is if all PLLs use diodes like that or if some use other types of VCOs such as the multivibrator example you gave or an inverter chain oscillator. Since MOSFETs switch faster at higher voltages the inverter chain will oscillate faster at higher supply voltages without any need for variable capacitance diodes. A PLL could be constructed from this. So, what I’m asking is if any PLLs are constructed like this.

Hmm, not sure to be honest. Analog PLL's don't need MOSFETs most of the time, as they are just plain old Colpitts or Hartley oscillators, but I guess other applications may use MOS