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Valery Bespalov
Valery Bespalov

FM Signal Detection The Pulse-Counting Way [Extra Quality]



The description here seems a bit off. You generate fixed length pulses, but as t he incoming signal varies in frequency t he number of pulses in a guven time increase or decrease. An RC network converts these pulses to a varying DC, representing the incoming signal.




FM Signal Detection The Pulse-Counting Way


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For communication to work, both the sender and the receiver must agree on what communication channel to use. After which, the sender encodes the message and transmits it to the receiver. Then, the receiver receives the message and decodes it. This holds true to FM: the transmitted FM signal is received and must be demodulated to take the information. This is what FM detectors do.


The carrier signal frequency is 1kHz, the modulating frequency 100Hz, and the modulation index is 3. Taking note of the modulation index, this makes the peak frequency deviation 300Hz. The frequency will swing between 700 and 1300 Hz. On the other hand, the function of the modulating frequency is to know how fast the cycle is completed.


Open the simulation file. In the circuit, an FM signal with a 1kHz modulating frequency, 5V-20kHz carrier signal, and a modulating index of 5 is fed to the input. The tuned circuit formed by C1 and L1 performs the FM-to-AM conversion and the peak detector formed by D1, R2, and C2 extract the information from the AM envelope. Run the simulation file and observe the waveforms.


A Pulse-Averaging discriminator uses a zero-crossing detector, a one shot multivibrator and a low-pass filter to recover the original modulating signal. Figure 10 shows a block diagram of the pulse-averaging discriminator.


The phase-locked loop (PLL) is a frequency- or phase-sensitive feedback control circuit. All PLLs have the three basic elements: Phase detector, low-pass filter, and voltage-controlled oscillator. Phase-locked loops are used in frequency demodulation, frequency synthesizers, and various filtering and signal detection applications. Figure 17 shows the block diagram of a PLL.


The phase-locked loop used as an FM demodulator, though the operation of a PLL is involved, is probably the simplest and easiest to understand. The ability of a phase-locked loop to provide frequency selectivity and filtering gives it a signal-to noise ratio superior to that of any other type of FM detector. For a more in-depth study of its operation, check the Phase-Locked Loop Laboratory Activity.


An RF Jammer is a static, mobile, or handheld device which transmits a large amount of RF energy towards the drone, masking the controller signal. This results in one of four scenarios, depending on the drone:


16. The counter according to claim 13, wherein the number of electronic pulses is summed and above a predetermined sum the processing device triggers a radio transmit signal in the radio transmit device, by means of which the number of pulses is transmitted.


31. The counting method according to claim 28, wherein the number of electronic pulses is summed and above a predetermined sum the processing device triggers a radio transmit signal in the radio transmit device, with which the number of pulses is transmitted.


Months ago when I was working in Cronulla I had plenty of time on the daily rail commute to design a pulse-integration circuit in LT Spice. At one point I threw the result together on a solderless breadboard and tested it with the signal generator - it worked well, but was far from a complete WBFM receiver. I forgot the idea for a while and built the Fremodyne instead. Last weekend I decided to revisit the idea. This radio receiver is the result.


What is pulse-counting FM detection? Basically you generate a constant-width (narrow) pulse for each cycle of a signal (say at the zero crossing) and integrate the pulses to produce a "DC" voltage directly proportional to the frequency of the signal. This is the familiar "tachometer" circuit used in many places. Changes in frequency produce corresponding changes in the voltage stored in the integrator capacitor - essentially an ideal FM discriminator. We don't care about the amplitude of the signal only its frequency so the IF can be class-C and is ideally driven into limiting to reject amplitude-related noise.


This radio uses a low-IF with a single conversion front-end. FM broadcast uses +/- 75 kHz peak deviation, so the IF needs to be at least 100 kHz or so and have a bandwidth of about 200 kHz. Channel spacing is 200 kHz too, which presents a bit of a problem, but in practice even a roughly designed IF/Limiter will perform fairly well. The FM multiplex signal has energy to at least 53 kHz so if you want to recover it you need to be a bit more careful about the IF system. This radio is not so careful and uses a very brute-force topology.


This simplistic design means there are two "optimal" local oscillator tuning spots per channel (+/- the IF relative to the carrier). In between the LO mixes down and/or aliases the IF into the audible range which is undesirable (sounds horrible). Ideally the IF amplifier only amplifies signals above "baseband" up to the peak IF frequency and the detector is followed by a filter that rejects the IF. This radio does the later with a simple Sallen Key filter, but the former isn't really implemented beyond the inter-stage coupling of the IF stages which roll off its gain towards lower frequencies. None the less the result is quite acceptable at the expense of a poorer signal to noise ratio at baseband where a bit of "out of band IF" leaks through the detector on weak signals. Once in full limiting it isn't really a problem as only intermodulation can mix IF energy down into baseband.


The front-end is a simplistic single-JFET mixer with a tuned-loop antenna on the gate. A Hartley JFET oscillator implements the LO, and injects into the low-Z source of the mixer JFET from a tap on the LO tank quite close to the cold end. The IF is extracted from the drain of the mixer. A choke rejects the higher frequency bits of the mess at the drain, but the IF amplifier does the bulk of the band-limiting - an area that probably needs improvement. The front-end is not particularly sensitive, but I live in a high-signal area so this is of little consequence. The pre-selection tunes sharply which is actually a bit annoying usability wise. Also severely detuned the pre-selector lets you tune the mixer to powerful out-of-band signals, this clobbers the signal of interest at the IF. In a better design the pre-selector would be track-tuned with the LO.


The IF limiter is just three cascaded "minimal" common-emitter BJT amplifiers. It is works well enough, but I am not very happy with the design. The pulse-generator was originally designed to be biased slightly on, but I encountered instability problems doing so. It is now "auto-threshold biasing" class-C which decreases the sensitivity to non-limiting IF signals but it behaves... The preceding IF amplifier is largely to blame. In the absence of an input signal it can oscillate. This is a hack. The topology of the pulse-generator may look a little weird with the grounded base, but it is easy to understand if you simulate it in LT Spice - basically the emitter gets pulled below ground by the 39 pF cap on the edge of the previous stage's collector signal. This produces a very well defined narrow spike of collector conduction, pulling down the collector voltage proportional to the frequency of the IF signal.


The radio was first built on a solderless breadboard as just the mixer, IF and pulse integrator, using my signal generator as the LO. In this form is was actually more forgiving than once built on a PCB. Most of the development time was spent keeping it stable on the PCB through addition of RF-hygiene. The filtering of the IF rail supply could be better, 100 nF is probably too small. In hind-site, the lack of reasonable filtering down to audio frequencies at the mixer drain is probably a major problem...


The receiver sounds good enough to actually use for listening to ABC Classic FM (92.9 MHz here in Sydney). The response is quite flat and uncoloured despite little attempt being made to implement de-emphasis properly. The LF response is excellent. The signal to noise ratio could be improved a bit, you don't notice this on most channels, only in the quiet passages of classical content can you notice the noise floor. On a spectrogram you can see the 19 kHz pilot signal of the stereo MPX (it is about 30 dB down once it makes it though the filtering), with a bit of effort it might be possible to build a stereo version...


The loop antenna makes the receiver a bit fragile for my liking. A solenoidal inductor could replace it at some loss of capture aperture (see breadboard version photo - worked fine). The loop antenna interacts with the headphone lead a bit (the headphones act as a parasitic antenna too and not always for better signal as the phase relationship between them is uncontrolled). One might wish to add chokes to the headphone line and tap off the RF it collects for injection into the mixer instead of the using the loop antenna.


Some flow measurement methods may generate analog signals that must be converted to digital signals before being used by a Micro-controller unit (MCU). A fast flow generates pulses/waveform of higher frequency while a slow flow generates low frequency pulses/waveforms.


A basic Hall Effect sensor comprises a Hall Element which is simply a magnetic field sensor. The Hall element is connected to a signal conditioning block to make the output usable for the application. These sensors are placed near a magnet on a rotating disc in an arrangement similar to GMR sensors. Figures 5 and 6 show the basic Hall Effect sensor and placement of magnets and sensor on a PCB.


The high-resolution LED bargraph displays are easy to read, and a "floating dot" program peak marker eliminates any ambiguity in the total-mod measurement. Off-air readings are qualified by Inovonics' exclusive multipath indicator, which also aids in antenna alignment during initial station build-outs. In addition, readouts of signal strength and synchronous AM noise qualify the incoming signal and validate measurements.


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