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E4C

AMATEUR PRACTICES

Receiver performance: phase noise, noise floor, image rejection, minimum detectable signal (MDS), increasing signal-to-noise ratio and dynamic range, noise figure, reciprocal mixing; selectivity; SDR non-linearity; use of attenuators at low frequencies

E4C011 of 14

What is an effect of excessive phase noise in an SDR receiver's master clock oscillator?

Why An oscillator with phase noise is not a pure tone; it has noise sidebands spread out on both sides of its nominal frequency. In a receiver, a strong signal on a nearby frequency mixes with those sidebands and lands right in the passband as broadband noise or hash, an effect called reciprocal mixing. The result is that the receiver's effective noise floor rises whenever a strong nearby signal is present, masking the weak signal you are trying to hear.

Watch out Frequency calibration depends on the oscillator's long-term accuracy and drift, not its short-term phase noise, and third-order intercept point is set by the non-linearity of mixers and amplifiers, not by clock purity.

Phase noise = reciprocal mixing: a strong neighbor plus a noisy clock puts noise right in your passband.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C022 of 14

Which of the following receiver circuits can be effective in eliminating interference from strong out-of-band signals?

Why Strong signals outside the band you are listening to cause trouble by overloading the RF amplifier and mixer, generating intermodulation products and blocking before any later filtering can help. A front-end filter or preselector sits ahead of those stages and attenuates the offending energy while passing the band of interest, so the first active devices never see it. The rule of thumb is that overload must be cured at the earliest point in the signal chain.

Watch out A narrow IF filter comes after the mixer, so it sharpens selectivity between signals in the passband but cannot undo distortion already created by an out-of-band signal overloading the front end. A notch filter removes a single interfering carrier inside the passband, and a product detector only demodulates SSB and CW.

Out-of-band trouble needs an out-front fix: preselector before the mixer, not IF filtering after it.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C033 of 14

What is the term for the suppression in an FM receiver of one signal by another stronger signal on the same frequency?

Why FM detectors respond to the instantaneous frequency of whatever signal dominates the limiter stage, so when two signals share a channel the stronger one takes over the detector and the weaker one is largely suppressed. This is the capture effect, and only a few dB of difference is usually enough for the stronger station to completely take the channel. It is a property of FM demodulation and is why you rarely hear two FM stations mixed together the way you do with AM or SSB.

Watch out Desensitization is a different problem: a strong signal on a nearby but different frequency drives the receiver front end toward compression and reduces sensitivity to the desired signal. Cross modulation transfers the modulation of a strong signal onto another carrier.

Capture = same frequency, strongest wins. Desense = nearby frequency, everything gets weaker.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C044 of 14

What is the noise figure of a receiver?

Why Noise figure measures how much noise a receiver adds beyond the unavoidable thermal noise floor, expressed in dB. The theoretical minimum is the thermal noise power kTB (about -174 dBm per hertz at 290 K), so a perfect, noiseless receiver would have a noise figure of 0 dB. Equivalently, noise figure is the decibel ratio of the signal-to-noise ratio at the input to the signal-to-noise ratio at the output: a 3 dB noise figure means the receiver degrades SNR by 3 dB.

Watch out The choice comparing receiver noise to atmospheric noise is wrong because the reference is the theoretical thermal noise minimum, not external noise; atmospheric noise varies enormously with band and time and dominates on HF, which is why a very low noise figure buys little below about 20 MHz.

Noise figure = how many dB worse than a perfect receiver. Perfect = 0 dB, reference is thermal kTB, not the atmosphere.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C055 of 14

What does a receiver noise floor of -174 dBm represent?

Why Thermal noise power is P = kTB, where k is Boltzmann's constant and T is absolute temperature. At room temperature (290 K) and a 1 Hz bandwidth, that works out to about -174 dBm, which is the absolute floor a perfect, noiseless receiver would see at its input. Any real receiver adds its own noise on top of this, and the amount it adds is its noise figure. Widening the bandwidth raises the floor by 10 log(B), so 500 Hz of CW bandwidth sits about 27 dB higher, near -147 dBm.

Watch out The choices citing 3 dB or 6 dB above theoretical minimum describe noise figure, the excess noise a real receiver contributes, not the reference floor itself. -174 dBm is the reference point, not an offset from one.

-174 dBm = kTB at 290 K in 1 Hz. Add 10 log(bandwidth) to get the floor in a real filter.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C066 of 14

How much does increasing a receiver's bandwidth from 50 Hz to 1,000 Hz increase the receiver's noise floor?

Why Thermal noise power reaching the detector is proportional to bandwidth, so the noise floor changes by 10 log(B2/B1). Here the ratio is 1000/50 = 20, and 10 log 20 = 13 dB. Note it is 10 log, not 20 log, because we are comparing powers, not voltages.

Watch out The choice of 10 dB would be right only for a 10:1 bandwidth change, such as 100 Hz to 1,000 Hz; 3 dB corresponds to merely doubling the bandwidth.

10 log(bandwidth ratio). 20x wider = 13 dB noisier. Remember 2x=3 dB, 10x=10 dB, so 20x=13 dB.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C077 of 14

What does the MDS of a receiver represent?

Why MDS stands for minimum discernible signal, sometimes called minimum detectable signal. It is essentially the receiver's noise floor: the weakest signal that produces a detectable output above the receiver's own internally generated noise, conventionally the level that raises the output about 3 dB above the noise. It is calculated from the thermal noise floor of -174 dBm/Hz plus the receiver's noise figure plus 10 log of the bandwidth in hertz, so narrower bandwidth and lower noise figure both improve (lower) the MDS.

Watch out The choices about meter sensitivity and modulation distortion are just plausible-sounding expansions of the letters; MDS is a sensitivity specification, not anything to do with an S-meter display or distortion measurement.

MDS = Minimum Discernible Signal = the noise floor. Think -174 dBm/Hz + NF + 10 log(BW).

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C088 of 14

An SDR receiver is overloaded when input signals exceed what level?

Why In a direct-sampling SDR, the analog-to-digital converter can only represent voltages up to its full-scale reference voltage. Once the RF input swings beyond that reference, the converter simply pegs at its highest (or lowest) code and the waveform is clipped, generating harmonics and intermodulation products across the passband. That reference voltage, together with the ADC's bit depth and any front-end gain, sets the receiver's maximum input level, which is why attenuation ahead of the ADC is the usual fix for overload.

Watch out The maximum count value of the converter is the digital output number that results from hitting full scale, not an input signal level you can exceed; the sample rate choices concern Nyquist and alias frequencies, which is a bandwidth issue rather than an amplitude limit.

Overload is about amplitude, not speed: the ADC's reference voltage is the ceiling. Go above Vref and you clip.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C099 of 14

Which of the following choices is a good reason for selecting a high IF for a superheterodyne HF or VHF communications receiver?

Why In a superheterodyne receiver the image frequency sits 2 x IF away from the desired signal. Raising the IF pushes that image further from the wanted frequency, so the relatively broad front-end preselector filter can attenuate it much more easily. That is why many HF and VHF designs up-convert to an IF above the tuning range, for example 45 MHz or 70 MHz, before down-converting to a lower IF for selectivity.

Watch out A high IF actually tends to make narrow-bandwidth crystal filtering harder and does nothing for oscillator drift; drift and tight selectivity are the arguments for a low IF, which is why dual-conversion designs use both.

Image = desired +/- 2 x IF. Bigger IF, farther image, easier to filter out.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C1010 of 14

What is an advantage of having a variety of receiver bandwidths from which to select?

Why Noise power reaching the detector is proportional to the receiver's bandwidth, so every extra hertz of passband beyond what the signal needs adds noise and lets in adjacent signals without adding any wanted energy. Setting the IF or DSP filter to roughly the modulation bandwidth (about 500 Hz or less for CW, 2.4 kHz for SSB, 6 to 15 kHz for FM) gives the best signal-to-noise ratio and rejects nearby QRM. Too narrow a filter, on the other hand, clips sidebands and distorts the signal, which is why a selection of bandwidths is useful.

Watch out Power consumption is irrelevant here, and noise figure is a property of the RF front end that filter selection does not change; likewise, a wide filter does not let you demodulate several signals at once in a conventional receiver.

Noise power rises with bandwidth: match the filter to the mode (CW 500 Hz, SSB 2.4 kHz, FM 6-15 kHz).

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C1111 of 14

Why does input attenuation reduce receiver overload on the lower frequency HF bands with little or no impact on signal-to-noise ratio?

Why On 160, 80 and 40 meters the noise you hear is external: atmospheric static, lightning, and man-made noise arriving through the antenna. That external noise is typically far stronger than the receiver's own internally generated noise, so inserting 10 or 20 dB of attenuation knocks down signal and band noise together and the signal-to-noise ratio stays essentially the same, while the reduced total power keeps the front end out of compression and away from intermod. Attenuation only starts hurting SNR when the band noise drops close to the receiver's own noise floor, which is typically the case only at VHF and above or on very quiet high bands.

Watch out The idea that signals are attenuated separately from the noise is wrong because a pad is a simple broadband resistive network, it cannot distinguish signal from noise and reduces both by the same number of dB. There is no filter of any kind in a plain attenuator.

Low HF: the band is noisier than the radio, so pad both down equally and SNR doesn't change, but overload does.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C1212 of 14

How does a narrow-band roofing filter affect receiver performance?

Why A roofing filter sits right after the first mixer, at the first IF, so it narrows the signal bandwidth before the later IF amplifiers, AGC detector and product detector ever see it. Strong signals a few kHz away from your frequency get knocked down there instead of overloading the downstream stages, which is exactly what blocking (and close-in intermodulation) dynamic range measures. The narrower the roofing filter, the better the receiver holds up on a crowded band during contests or pileups.

Watch out The sensitivity claim is the common trap: the roofing filter comes after the front end and mixer, so it cannot reduce front-end noise or improve noise figure, and its insertion loss slightly hurts it. The low-Q claim is backwards too, since narrow filters are high-Q and ringing is a side effect to be managed, not a benefit.

Roofing filter = the roof over the IF: it keeps loud neighbors out, improving blocking dynamic range, not sensitivity.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C1313 of 14

What is reciprocal mixing?

Why A receiver's local oscillator is never a pure single frequency; its output carries phase noise skirts on either side of the carrier. When a strong signal appears near the frequency you are listening to, that signal mixes with the LO's noise sidebands and the noise gets translated right into the IF passband, raising the noise floor and masking the weak signal you want. This is called reciprocal mixing because the roles are reversed: the unwanted signal mixes with LO noise rather than the LO carrier. A cleaner, lower phase noise oscillator is the cure, since no amount of IF filtering can remove noise that is already inside the passband.

Watch out Two out-of-band signals combining to make an in-band product describes intermodulation distortion, a non-linearity problem in the mixer or front end, not a phase noise problem in the oscillator.

Reciprocal mixing = LO phase noise plus a strong neighbor. Fix the oscillator, not the filter.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4C1414 of 14

What is the purpose of the receiver IF Shift control?

Why IF Shift moves the receiver's IF filter passband up or down relative to the incoming signal while the BFO tracks along with it, so the recovered audio pitch stays the same. By sliding the passband window you can push an interfering heterodyne or an adjacent signal outside the filter skirts while keeping the desired signal inside. It is a selectivity tool: same bandwidth, different placement of that bandwidth.

Watch out Tuning in a station that is slightly off frequency without moving your transmit frequency is what RIT (receiver incremental tuning, or the clarifier) does, and listening on a frequency different from your transmit frequency is split operation, not IF Shift.

IF Shift slides the filter window, not the frequency. Shift = shove the QRM out of the passband.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

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