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This discusses several issues with receivers and transmitters.
A pre-selector is a tuneable filter placed ahead of the receiver circuit, which only passes frequencies in the desired band, meaning it filters out strong out-of-band signals, so they do not adversely affect receiver sensitivity, etc. A front end filter might be used in a single band transceiver to help protect it from interference, especially when driving past communications sites.
These typically consist of resistors. Like a voltage divider they reduce the level of the signal going from the input to the output. In most cases they have an input and output impedance to match the system, such as 50 or 75 ohms. They can also be made for balanced systems, such as 300 ohms. It is possible to make attenuators which can match impedance between circuit sections, or between devices.
They can be used to prevent receivers from being overloaded, and may be built into radios, switched in and out via a switch, or by a relay under software control. Other uses include reducing a transmitter output before feeding a power amplifier with a sensitive input, or in the transceiver port of a transverter.
Suppose we have a public service station set up on 2 metres FM voice, and either a 2 metre packet station, or a marine VHF station set up on the same site, say for a canoe paddle event. Transmitting on one band will "desensitise" the receiver on the other unit. The extent to which this occurs is down to the dynamic range of the receiver(s). This can also occur with multiple HF transceivers on a field day site.
This can also happen if you are trying to work a distant station on 14.226 while a bunch of lunatics are running high power on 14.222 MHz, swearing / cursing, and preaching Q-Anon and other conspiracy theories. Solutions include high quality, narrow filters, and roofing filters. I meant quality in the generic sense, but a high Q, or quality factor, is beneficial.
If you are trying to hear your own signal returning from a repeater using a second hand-held, or are testing a repeater the local transmitter may overload the receiver. It is best to get a friend to listen a few 10s of metres or more away. This person does not need to be licensed if they don't transmit back on amateur frequencies.
A radio will generate a sequence of signals called harmonics, especially at odd multiples of their design frequency, but also on even multiples. These are supposed to be well suppressed, but 43 dB below 100 watts is 5 milliwatts, enough to be heard a fair distance.
Not mentioned on the exam, if you transmit on 146.500 kHz, and a station on site is listening on 439.500 MHz, then the 70 cm station will likely hear your harmonics. Usually annoying, this can be a quick way to test a 70 cm receiver, with a similar relationship existing to use a 70 cm HT to make a basic test of a 23 cm radio.
Given this typical factor of multiplying by 3, you could also determine the 6 metre frequency which might interfere with a Coastal Station on 156.800 MHz, Marine Ch 16, at a Lighthouse & Lightship Weekend event.
156.800 / 3 = 52.26666667 which means something like 52.255 to 52.275 MHz should be avoided at this site.
The numbering of harmonics is: The base frequency, say 1 MHz, is termed the fundamental. The second harmonic is twice the fundamental, at 2 MHz; the third is three times it, at 3 MHz; the fourth, 4 MHz; the fifth, 5 MHz; and so on. The fundamental can be termed the first harmonic, although this is not a common term.
Intermodulation, or "Intermod" is a form of interference which can be an A-grade pain in the posterior, as finding the source can be very difficult. The classic version is f = 2a-b, where a is the frequency of one transmitter, and b is that of the other, and if b is greater than a, we can end up with a signal lower in frequency than either a or b, so services above, say 2 metres can end up generating intermod on 2m.
Suppose a poor quality narrow-casting transmitter is operating on 152 MHz, and a voice communications or data link on 156.2 MHz keys up periodically. Energy from the higher frequency transmitter enters the narrow-casting transmitter, and interacts with its frequency, in the diode junctions in its transistors. This creates a third frequency, thus: f = (a x 2) - b = 152 x 2 - 156.2 = 304 - 156.2 = 147.8 MHz, a 2 metre repeater input channel.
To fix the problem the dodgy transmitter needs to be taken out of service, and either fixed, or replaced with a quality product. The exam asks about two repeaters on a site causing the same problem, with the answer being to use a circulator on the transmitter in which the mixing occurs.
A circulator is a 3 or 4 port device, in which RF or microwave energy can only travel in one direction. They typically use a strong ferrite magnet. The configuration they are discussing is a 3 port unit, so power goes from the transmitter on Port 1 to the antenna on Port 2, but energy coming into Port 2 from the other transmitters on the side (via the antenna) is sent to Port 3, as long as there is a proper dummy load (termination) on that port. Another use is to divert power reflected from a faulty antenna into a dummy load, to protect the transmitter. (I noticed a couple of them in the "flea market" at Mayham in Wyong, which had only 2 external connectors, with the dummy load build into the enclosure).
The problem is that even if the transmitters are all clean, quality products, and all appropriate isolation devices are used, a naturally occurring diode in the fence, or on the tower can act as a rectifier, causing intermodulation to occur. It can even be something like an old rusty fence or telephone wire, fallen on a galvanised water trough, occurring randomly, depending on the wind. This can be termed the "rusty bolt effect". Other points can be real diodes, such as LED based tower lights or illuminations could be an issue. Even micro-diodes between braid and foil in coaxial cable are potential problems, with cable such as RG-223 (dual braid shield) or FSJ1-50 (corrugated copper shield) being ideal cables for shorter links in duplex systems. Diode junctions, intended or otherwise, are termed "non-linear devices" in the exam. Transistors junctions also appear to be diodes.
Normally the problematic transmitters are on the same site as the impacted receiver, but in one case, intermod on a site at Pennant Hills (northern Sydney) impacted a repeater on the Central Coast, over 50 km away.
There are other combinations, some involving 3 transmitters, typically f = a + b - c.
Further combinations are listed here: Wikipedia: Intermodulation
Given many involve 3 factors, either something like twice the "a" frequency minus the "b" one; or and interactions of three frequencies, these are termed "third-order intermodulation products". These are of interest as they often land the interfering signal in the same band as the two or 3 signals which contribute to it. Taking 2 metres as an example this may include nearby parts of the "VHF-High" band, beyond 2 metres itself, although the exam uses frequencies within 2 metres. These are an example of "odd-order" products, a term introduced to the latest version of the exam.
Pro-tip for Australian hams: Narrowcasting transmitters around 152 MHz may be of questionable quality, and can be a source of interference or intermods. Searching the ACMA database, and nutting out possible combinations on the site, or on nearby sites is often part of the investigation, either by Hams, or by the ACMA. Other nearby users include government and commercial bodies (voice and data), and the VHF Marine Band. Example of data bursts may be telemetry of river heights, or telecommand of pumps in a municipal water supply system.
A large dynamic range is useful, as it allows a receiver (or the amplifiers within it) to work with signals over a wide range of levels. It can also amplify a weak signal of interest in the presence of a much stronger signal, without generating intermodulation products.
Way off the exam, valve (tube) input stages often had very good dynamic range, partly because the large supply voltage allowed large swings in output voltage without compression or distortion.
As we know, decibels are a means to compare two signals, using a logarithmic scale. This is used for dynamic range, among other things.
If a reference level is specified, a single figure can provide an absolute value. A figure in dBm is relative to 1 milliwatt, or 1 mW. Just as freezing point is 0°C, the reference level of 1 mW is 0 dBm. Home audio is standardised around a signal of -10 dB, while professional audio uses +4 dBm. In the RF world, a weak FM signal might present -128 dBm at the receiver's input socket.
Off the exam, as the US FCC specifies power limits in watts, some overseas administrations use dBW, decibels relative to 1 watt. For example, 27 dBW is 500 watts, this being 3dB less than, or half of +30 dBW or 1 kW.
1 watt is 0 dBW or 30 dBm. 10 watts is 10 dBW or 40 dBm.
A parameter by which immunity to interaction between strong nearby signals can be quantified is the Third-order Intercept Point, or IP3.
Suppose there is are signals on 14.110 and 14.120 MHz. As these pass through an amplifier, harmonics, intermodulation products of the fundamentals, and intermodulation products of the harmonics, are all created in non-linear components. The two of most interest are those close-by, at 14.100 and 14.130 MHz, these being 2f₁-f₂ and 2f₂-f₁.
An increase in the signals of 1 dB results in an increase of 1 dB in their representation at the output. However, the intermodulation products increase by 3 dB in strength.
The levels of real signals and the intermodulation products can be graphed in such as way that the lines can be extrapolated to an intersection point. The intersection often occurs at 10 dB above the 1 dB gain reduction point, and often beyond the point at which the device under test would be destroyed.
A good figure for this parameter is often represented as a selling point for the radio.
It is important to emphasise that this is a theoretical figure.
This video is most helpful in understanding these processes:
See: Wikipedia: Third-order Intercept Point
While a $10,000 plus transceiver may boast a somewhat greater power output and/or other transmitter features, a high quality receiver capable of copying weak signals under arduous conditions is a significant part of the price, and of its attraction to those able to afford one. These would also be valuable if on the DX side of a dog-pile. The 40 dBm IP3 figure on the exam apparently relates to a very high-dollar Icom unit, with radios for us mortals having a 15 to 22 dBm figure.
Software defined radios range from a low cost USB dongle designed for receiving DVB-T digital TV to a low budget "Soft Rock" device to feed a PC soundcard to a receiver of transceiver costing thousands of dollars. The latter can be a PC controlled, or have a conventional modern transceiver user interface.
A card or USB device sold to digitise home videos can apparently accept a roughly 7 MHz wide chunk of HF spectrum, or other spectrum down-converted. Amplification is needed.
While digitisation may occur at the RF frequency, it is often done at Intermediate Frequency. Whatever the system, all rely on a digital to analogue converter.
In each case an instantaneous sample of the voltage is taken, and this is converted to a numerical value.
In PCM, or pulse-coded modulation, the number of values which can be counted are 2 to the power of the number of bits, or 2≳. This may be split between positive and negative values.
An 8 bit ADC allows 256 values to be measured. 16 bits allows 65,636 to be used. 20 bits gives 1,048,576 levels, and 24 bits allows 16,777,216 values. In these audio world these correlate to telephone systems; to CD and DAT; and the last two to "high definition" or "high resolution" consumer systems such as Blu-Ray and DVD-Audio.
Off the exam, there is however a trick with 8 bit used in telephony is a logarithmic system which accurately transfers lower level speech while also being able to use courser steps for higher level signals, which parallels human hearing. The method is to quantify at 12 bits, and convert to 8 bits using either the global A-law or US-centric μ-law algorithms in the ITU G.711 standard.
Say we want a system to quantify a maximum signal of 1 mV of RF. At 8 bits a single bit will have a value of 1/128 of a millivolt, or 7.8125 microvolts. Many real-world radio signal are well below this level. In a 16 bit system, divide 1 mV by 32,768, and 1 bit is about 0.0305 μV. 20 bit gives a resolution of 0.0019 μV. Thus, a greater number of bits increases the dynamic range of the system.
The second parameter is the sample rate, varying from 8 kHz for telephony to 48 kHz for professional audio, with 96 and 196 kHz for "HD" consumer products. Various higher rates are used for SDRs. The RTL-SDR, using DVB-T dongles, down-converts a section of spectrum, and samples it at 2.4 to 3.2 mega-samples per second. Software then extracts the desired signals, be that one af a range of voice modes, or all manner of data modes.
The sample rate must be twice the frequency of the highest frequency signal. There is often a filter placed before the A-to-D converter, to prevent false values being recorded.
The various digital voice modes use compression techniques to reduce the data-rate, and thus bandwidth of the signal, such as AMBE2. CODEC2 is an open-source option.
Many ham repeaters consist of a couple of used ham or commercial radios, or a retired commercial repeater, or perhaps a dedicated ham device for things like Fusion or D-Star. These feed a dedicated antenna or antennas via a bunch of cavity filters or "cans".
Not on the exam, but useful: The alternative implemented by several hams who work for or own communications companies is to integrate current commercial quality repeaters tuned to 70 cm into a commercial UHF site antenna system. This consists of a receiver antenna system, typically folded dipoles or a "binary array". This is filtered, amplified and distributed to multiple receivers. Transmitter outputs are combined, often into a separate transmit antenna array. Circulators are often used, to ensure no RF power ends up where it shouldn't. This applies to modes such as FM, P25, and DMR. Sites may be linked via their corporate microwave-based IP system.
Vertical separation of antennas helps to increase isolation between transmit and receive antenna arrays.
These are the actual questions from the Extra licence exam pool, as published by the NCVEC.
What is an effect of excessive phase noise in a receiver's local oscillator?
A. It limits the receiver's ability to receive strong signals
B. It can affect the receiver's frequency calibration
C. It decreases receiver third-order intercept point
D. It can combine with strong signals on nearby frequencies to generate interference
This can cause strong signals on nearby frequencies to interfere with reception of weak signals, answer D.
Which of the following receiver circuits can be effective in eliminating interference from strong out-of-band signals?
A. A front-end filter or pre-selector
B. A narrow IF filter
C. A notch filter
D. A properly adjusted product detector
Use a front-end filter, or a pre-selector, answer A.
What is the term for the blocking of one FM phone signal by another, stronger FM phone signal?
B. Cross-modulation interference
C. Capture effect
D. Frequency discrimination
This is the capture effect, answer C.
AM suffers much less from the capture effect, which is one reason that it is still used in aviation VHF.
What is the noise figure of a receiver?
A. The ratio of atmospheric noise to phase noise
B. The ratio of the noise bandwidth in Hertz to the theoretical bandwidth of a resistive network
C. The ratio of thermal noise to atmospheric noise
D. The ratio in dB of the noise generated by the receiver to the theoretical minimum noise
This is the ratio in dB of the noise generated by the receiver to the theoretical minimum noise, answer D.
Amplifiers with a low noise factor are important when receiving weak signals. Even a 1 dB improvement is significant is systems such as EME / Moonbounce station pre-amplifiers. In radio-telescopes receivers are cryogenically refrigerated to reduce noise production.
What does a receiver noise floor of -174 dBm represent?
A. The minimum detectable signal as a function of receive frequency
B. The theoretical noise in a 1 Hz bandwidth at the input of a perfect receiver at room temperature
C. The noise figure of a 1 Hz bandwidth receiver
D. The galactic noise contribution to minimum detectable signal
This figure is the theoretical noise in 1 Hz of bandwidth at the input of a perfect receiver, at room temperature, answer B.
A CW receiver with the AGC off has an equivalent input noise power density of -174 dBm/Hz. What would be the level of an unmodulated carrier input to this receiver that would yield an audio output SNR of 0 dB in a 400 Hz noise bandwidth?
A. -174 dBm
B. -164 dBm
C. -155 dBm
D. -148 dBm
400 Hz is clearly 400 times the bandwidth of 1 Hz. 400 times expressed in dB is 26 dB. This is because 400 being 10 × 10 × 2 × 2 translates to 10 + 10 + 3 + 3 being 26 dB. -174 dB + 26 dB = -148 dBm, answer D.
What does the MDS of a receiver represent?
A. The meter display sensitivity
B. The minimum discernible signal
C. The multiplex distortion stability
D. The maximum detectable spectrum
This is the minimum discernible signal, answer B.
An SDR receiver is overloaded when input signals exceed what level?
A. One-half the maximum sample rate
B. One-half the maximum sampling buffer size
C. The maximum count value of the analog-to-digital converter
D. The reference voltage of the analog-to-digital converter
The reference voltage of the A-to-D converter also represents the maximum input level and the maximum count. The current "correct" answer that it is "overloaded" when the voltage of the input exceeds the reference voltage, answer D.
Note that there is probably a margin between the maximum input that can be quantified, and that which causes damage to the converter.
Which of the following choices is a good reason for selecting a high frequency for the design of the IF in a superheterodyne HF or VHF communications receiver?
A. Fewer components in the receiver
B. Reduced drift
C. Easier for front-end circuitry to eliminate image responses
D. Improved receiver noise figure
It is easier for the front-end to filter out the image, as it is further away in frequency, answer C.
To filter an unwanted signal under 1 MHz away from a signal of interest is harder than one tens of Megahertz away. Something like a radio-cassette player including a SW band (or SW1 & SW2) probably had a 455 kHz IF, meaning that a broadcast signal at or near 6.29 MHz or 8.11 MHz may over-power a ham one on 7.2 MHz. A 10.7 MHz IF would mean that the singal we need to reject is all the way up at 7.2 + (10.7 × 2) = 28.6 MHz, much easier to filter out.
What is an advantage of having a variety of receiver IF bandwidths from which to select?
A. The noise figure of the RF amplifier can be adjusted to match the modulation type, thus increasing receiver sensitivity
B. Receiver power consumption can be reduced when wider bandwidth is not required
C. Receive bandwidth can be set to match the modulation bandwidth, maximizing signal-to-noise ratio and minimizing interference
D. Multiple frequencies can be received simultaneously if desired
The correct filter can be selected for the mode being received, answer C.
CW, SSB, AM, etc each have different bandwidths. Ceramic, crystal, or idealy, Collins mechanical filters of various suitable bandwidth can be added to most HF transceivers. The last stocks of Collins filters are available from InRad, typically fitted to small "daughter boards" to plug into radios. Yaesu or its dealers may also have some left for things like the FT-857D.
Why can an attenuator be used to reduce receiver overload on the lower frequency HF bands with little or no impact on signal-to-noise ratio?
A. The attenuator has a low-pass filter to increase the strength of lower frequency signals
B. The attenuator has a noise filter to suppress interference
C. Signals are attenuated separately from the noise
D. Atmospheric noise is generally greater than internally generated noise even after attenuation
Distant lightning crashes, "static" and other atmospheric noise exceeds noise within the receiver, answer D.
Which of the following has the largest effect on an SDR receiver's dynamic range?
A. CPU register width in bits
B. Anti-aliasing input filter bandwidth
C. RAM speed used for data storage
D. Analog-to-digital converter sample width in bits
An A-to-D with many bits can quantify a very small or a very large signal, answer D.
How does a narrow-band roofing filter affect receiver performance?
A. It improves sensitivity by reducing front end noise
B. It improves intelligibility by using low Q circuitry to reduce ringing
C. It improves dynamic range by attenuating strong signals near the receive frequency
D. All of these choices are correct
A feature in more modern gear, these improve dynamic range by attenuating strong signals near the receive frequency, answer C.
What transmit frequency might generate an image response signal in a receiver tuned to 14.300 MHz and which uses a 455 kHz IF frequency?
A. 13.845 MHz
B. 14.755 MHz
C. 14.445 MHz
D. 15.210 MHz
The image occurs twice the IF, and 14.300 + 0.910 = 15.210 MHz, answer D.
The local oscillator is 455 kHz above the desired frequency, and in this case it is set to 14.755 MHz. Adding 455 kHz (0.455 MHz) to this we get 15.210 MHz. This is a frequency in the 19 metre broadcast band. 19 metres is used in the daytime, and in the evening in summer. Power can be tens or hundreds of kilowatts.
What is reciprocal mixing?
A. Two out-of-band signals mixing to generate an in-band spurious signal
B. In-phase signals cancelling in a mixer resulting in loss of receiver sensitivity
C. Two digital signals combining from alternate time slots
D. Local oscillator phase noise mixing with adjacent strong signals to create interference to desired signals
Phase noise in the local oscillator can mix with adjacent strong signals to create interference to desired signals, answer D.
What is meant by the blocking dynamic range of a receiver?
A. The difference in dB between the noise floor and the level of an incoming signal which will cause 1 dB of gain compression
B. The minimum difference in dB between the levels of two FM signals which will cause one signal to block the other
C. The difference in dB between the noise floor and the third order intercept point
D. The minimum difference in dB between two signals which produce third order intermodulation products greater than the noise floor
Within the design frequency band an amplifier normally amplifies a signal, be it weak or strong, by a certain number of dB, determined by the nature of the active device, and things like resistors around it. However, once a certain level is reached, the amplifier "runs out of puff", and the gain is reduced. In measuring the dynamic range a common parameter must be agreed to, and this is the point where the output is 1 dB less that a straight line trend predicts, relative to the noise floor, answer A.
Which of the following describes two problems caused by poor dynamic range in a receiver?
A. Spurious signals caused by cross-modulation and desensitization from strong adjacent signals
B. Oscillator instability requiring frequent retuning and loss of ability to recover the opposite sideband
C. Cross-modulation of the desired signal and insufficient audio power to operate the speaker
D. Oscillator instability and severe audio distortion of all but the strongest received signals
Cross-modulation of the desired signal, and desensitisation from strong adjacent signals, answer A.
How can intermodulation interference between two repeaters occur?
A. When the repeaters are in close proximity and the signals cause feedback in the final amplifier of one or both transmitters
B. When the repeaters are in close proximity and the signals mix in the final amplifier of one or both transmitters
C. When the signals from the transmitters are reflected out of phase from airplanes passing overhead
D. When the signals from the transmitters are reflected in phase from airplanes passing overhead
This can occur when two or more repeaters (or other transmitters) are in close proximity, and the signals mix in the final amplifier of one or both transmitters, answer B.
Which of the following may reduce or eliminate intermodulation interference in a repeater caused by another transmitter operating in close proximity?
A. A band-pass filter in the feed line between the transmitter and receiver
B. A properly terminated circulator at the output of the transmitter
C. A Class C final amplifier
D. A Class D final amplifier
A circulator ensures the transmitter's power goes to the antenna, and any signal coming in from the antenna goes to the dummy load, answer B.
What transmitter frequencies would cause an intermodulation-product signal in a receiver tuned to 146.70 MHz when a nearby station transmits on 146.52 MHz?
A. 146.34 MHz and 146.61 MHz
B. 146.88 MHz and 146.34 MHz
C. 146.10 MHz and 147.30 MHz
D. 173.35 MHz and 139.40 MHz
Assume they are using f = 2a - b, and plug 146.52 into each of a and b in turn, with 146.70 as f.
2a = f + b = 146.70 + 146.52 = 293.22; so a = 293.22 / 2 = 146.61, so answer A, but better confirm this.
-b = f - 2a = 146.70 - (146.52 x 2) = 146.70 - 293.04 = -146.34, so b = 146.34, yep, answer A.
What is the term for unwanted signals generated by the mixing of two or more signals?
A. Amplifier desensitization
C. Adjacent channel interference
This is intermodulation interference, answer D.
Which of the following reduces the likelihood of receiver desensitization?
A. Decrease the RF bandwidth of the receiver
B. Raise the receiver IF frequency
C. Increase the receiver front end gain
D. Switch from fast AGC to slow AGC
Decreasing the RF bandwidth hopefully helps to exclude the strong signal, answer A.
This may require some form of external filter. In more challenging conditions this may be a cavity filter.
What causes intermodulation in an electronic circuit?
A. Too little gain
B. Lack of neutralization
C. Nonlinear circuits or devices
D. Positive feedback
These are non-linear devices, being an intentional or non-intentional semiconductor junction (diode), answer C.
What is the purpose of the preselector in a communications receiver?
A. To store often-used frequencies
B. To provide a range of AGC time constants
C. To increase rejection of signals outside the desired band
D. To allow selection of the optimum RF amplifier device
A pre-selector helps the receiver to reject signals outside the desired band, answer C.
What does a third-order intercept level of 40 dBm mean with respect to receiver performance?
A. Signals less than 40 dBm will not generate audible third-order intermodulation products
B. The receiver can tolerate signals up to 40 dB above the noise floor without producing third-order intermodulation products
C. A pair of 40 dBm input signals will theoretically generate a third-order intermodulation product that has the same output amplitude as either of the input signals
D. A pair of 1 mW input signals will produce a third-order intermodulation product which is 40 dB stronger than the input signal
Given applying two 10 watt signals (40 dBm is 10,000 mW) to a tiny FET will destroy it, the operative term is "theoretical", answer C.
This is somewhat flippant comment, but an easy way to remember the answer. An IP3 of 40 dBm means that it has a very high resistance to generation of intermodulation products within its internal amplifiers and other components. 40 dBm is the level at which graphing indicates the front-end amplifier will theoretically generate third-order intermodulation products with the same level as the input signals.
Why are odd-order intermodulation products, created within a receiver, of particular interest compared to other products?
A. Odd-order products of two signals in the band of interest are also likely to be within the band
B. Odd-order products overload the IF filters
C. Odd-order products are an indication of poor image rejection
D. Odd-order intermodulation produces three products for every input signal within the band of interest
Odd order products, such as third-order ones, are often within the band we are operating in. As and example, if two strong signals are in or near the 2 metre band, they are likely to produce products which cause interference in that band, answer A.
In the 1970s to early 2000s in Australia high power pager transmitters operated just above 2 metres, from 148.0125 MHz and up, and these caused interference to 2 metres, through various mechanisms.
What is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency?
C. Cross-modulation interference
D. Squelch gain rollback
This is desensitisation, answer A.
On to: Practices 3 - Noise suppression
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, June 2022.
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