Tiny Elephant's Contest CornerThe Latest Contest News -
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Tiny Elephant's Contest CornerThe Latest Contest News -
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My anxiety got me to thinking about receiver performance and whether I could improve things. I’ve decided over the next few months to discuss receiver performance capabilities from an operator’s perspective. I will also touch on my operating experiences with several of today’s transceivers.
As background, receiver technology started with the regenerative receiver design. The regeneration receiver design is basically an oscillator circuit that is not quite oscillating. In a normal oscillator circuit, enough energy is fed back from the output to the input to sustain an oscillation (regeneration) at a frequency determined by the LC components at the input. I’m sure many of us struggled with the different oscillator circuit designs at one time or other as we studied for license upgrades. An oscillator before oscillation acts as a tuned circuit exhibiting some selectivity at its intended oscillation. Early radio developers found that this condition could be used to detect signals and input them to an amplifier. However, the design was fussy in that feedback invariably would kick the circuit into oscillation, resulting in no signal.
Radio engineers went back to tinkering and developed the heterodyne receiver. I don’t know who came up with the term “heterodyne” but it describes a technique for converting the frequency of the received signal down to a lower or intermediate frequency (IF) where filtering could be applied. Another term that better describes what is happening is conversion receiver. A conversion receiver could have more than one intermediate frequency stage; hence, the name “dual-conversion” or “triple- conversion” receiver.
The conversion design uses a separate oscillator known as a local oscillator that feeds into a mixer to convert the incoming frequency to a lower IF. The output of the mixer is tuned for the difference frequency between the incoming (carrier) frequency and the local oscillator. Because it is a non- linear device, the mixer generates output at a lot of other frequencies. Fortunately, filtering takes care of them and subsequent amplifier circuits are tuned to the desired IF.
The conversion receiver design is widely used today in communication receiver applications. The long history of the design has seen many improvements as solid state and integrated circuit technologies matured. These improvements have led to an overwhelming list of “features” that are part of the technical details for today’s amateur receivers. A look at a QST product review of a ham transceiver usually contains a daunting list of numbers resulting from a variety of measurements made in the ARRL lab. What do they all mean?
The first parameter that every ham knows is sensitivity, which is to a receiver what horsepower is to a car. Very simply, it is the minimum detectable signal (MDS) level in a given bandwidth at a specified signal-to- noise (S/N) ratio. For the ARRL lab, measurements are made at each filter setting available in a rig so that one can determine sensitivity for CW, SSB, AM, or FM signals. A S/N level of 10 dB is generally used for all modulations except FM where a S/N of 12 dB referenced to distortion is used. Many of today’s high-end rigs feature selectable filters for the 2nd and 3rd IF stages with 4 to 5 different bandwidths available. On top of this, throw in measurements with the pre-amplifier on or off and the table gets quite long. My FT- 1000MP, for example, has 5 selectable filters for each IF although I do not have all the available slots populated (yet).
Ok, so all I need is to buy the rig with the best sensitivity numbers and I’m all set, right? Well, it depends. If your operating style is one of hunting for weak signals on “dead” bands and running in terror at the first indication of an S3 signal, fine. However, a very sensitive receiver can be almost useless in a major contest if it can’t tolerate off-frequency strong signals. The basic receiving phenomenon relies on diode detection, which is a non-linear function. This means that you get out what you expected plus more than you bargained for. The non-linearity results in mixing of off- frequency signals in what is known as intermodulation distortion (IMD).
A parameter that is used to measure a receiver’s large signal handling capability is known as 3rd-order intercept point (IP). The name comes from a second harmonic signal mixing with its fundamental to produce a nasty offspring known as a 3rd-order IMD signal. To measure the IMD dynamic range of a receiver, two tones of the same strength are fed into the receiver, one tone being the second harmonic of the other, and both tones outside the receiver’s passband. An offset of 20 KHz for the fundamental tone is generally used so that the receiver’s automatic gain control (AGC) is not activated that could lead to false readings. The two signals are pumped up until the 3rd- order IMD fellow is detected in the receiver’s passband. The difference between the sensitivity level of the receiver and the level of the interfering signal is the IMD dynamic range. The higher the IP, the better the IMD dynamic range and strong signal tolerance. Typical IP values range from 15 to 20 dBm for the 20-meter band. Many a receiver design engineer has earned his fame and gray hair trying to nudge the IP higher without giving up something elsewhere.
What does this look like in practice? Well, one measure I use for CW is to see how far I have to move away from two 50 dB-over-S9 Italian contest stations that are about 10 KHz apart, each running “only” 300 watts, so that I no longer sense them modulating my noise floor. All things being equal, a station whose receiver has a better IMD dynamic range can operate closer to the interfering station and still hear other stations.
Another parameter that is easier to associate with operating is the blocking dynamic range (BDR) of a receiver. Another name for BDR would be QRM rejection. A test signal is set up typically 20 KHz away from a desired weak signal. This signal is well away from the receiver passband so that filtering behavior and AGC don’t come into play. The level of the test signal is increased until there is some degradation in the received signal output level. The level of the test signal is compared to the level of the desired signal - usually at MDS level - is the BDR.
A real-life illustration of BDR performance is to find a strong local SSB contest station at the edge of the band. Tune away from this station and see how far you need to tune before you can hear weak signals without difficulty. You probably won’t have to go as far as 20 KHz away, and you are not likely to find a PW signal, either, but you will get the idea. The receiver with the better BDR performance will allow you to get closer to the strong station and work that weak one that would otherwise get covered up.
A final parameter for receiver performance is filter selectivity. This is not usually a measurable parameter but I feel it is a characteristic that should be included in a rig’s specifications. The filters used in amateur rigs range from 250 Hz for weak- signal CW reception up to 6.0 KHz for AM signals. A filter is often graphically shown as a trapezoidal figure where the flat part is the passband and the sides are the filter skirts. The goal is to hear everything inside the trapezoid and silence everything outside of it. The closer to vertical the skirts are, the better the filter is at doing this.
Filter design engineers achieve their fame and lose their gray hair trying to make filter skirts as steep as possible in the smallest physical package possible at the cheapest price. For many years, crystal filters have been the mainstay of receiver filters to achieve narrow bandwidth and good skirt performance. One of the tradeoffs with filter design, however, is a tendency for the filter characteristic to degrade at some point outside the filter passband. At certain frequencies outside the passband the rejection level can degrade as much as 20 dB or more, a “bump” that can allow strong signals to get through the filter. Minimizing this behavior becomes an exercise in design tweaking and physical placement with a little bit of black magic thrown in.
This simplistic overview of receiver performance may be “fluff and buff” to RF engineers. It is intended to provide some rules of thumb to remember when using a receiver in a contest or DX pileup. It also sets the stage to go into detail on some of today’s rigs in later columns. For now, why not tune in to some of these contests and observe how your receiver behaves?
Closely following CT, the Massachusetts QSO party runs from 1800Z Saturday to 0400Z Sunday and 1100Z to 2100Z Sunday. Contacts may be made on SSB and any digital mode, including CW. Phone QSOs count as 1 point while all other types are worth 2 points. The exchange and scoring method is the same as for the CT contest. Logs for the MA QSO party go to the Framingham ARA by 7 June.
For more of a challenge, the Indiana QSO party over the same weekend starts at 1400Z on Saturday and goes till 2300 Sunday. The exchange and scoring scheme is the same as CT or MA, except SSB QSOs count 2 points and CW QSOs are 3 points. Logs may be sent to sharon.l.brown@gte.net by 11 June.
Heading further west, the Nevada QSO party kicks off on Saturday at 0000Z and ends 0600Z on Sunday. Operation covers 160 thru 6 meters on CW, RTTY, and SSB. Stations may be worked once per band and mode. Points are 1/SSB and 2/others. Scoring is points x NV counties worked.
73 and have fun at Rochester! de K2TE