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Hardware review

Close-up on Yaesu FT-950 HF+6 meters transceiver ($1200-1400 in 2015). Discontinued in 2013, it was replaced advantageously by Yaesu FT-DX1200 ($1200-1500 in 2015). Document T.Lombry.

How to select a HF transceiver ? (I)

On what basis can we select a HF transceiver ? What features made the difference between a mid-range and a high-end model ? Is the price a good indicator ? Are DSP functions useful ? Is it better to install mechanical filters or a DSP filtering ? Is the selectivity and 3d-order IMD important ? Here are some among the thousand questions that run through our mind when we decide to buy a new RTX.

We will discuss in this article about desktop HF transceivers, and what makes the difference between a mid-range and a high-end transceiver.

If you are not at the top of the novelties in reading regularly ham magazines or speaking with friends in radio clubs, you will have difficulties to have a deep knowledge of the technology used in new transceivers, how it performs against the older models, especially if you are not interested in the new developments in both hardware and built-in software.

In a transceiver, the most important module after the transmitter is of course the receiver. Emitting a signal is not really a problem. It requires few components and sometimes even not much power (QRP). The receiver on the contrary is a very complex module. If you can quite easily send a message using the transmitter, you need a performing receiver to detect the weakest signals emitted by your correspondent, and the few microvolts displayed on the S-meter are really not much energy to pick up.

Therefore the performances of a transceiver depend first of all on the quality of its receiver. There is first its sensitivity, the minimum input signal required to produce a specified output signal, under which threshold you listen only to the electronic noise of components or, at best, the noise on the frequency but no readable signal. The second quality factor is the selectivity.

The receiver selectivity

The selectivity of the receiver represents its ability to respond to a tuned station and to reject any nearby undesired signal. This is probably the first feature where your receiver will show its limitations, or its performance. The next one will be the DSP.

We have also to make the distinction between modes of traffic. The response curve of a filter is defined by its bandwidth at different characteristic attenuation levels (0, -6, -40, -60 dB). Such a curve displays scarcly the rectangular shape of the theoretical model, but a sort of bell shape which wings are more or less abrupt with skirts more of less short. Some of the best mechanical filters are Collins. These filters are inserted all through the receiving chain at each IF stage.

If we take the example of Yaesu FT-1000MP Mark V, in CW a narrow filter of 250 Hz of bandwidth at -6 dB (700 Hz at -60 dB) is enough to work all pile-ups and weak stations submerged by QRM. But in SSB, the signal is much wider. The narrowest IF passband is 1.8 kHz wide at -6 dB and 3.6 kHz at -60 dB. You can't thus get similar results in SSB and in CW.

Three of the many ways to suppress QRM on the Yaesu FT-1000MP Mark V : with IF WIDTH, IF SHIFT and DSP NOTCH controls. Clic on the drawing to enlarge.

On the Yaesu these filters are installed at the second IF at 8.215 MHz and at the third IF at 455 kHz. Like on all transceivers they can be combined in cascade to form the equivalent of one filter showing a variable bandwidth (called VBT to Kenwood).

In some books we read that a filter would works better if signals applied to it are weak. This is not true because the response of a crystal or ceramic filter is very linear; it responds exactly the same way to large signals as it does to small signals. 

This means that whether the signal applied to the filter in within the filter's passband or not, the amount of signal appearing at the output of the filter is proportional to the input signal applied. 

The method to suppress these undesired noises (they are static noise or RFI) consists in rejecting them outside the passband or, if they are on the same frequency, to attenuate the interfering signals as much as possible before they reach the RF amplification stage.

As you cannot change the filter passband, you need to apply one to three cascading methods to remove all interference and noise that prevent you listening your correspondent in good conditions. Let's see each of them in a few words illustrated with some graphs.

High and Low cut settings

The first method consists in shifting the passband of each filter up or down in frequency until the noise has disappeared. So the signal is now in sandwitch between the two filters, rejecting the interfering signals outside the passband (see graph B. below). This technique does not modify the beat frequency oscillator (BFO) tuned on the remote station frequency, so the signal sounds normal. You can also use the High and Low cut controls to remove undesired audio high and low pitch from the desired signal and thus reduce or remove interference as well. 

These graphs explain better that any comment the effects of filters on interference and noise interfering with a signal in the passband. The first graph describes the default bandwidth of an SSB filter, all controls off. Note that the BFO frequency (red) stands close to the filter center frequency (+1800 Hz). The second graph displays the effect of Low and High cut controls on two tunes interfering with a signal (yellow) in the center of the passband. In sliding the High/Low cut the effective passband became narrower, rejecting the interferences. The third graph displays the effect of Attenuation (30 dB) and High/Low cut on two strong interfering signals adjacent to the desired signal. The fourth graph displays two interferences on the same frequency as the signal. This time the IF shift control was set to +500 Hz and the RIT was adjusted so that the BFO is properly positioned in the passband. In addition the RF gain was reduced, and the attenuation and High/Low cut controls were adjusted to isolate the desired signal. In playing cleverly with these controls you can reject all residual RFI and noise. In high-end receivers these controls are completed with a Notch and DSP.

Attenuation and RF gain

But this solution does not work if the interference is at the same frequency as the desired signal. To remove the interference without rejecting the signal, you need to apply a second method, the attenuation, displayed above in the center graph. Thanks to the RF attenuator (ATT control) adjusted to 20 or 40 dB for example depending on the strenght of the noise, you could lower the intensity of all signals until the interference reaches the background noise hash. In these conditions you will reduce the generation of undesired products.

If this is not enough, you can  cascade it with a low RF gain until the interfering signal is removed from the passband, offering you a chance to hear your correspondent without QRM. If you still heard the RFI you can in addition adjust the High and Low cut controls to reduce the passband to less than 1.5 kHz or so, while keeping the BFO frequency close to the passband.

Elecraft series.

However, adding more attenuation and reducing the RF Gain have one drawback. In many receivers there is a poor audio circuitry following the detection stage, which results in excessive noise (hiss) being added to the desired signals. The gain that you reduced on one side has to be made up for elsewhere. If the signals are not already above the AGC attack threshold for a same amount, a significant amount of audio gain must be applied to maintain the desired listening levels at the speaker output. Consequently, this audio gain will increase the amount of hiss present in most receivers. This means that the operator has to manually adjust the AF or RF stage in order to maintain the desired signal levels under fading conditions.

IF Shift

Till now we haven't modified the BFO frequency that was fixed in relation to the filter center frequency. The BFO was set at about 1 kHz or a bit more up or down from the IF center frequency.

Now, in some conditions, you can hear both interference and noise on the frequency, tunes as well as static nosies. In the worst case all signals are on the same frequency. To remove these undesired noises you have to use a third method, this time the IF shift as displayed above in the third graph at right. In most cases you will have to cascade all three methods, playing with together a string attenuation, a low RF gain, adjusting the High/Low cut, and at last setting the IF shift to some hundreds of Hz above the filter center frequency, and adjusting the RIT to properly position the BFO close to the passband.

In most conditions, in using this approach you will get a reception free of noise and interference. However, if a strong interference gain access to the amplification stage located behind the last IF stage, you will hear it loud and clear in your headset. To remove it modern transceivers are equipped of a fourth IF stage, called the DSP. We will come back on this technology later when we will discuss about high-end transceivers.

To check : Run WebSDR to test the IF Shift on real QSOs

Yaesu FT-950 and all its controls. Document EC1DJ "MrDJ".

Competition between transceivers

Now that our transceiver is equipped of two or more IF stages, each of them taking advantage of the finest filters, let's see what are the different features offered in some well-known transceivers, and some of their respective performances in order to well understand the difficulty of selecting a transceiver suited to your needs.

To clarify the problem, we will compare different transceivers used by amateurs. On one side the mid range Kenwood TS-570D, famous of its performances and sold at an affordable price ($1100 or 1700 in 2002), and on the other side high-end models like Icom IC-756PROIII and Yaesu FT-DX5000MP at prices close to $3000 up to Kenwood TS-990S at $8000 and even the overquoted Icom IC-7851 at $17000 plus options in 2014 !

Are these models worth their price and do they display all similar performances ? We have a tendency to believe that in a same category all transceivers are on par, even if their price fluctuates somewhat from one brand to another. But this is an opinion put a priori, and a subjective feeling far to reflect the reality of the field.

Mid-end vs. high-end transceivers

Take Kenwood TS-570D for example. It is a compact desktop HF transceiver that received good reviews, efficient for casual, non-contest operations. In short, its DSP noise reduction is great, and with the optional 1.8 kHz SSB filter, selectivity is good a reason I bought it contrarily to the opinion of some hams !

But under stress, to carve your hole in a crowded band, to work a pile-up or when the atmospherics (QRN) are strong, TS-570D shows its limits due to its receiver noise level and its limited number of filtering means to discriminate two nearby signals, one being much weaker than the other. 

Kenwood TS-590SG (2014)

Kenwood TS-590S (2010)

Kenwood TS-570D (1998)

In this context, its successors TS-590S released in 2010 and specially TS-590SG released in 2014 are much more efficient with a larger dynamic range, TS-590SG including the IF DSP AGC technology developed for the Kenwood TS-990 transceiver.

However, even this TS-590SG cannot struggle against an Elecraft K3 that displays better figures, a better audio, and is more accurate for a lower price.

In fact, three factors limit the receiver performance :

- the blocking dynamic range : also called "blocking gain compression" is for short the range between the weakest and the strongest signals that a system can handle simultaneously without noticeable degradation in performance (thus without activating any filtering or attenuator). The value is good from 100 dB and excellent from 125 dB for an offset of 2 kHz.

- the 3d-order dynamic range (IMD) : related to the intermodulation distortion, it is a measure of the 3d-order distortion generated when two tones are closely spaced in frequency and are combined into the same input. If the filtering system is unable to separate both signals, it causes interferences and signals cannot be discriminated. 

IMD should be read in association with RMDR. Usually IMD is a bit lower than RMDR at the same offset. The dynamic range is good from 90 dB and excellent from 100 dB for a offset of 2 kHz.

- the reciprocal mixing dynamic range (RMDR) : it defines the strength at which the filtering system can cope with a strong signal near the current frequency, taking into account the (low) noise generated by local oscillators. Higher it is, better is the gain from the phase noise. The value is very good from 100 dB and excellent from 120 dB. Up to these last years, RMDR was rarely listed in reviews.

Dynamic range of some transceivers and receivers (R)

(at 5 kHz or 2 kHz spacing, n=14 MHz, preamp off, in dB)

Model

Blocking

IMD

Model

Blocking

IMD

Icom IC-7851

≥135*

116

Yaesu FT-DX9000MP

102

85

Yaesu FT-DX5000D

136

114

Yaesu FT-DX1200

123

83

WinRadio WR-G31DDC (R)

128

107

Yaesu FT-1000MP

119

83

Elecraft K3

140

103

Ten-Tec Omni VII

134

82

Kenwood TS-990S

133

101

Icom IC-756PRO

104

80

Elecraft KX3

128

100

Yaesu FT-1000MP Mark V

106

78

Yaesu FT-DX3000

127

100

Icom IC-746

88

78

Icom IC-756PROIII +Inrad^

119

100

Icom IC-775DSP

104

77

FlexRadio FLEX-5000A (R)

123

99

Icom IC-756PROIII

101

77

TenTec 599AT Eagle

121

98

Icom IC-756PROII

100

76

Perseus SDR (R)

129

97

Yaesu FT-450D

88

76

Kenwood TS-590S

126

97

Kenwood TS-480SAT

98

75

Icom IC-7800 +Inrad^

127

96

Icom IC-706MKIIG

86

74

Ten-Tec ORION II

136

95

Yaesu FT-847

82

73

WinRadio G303i (R)

113

93

Kenwood TS-570D

87

72

Elecraft K2

126

88

Yaesu FT-950

98

71

Icom IC-7410

111

88

Alinco DX-SR8T

83

70

Icom IC-7600

102

88

Kenwood TS-2000

99

67

Ten-Tec Omni 6+

119

86

Yaesu FT-2000

92

64

Icom IC-7800 V2

117

86

Icom IC-7200

83

67

Blocking gain compression (Blocking GC) and 3d-order dynamic range (IMD) measurements at 5 kHz or 2 kHz () spacing for several HF transceivers or receivers with factory settings tested by ARRL. The blocking gain compression is the signal level required to block or to reduce the sensitivity of a receiver to weak signals. When this occurs the strongest signal from an adjacent frequency tends to "capture" the amplifier reducing the strength of the other signals. IMD quantify the intermodulation, non-linearities by a single number, the intercept point (IP3). This is a measurement of the mixing products produced when two strong signals are fed to the receiver simultaneously. Excellent models (deep green) have a Blocking GC > 124 dB and an IMD ≥ 100 dB (2 kHz, 3d order). Models in the average (light green) have an Blocking GC between 100-124 dB and an IMD between 90-99 dB. Below these figures, the model is just "good" (and many amateur can appreciate it) but it is not very performing in stress conditions even if it includes some DSP functions.

See additional comparisons on PA1HR, Elecraft, Sherweng and F6CRP websites. 

(^) Calculated with the optional Inrad's 2 or 3 kHz roofing filter depending on the RTX. 

(*) BDR=135 dB with Preamp2 ON according to ARRL. So, set OFF it is either the same or higher. To confirm.

While many systems use DSP technology, in TS-570D among others this processing is installed on the AF stage rather that on the IF one. This presents some drawbacks when the receiver tries to remove interferences or even simple tunes on a near frequency that successfully passed through the all chain of detection.

Take now the Icom IC-775DSP, a high-end HF transceiver recently discontinued but from time to time available on secondhand. The main reason to purchase this model with its options is to participate in more contests and penetrate through pile-ups. This is where the Icom excels, and still more equipped with all optional filters.

But even using its default settings, IC-775DSP allows narrow filters at two IF-stages where TS-570D allows only one. What is the result? The Twin Passband Tuning used in conjunction with the two 1.8 kHz filters of IC-775DSP is far superior to the IF Shift method offered on TS-570D. With the manual notch filter, there is almost nothing you cannot filter out, another feature not available on TS-570D. On 160 m also, when adding attenuation, turn off DSP, you can open the filters to 8 kHz for hi-fi audio, what TS-570D could never perform.

Icom IC-7600

Icom IC-775DSP

Under strong Q-signals, IC-775DSP receiver does not overload or desense like the much cheaper TS-570D. The sensitivity is about the same, as you would expect because TS-570D is really a great rig in its category, providing an excellent signal and a good reception in usual working conditions.

As I explained previously, when the propagation is open and no QRN at all on the band, filtering is even useless and both rigs yield the same results ; they are really on par. So, before reselling your TS-570D, think rather twice than one as it can be a great performer for working abroad, e.g. on holidays or in the field as a portable RTX (even on dry battery) although IC-775DSP is still small enough to be portable but it is much heavier.

On the other hand, while contesting or trying to penetrate into pile-ups against world class competitors, you need together powerful tools to remove the QRN generated by near amateurs calling, possible QRM (weather conditions and specially static), and preferably a directional antenna to pick up the weakest signals (note also that contrary to a general opinion, in CW you need a better receiver than for SSB, specially during pile-ups).

Icom IC-756PROII

Icom IC-756PROIII

In practice your system must be able to discrimine any incoming signal without be bored by near strong QRM. To reach these two objectives you need a high sensitivity, excellent 3d-order IMD, a high RMDR and the best selectivity. With these objectives in mind, the small TS-570D will think seriously to give up the fight.

Contrary to IC-775DSP, that once equipped with all optional filters in its two IF-stages can have unbelievable selectivity and audio compared to many other rigs. But it is however not as selective as Yaesu FT-1000MP Mark-V or the more recent Yaesu FT-DX3000. This last has indeed more IF stages and more filtering features to fight against interferences.

But take also care to the frequencies used by IF stages. In many high-end transceivers, and even TS-570D, the IF stages are outside amateur bands (around 73 MHz, 8.8 MHz and 455 kHz) and they cannot disturb your QSO. But is some so-called performing mid-ranges, like the Elecraft K2 to name one, there are a lot of images right in the 15 and 17 meter bands... This is a problem too rarely brought to the fore but that spoils all the interest of this later model.

Yaesu FT-2000

Yaesu FT-DX1200

At last, as far as ergonomy is concerned, the cheap TS-570D is menu driven, without many direct functions, which slows down operation in contest or strong pile-ups. On the contrary, in most high-end transceivers all common settings have a control, even of small size, on the front panel, sometimes highlighted with a LED, giving them a very "high-tech" look ,what will not deny James Bond, Hi!

Filtering : DSP or Audio stage ?

If you are only interested in CW, a filter placed on the audio stage is more than enough to get good results in most traffic conditions. Autek QF-1A is for example an excellent product.

But as we explained in reviewing Yaesu FT-1000MP Mark-V, a DSP doesn't work the same way, and in SSB for example it is able to remove automatically carriers that fall in the current passband, and this without using a notch circuit... a great improvement.

The passband of a DSP is also programmable to get abrupt wings contrary to an audio filter that displays usually a bell shape. The first is thus "better", in theory.

To read : Intermodulation range and roofing filters, by QTH.com (PDF)

Receiver Performance Transmitted BW Contest Fatigue,  by R.Sherwood, NC0B (10 MB PPT)

Block diagram of the Interlocked Digital Bandwidth Alignment Technique, for short IDBT, one of the best interference-fighting system developed by Yaesu for FT-1000 MP Mark V to get the sharpest signal at reception. An analog IF includes both IF WIDTH and IF SHIFT controls in conjunction with cascaded filters to modify the IF passband width and center frequency. Thanks to this technique, the DSP filter is automatically re-programmed so as to match the custom bandwidth you set to extract the weakest signals from QRM.

At last, the elimination of noise is ruled by signal processing protocols (maths), whereas an audio filter depends on its bandwidth... which affects the audio and therefore, it is not always suited to the mode used.

In selecting a filter, keep in mind these reference parameters when comparing performances of your filters. Remember also that not all mechanical filters display the short skirts of the Collins and many are not worth their price.

Generally speaking we can thus say that a DSP is more versatile as it can be used with most modes of traffic than mechanical filters. Let's see how perform these DSP when they are used in high-end transceivers.

Second part

The competition between high-ends

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