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Receiving equipment
Narrowband "SoftRock" receivers

SoftRock receivers:

These receivers are the "High Performance" receivers used at the Northern Utah WebSDR and are based on the so-called "QSD" (Quadrature Sampling Detector) mixers (sometimes known as a "Tayloe" detector) 1  2  3  4 that use analog MUX switches to provide the "mixing" action.  This mixer topology is well-used in commercial amateur gear - notably more recent Elecraft gear - and it has the advantage of being both simple and capable high-performance as well as being easy to interface with conventional hardware - such as a standard audio-frequency sound card - for final digitization.  Essentially a direct-conversion receiver, the RF energy that is present +/- the center (local oscillator) frequency is digitized up to the Nyquist limit (half the sample rate).  The receiver itself has two conversion channels that are identical aside from the fact that the local oscillators are 90 degrees out of phase with each other ("I" in-phase and "Q" - quadrature) which, with a bit of math (addition and subtraction and phase-shifting) allow the entire spectrum above and below the center frequency to be represented in software.
Figure 1:
Three SoftRock Ensemble II receivers in service.
Click on the image for a larger version.
The three Softrock Ensemble II receivers on the RF shelf

There are two "SoftRock" receiver types in use on this WebSDR system:
Comment:  Five-Dash seems to be winding down its operations, so one or more of the above kits may not be available now or in the future.
For our purposes, the two kits function identically in their role in converting RF to audio as the mixer and audio amplification circuitry of the two are pretty much the same.  The "Softrock Ensemble II" kit has the obvious advantage of having a built in, tunable local oscillator - but it is a much more expensive and complicated kit.  The Si570 synthesizer, while convenient to use, has the disadvantage that it is not particularly stable with frequency - indeed, the Elecraft KX3 uses a temperature sensor and a computer lookup table to maintain frequency stability, but the Ensemble II lacks this so its absolute frequency stability can be affected by changes in ambient temperature.  On lower bands like 160 meters this is not much of an issue, but on a higher band like 15, 12 or 10 meters this can amount to several 10s of Hz change.  Because we are using an "outboard" synthesizer for the "Softrock II Lite" kits, we were able to substitute a TCXO (Temperature-Controlled Crystal Oscillator) to obtain excellent frequency stability - more on this later.

Initially, we used three Softrock Ensemble II receivers for 160, 80 meter phone and 75 meter phone coverage, but a later upgrade included the construction of a module with three Softrock II Lite receivers to cover the entire 80/75 meter band, freeing those two units for coverage of other bands as described below.

Both the "Ensemble" and the "Lite" receivers feed the RF directly into the mixer (after filtering, of course) which is then passed to low-noise audio amplifiers - but this also means that these receivers are a bit "deaf".  For low bands like 160 and 75 meters - where both noise and signal levels are quite high on a "full-sized" antenna like a dipole this isn't too much of a problem, but on higher bands where the intrinsic noise is lower - and the losses of the receivers' circuitry is higher - it is increasingly important that some sort of RF amplification be used:  This, too, will be discussed later.

Receiver modules using the Softrock II Lite receivers:

Figure 2:
Inside the dual 40 meter receiver module.  In the center is power supply filtering (on the left) and a passive 2-way signal splitter (on the right) with the pair of Softrock II Lite receiver modules on each side.  In the center at the top is the ProgRock synthesizer, configured for two outputs.  The receivers have been slightly modified to accept an external frequency source in lieu of the original quartz crystals.  Remember that the local oscillator is four times
the actual receiver center frequency due to the on-board divide-by-four
counters to produce the needed quadrature signal.
Click on the image for a larger version.
Inside the dual 40 meter receiver
As noted above, the "Softrock II Lite" kit uses quartz crystals and the frequency selection is rather limited, so if we wish to have flexibility in our frequency coverage we'll need to use something else to provide our local oscillator.  The device chosen for this role is the inexpensive ($18) "Progrock" kit sold by QRP Labs (link).  As the name implies, this functions as a sort of programmable crystal (e.g. "rock") and these devices, based on the Silicon Labs Si5351a (the same chip used to provide the local oscillator of the Elecraft KX-2) can easily cover from about 8 kHz to well over 150 MHz - and may be coaxed even lower/higher than that if one pushes it beyond its official specs!  What's more, these devices can output up to three programmable frequencies at once (with some limitations) which means that a single unit can provide the local oscillators for up to three different receivers.

As delivered, these devices come with inexpensive quartz crystals that are prone to drift with temperature, but for less than $3 they can be retrofitted with a TCXO (Temperature Controlled Crystal Oscillator) that "nails" down the frequency with part-per-million stability, giving them better frequency accuracy and stability than many commercial HF rigs.  The ProgRock kits are typically programmed using DIP switches and a pushbutton for frequency entry using binary-coded decimal (BCD) but newer versions may be programmed via an asynchronous serial port.  For our purposes, only the DIP-switch programming is used as there is no reason to be able to change the center frequency of a receiver remotely.

Figure 2 shows the dual 40 meter receiver module.  On either side is a Softrock II Lite module built for 40 meters with the ProgRock synthesizer at the top, two outputs used to feed the receivers which have been slightly modified (the addition of a single capacitor, the removal of a different capacitor) to accept an external input in lieu of the original quartz crystal.  The ProgRock itself has been modified to use a TCXO as its frequency reference to provide a stability of approximately 1ppm over a wide temperature range which translates to an on-frequency stability of about +/- 7 Hz at 40 meters.  It's worth noting that the local oscillator frequencies being fed into the receiver modules operate at four times the center frequency of the receiver because there is digital divide-by-four circuitry to provide the quadrature local oscillator feeds for the QSD mixer.

In the lower-center, constructed "Manhattan" style - is a power supply filter on the left and a 2-way passive RF signal splitter on the right, the latter sending equal amounts of received RF to each of the two receivers.  As noted above, additional RF amplification is used to bring the signal levels up a bit and this will be discussed in greater detail later.

Comment:  Later multi-receiver modules contain one amplifier per receiver board to reduce possible spurious signals caused by local oscillator leakage from the receiver module itself.  Eventually, the 40 meter receiver will be modified so that each receiver has its own RF amplifier to improve isolation, for the reasons mentioned in the discussion of the 20 meter receiver module, below.

Potential issues with spurious signals:

As mentioned above, the ProgRock is capable of producing up to three independent frequency outputs at once, all from the same (tiny!) Si5351a chip - but there is basis for some concern if one does this.  Using just a single output, the signal is actually quite "clean", spectrally - far cleaner in non-harmonic content than that typically obtained using a typical DDS synthesizer module - which is why this same chip is the basis for many commercial and kit radios these days.  If more than one output is used, the spectral purity of the Si5351a chip suffers - but how much?

In the case of the dual receiver - where two outputs are used - if a single, strong CW signal were injected into the receiver (say, -20dBm - a "50 over" signal) a number of low-level spurious responses could be seen in the receiver's waterfall, the worst being on the order of 70dB (or more) down.  What this means is that if a "20 over" signal were present on the input (about -53dBm) a signal of S-1 or lower would result - but this would be at or below the noise floor on nearly any HF frequency, anyway.  Whether or not you might think that this was poor performance it's worth pointing out that many HF receivers do have similar spurious signal performance numbers, these "deficiencies" going unnoticed by the casual user - particularly on a busy band.
Figure 3:
Top:
 Close-up of the 2N5109 RF amplifier - one of three
 contained within a single amplifier module.  A later
modification of these amplifiers included the addition of a 2dB resistive pad on the output to assure unconditional stability
with reactive sources/loads.
Bottom:  Schematic diagram of the amplifier module
Click on an image for a larger version.
Close up of one of the broadband RF amplifier sections
Schematic diagram of the amplifier module

RF amplification using the 2N5109:


As noted previously, these "SoftRock" receivers - with no active devices in the signal path prior to the mixer - tend to be a bit "deaf".  In theory, their audio outputs are pretty low-noise with microvolt-level RF signals appearing nicely at the output, but the problem comes about when interfacing these same low-level audio signal to sound cards.  A typical good-quality sound card (such as those in the Asus Xonar series) is able to "see" weak signals like this - but there are two other issues that tend to show up:
By boosting the RF signal a bit the two noise sources can be submerged by RF noise coming in from the antenna - but there is a delicate balance:  Too much RF gain and the high-signal performance of the receiver will suffer, and too little gain, weak signals are lost in the noise.  "Barefoot" (e.g. without any amplification) these SoftRock receivers will start to saturate/clip at about -12 to -17dBm (a signal level of about "60 over") which means that one could "safely" add another 10-15dB gain without much worry about a few very strong signals, or the cumulative RF power of many signals on a band, causing overload.  For example, if the receiver were to start to overload at -25dBm (about "50 over") it would take about 100 "20 over" signals on the band (not including overall noise) to attain this much signal power:  While not impossible, this is unlikely to happen - even during contests.

To be "safe", one must keep in mind the following for any RF amplification that is to used:
A useful article on this topic (among many) is one written by Gary, WB9JPS 5  where he discusses various requirements of signal amplifiers, including gain and noise figure, in HF and VHF radio systems.   (An article that comes to similar conclusions is one written by AB4OJ - see reference 8.)   Gary's conclusion - which is not unique to this paper - indicates that conservative system noise figure requirements of HF receive systems are modest and along the lines of:
The amplifiers discussed in the article by WB9JPS reminded me of a November, 1984 article in Ham Radio magazine by Joe Reisert, W1JR 6 , where various topologies of amplifiers using the venerable 2N5109 transistor are discussed- a device designed specifically for broadband, low noise, linear operation and is, more importantly, still readily available!  While both the W1JR and the WB9JPS articles describe amplifiers with better signal-handling performance and lower noise than the common-emitter configuration that I used (see Figure 3 in the WB9JPS article, which references a design by W7ZOI) the performance of the amplifier is quite good and more than adequate for the task at hand.

Figure 4:
Top:  The dual 20 meter receiver.  This is very similar to the 40 meter
receiver except that there is an RF amplifier for each receiver to increase isolation of the LO bleedthrough between the two.
Bottom:  The schematic diagram of the dual receiver module.  This
module could be used on any HF amateur band -
it is only the receivers and the programming of the LO that
dictate the frequency of operation.  Remember:  The LO frequency is
four times the actual receiver center frequency!

Click on the image for a larger version.

Inside the dual 20 meter receiver
Diagram of the dual receiver with amplifiers
A set of three of these amplifiers were constructed and housed in a Hammond 1590D die-case enclosure using the circuit depicted in Figure 3 on this page.  Between each amplifier is a "wall" of double-sided, glass-epoxy circuit board material and each amplifier was built on its own, private board using "Manhattan" ("dead bug") techniques using "Me Squares" sold by QRPME (link) that were (literally!) glued down using high-quality cyanoacrylate adhesive (e.g. "super" glue.)   The usable frequency range of these amplifiers is on the order of 50kHz through 200 MHz, but they are flat to better than 1dB over the range of 1.5-30 MHz.  In testing these amplifiers they maintained very linear output (e.g. negligible intermodulation distortion) at power levels over +20dBm (100 milliwatts).  Even though there are three amplifiers in the same enclosure, the isolation between them was around 100dB at 10 MHz degrading to around 80dB at 30 MHz. 

The idea behind three amplifiers in one enclosure was that they could be used as general purpose gain blocks:  If extra gain was needed somewhere, these would be available to provide it - and upon installation of the WebSDR, one section was used with the 160 meter "Softrock Ensemble II" while another section was used for the dual 40 meter receiver module depicted in Figure 2:  This added gain (13-15dB) was about right to allow the receiver to "see" the noise floor during daylight hours, but not so much that receiver system performance was compromised even when there were a lot of "big" signals during contests.

RF amplification using the Mini-Circuits GALI74+:

More recently, amplifier "gain blocks" used at the WebSDR have been constructed using the Mini-Circuits GALI74+ MMIC.  This device is quite remarkable in its capability in that its gain is ruler-flat from DC through at least VHF and usable up to at least 1 GHz.  In terms of signal-handling capability, it compares very well with a 2N5109-based amplifier, having a typical 1dB compression level of about +19dBm and a 3rd-order intercept point of around +38dBm.  The noise figure of this device is also lower than that of a simple common-emitter 2N5109-based amplifier - typically a bit under 3dB whereas the 2N5109 has an approximately 6 dB noise figure.  Another property is that the Gali74+ amplifier has a gain of around 25dB at HF - about 10dB higher than that of a typical 2N5109 amplifier:  While this extra gain is useful, it can also be a liability if an appropriate signal-path analysis is not carried out.

Also important is that this device costs, in single quantities, about as much as a 2N5109 transistor - and it needs fewer support components to work.

At present, this circuit is being used in a few places where higher gain and/or lower noise figure is desirable as discussed later.

Additional receiver modules:

When the WebSDR system was first brought online the three Softrock Ensemble II receivers were used to cover a portion of 160 meters and most of the phone portions of the 80/75 meter band.  At this time additional equipment was in the works that would be used to provide coverage of allof 80/75 meters in three chunks and the entire 20 meter band in two - all of these using SoftRock Lite II modules.  As new modules were constructed using the Softrock Lite II receivers, the Softrock Ensemble II, being capable of being tuned anywhere, became available for general use - such as providing coverage for "new" bands or being used as a spare receiver in case of some sort of equipment failure.

20 meter coverage:


By observation, it was known that amplification would be required for the 20 meter SoftRock receivers so it was designed from the beginning to include it - but there was a bit of a twist:  One issue related to any receiver using a QSD mixer is that it can have a "significant" amount of local oscillator energy appearing on the antenna port, and on the 20 meter modules this signal level was on the order of -33dBm, or about "40 over" S-9.  To the receiver itself, this amount of signal is irrelevant as it cancels out and doesn't appear as a strong "zero Hz" component, but on the dual receiver modules the local oscillator for one receiver appeared in the other and this "big signal" could have potentially degraded performance - mostly in the form of a strong, off-frequency signal that could mix in various ways with low-level local oscillator spurious signals and the myriad of signals that might appear on a "busy" band.

A degree of isolation (15-20dB) between the receivers was provided by the passive 2-way splitter, but it was decided that each, individual receiver would sport its own, private RF amplifier, adding another 20dB or so of isolation, the end result being that one receiver would "see" a signal of -60dBm (about "10 over") or less from the local oscillator of the other.  Because these amplifiers are relatively simple - and the parts cheap - the construction of the added circuitry was not much of a burden.

Figure 5:
Top:
 The "triple" 80/75 meter receiver module.  Because there
are three receivers, the physical layout is different from the
dual receiver modules and like the 20 meter receiver, each
receiver has its own, private RF amplifier - both for gain
and LO isolation.
Bottom:  The schematic diagram of the triple
receiver module.  Like the dual receiver, it's only the receiver
itself and the programming of the synthesizer that determine
the HF band on which it operates.  Remember:  The LO
frequency is four times the actual receiver center frequency.

Click on an image for a larger version.
The triple Softrock receiver module with RF amplifiers.
Schematic diagram of the triple Softrock receiver with amplifiers
Figure 4 shows the completed receiver module.  It is nearly identical to the dual 40 meter receiver module in that it uses a single ProgRock synthesizer to to provide the local oscillator signals for both receivers.  If you look closely, you can see that there are two transistor amplifiers on the right-hand side of the copper-clad board in the bottom-center, following the 2-way splitter and each one feeding a receiver.

In testing on the workbench, the "MDS" (Minimum Discernible Signal) of each 20 meter receiver was better than -127dBm (e.g. 0.1 microvolts in a 50 ohm system) indicating that they were as sensitive as they needed to be:  In other words, this receiver was more than capable of hearing its fair share of ionospheric noise when connected to an HF antenna and as such, more sensitivity would not improve the ability to "hear" weak signals!

80 and 75 meter coverage:

It is somewhat inconvenient that most amateur bands are sized in multiples of 100 kHz but audio sound cards have sample rates of 96 or 192 kHz.  In the case of the U.S. 40 meter band we would need two 192 kHz sound cards to fully-cover the 300 kHz-wide band.  A somewhat similar situation exists on the U.S. 80/75 meter band, which covers from 3.5 to 4.0 MHz where we would need three receivers to cover the entire band.  It is convenient that the ProgRock can output three simultaneous outputs, so another receiver assembly was constructed using three Softrock Lite II modules.

The picture in Figure 5 shows the layout.  In the upper-left corner is the ProgRock synthesizer and like its counterparts, it, too is equipped with a 1ppm TCXO.  Below it are the three, identical Softrock Lite II modules and to the right of those, one for each receiver, are three RF amplifiers.  In the upper-right corner is a three-way splitter that divides the signals to the receivers equally and provides a bit of additional LO isolation between the receivers and on the other side of the divider is the same type of power supply filtering found in the other receiver modules.

Figure 5 also shows the diagram of the triple receiver module, the circuitry being representative of that in the other two modules.  If you look carefully you will notice that the RF amplifiers in the receiver modules are slightly different than that depicted in the WB9JPS article - and is, in fact, a direct "quote" of one of the amplifiers discussed in the November 1984 W1JR article.  The main difference is that these amplifiers lack the output balun/transformer which somewhat reduces the large-signal performance, but because these amplifiers are placed "downstream" bandpass filtering that is specific to an amateur band, they will not be "seeing" much of the HF spectrum and will be dealing with fewer signals, overall.







References:

  1. Youngblood, Gerald (July 2002), "A Software Defined Radio for the Masses, Part 1" (PDF), QEX, American Radio Relay League: 1–9
  2. Youngblood, Gerald (Sep–Oct 2002), "A Software Defined Radio for the Masses, Part 2" (PDF), QEX, American Radio Relay League: 10–18
  3. Youngblood, Gerald (Nov–Dec 2002), "A Software Defined Radio for the Masses, Part 3" (PDF), QEX, American Radio Relay League: 1–10
  4. Youngblood, Gerald (Mar–Apr 2003), "A Software Defined Radio for the Masses, Part 4" (PDF), QEX, American Radio Relay League: 20–31
  5. Johnson, Gary, "Measurements on a Multiband R2Pro Low-Noise Amplifier System, Part 2" (PDF)
  6. Reisert, Joe, (November, 1984), "High Dynamic Range Receivers, Ham Radio.  An English translation of part of this article from a Dutch web site may be found here.
  7. Turner, Clint, (March, 2018), "Managing HF signal dynamics on an RTL-SDR receiver"
  8. Farson, Adam, "Antenna and Receiver Noise Figure"


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