As a sort of follow-up to the previous posting about the RX-888 (Mk2) I decided to make some measurements to help characterize the gain and attenuation settings.
The RX-888 (Mk2) has two mechanisms for adjusting gain and attenuation:
- The PE4312 attenuator. This is (more or less) right at the HF antenna input and it can be adjusted to provide up to 31.5dB of attenuation in 0.5dB steps.
- The AD8370 PGA. This PGA (Programmable Gain Amplifier) can be adjusted to provide a "gain" from -11dB to about 34dB.
While this blog posting has specific numbers related to the RX-888 (Mk2), its general principles apply to ALL receivers - particularly those operating as "Direct Sampling" HF receivers. A few examples of other receivers in this category include the KiwiSDR and Red Pitaya - to name but two.
Other article RX-888 article:
I recently posted another article about the RX-888 (Mk2) discussing the thermal properties of its mechanical construction - and ways to improve it to maximize reliability and durability. You can find that article here: Improving the thermal management of the RX-888 (Mk2) - link.
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To ascertain the signal path properties of an RX-888 (Mk2) I set its sample rate to 64 Msps and using both the "HDSDR" and "SDR Radio" programs (under Windows - because it was convenient) and a a known-accurate signal generator (Schlumberger Si4031) I made measurements at 17 MHz which follow:
|Gain setting (dB)||Noise floor (dBm/Hz)||Noise floor (dBm in 500Hz)||Apparent Clipping level (dBm)|
Figure 1: Measured performance of an RX-888 Mk2. Gain mode is "high" with 0dB attenuation selected.
For convenience, the noise floor is shown both in "dBm/Hz" and in dBm in a 500 Hz bandwidth - which matches the scaling used in the chart below. As the programs that I used have no direct indication of A/D converter clipping, I determined the "apparent" clipping level by noting the amplitude at which one additional dB of input power caused the sudden appearance of spurious signals. Spot-checking indicated that the measured values at 17 and 30 MHz were within 1 dB of each other on the unit being tested.
Determining the right amount of "gain"
It should be stated at the outset that most of the available range of gain and attenuation provided by the RX-888's PE4312 step attenuator and AD8370 variable gain amplifier are completely useless to us. To illustrate this point, let's consider a few examples.
Consider the chart below:
|Figure 2: ITU chart showing various noise environments versus frequency.|
This chart - from the ITU - shows predicted noise floor levels - in a 500 Hz bandwidth - that may be expected at different frequencies in different locations. Anecdotally, it is likely that in these days of proliferating switch-mode power supplies that we really need another line drawn above the top "Residential" curve, but let's be a bit optimistic and presume that it still holds true these days.
Let us consider the first entry in Figure 1 showing the gain setting of 0dB. If we look at the "Residential" chart, above, we see that the curve at 30 MHz indicates a value very close to the -113dBm value in the "dBm in 500 Hz" column. This tells us several things:
- Marginal sensitivity. Because the noise floor of the RX-888 (Mk2) and that of our hypothetical RF environment are very close to each other, we may not be able to "hear" our noise floor at 30 MHz (e.g. the 10 meter amateur band). One would need to do an "antenna versus no antenna" check of the S-meter/receiver to determine if the former causes an increase in signal level: If not, additional gain may be needed to be able to hear signals that are at the noise floor.
- More gain may not help. If we do perform the "antenna versus no antenna" test and see that with the antenna connected we get, say, an extra S-unit (6dB) of noise, we can conclude that under those conditions that more gain will not help in absolute system sensitivity.
Thinking about the above two statements a bit more, we can infer several important points about operating this or any receiver in a given receive environment:
- If we can already "hear" the noise floor, more gain won't help. In this situation, adding more gain would be akin to listening to a weak and noisy signal and expecting that increasing the volume would cause the signal to get louder - but not the noise.
- More gain than necessary will reduce the ability of the receiver to handle strong signals. The HF environment is prone to wild fluctuations and signals can go between well below the local noise floor and very strong, so having any more gain that you need to hear your local noise floor is simply wasteful of the receiver's signal handling capability. This fact is arguably more important with wide-band, direct-sampling receivers where the entire HF spectrum impinges on the analog-to-digital converter rather than a narrow section of a specific amateur band as is the case in "conventional" analog receivers.
Let us now consider what might happen if we were to place the same receiver in an ideal, quiet location - in this case, let's look at the "quiet rural" (bottom line) on the chart in Figure 2.
Again looking at the value at 30 MHz, we see that our line is now at about -133dBm (in 500 Hz) - but if we have our RX-888 gain set at 0 dB, we are now ((-133) - (-113) = ) 20 dB below the noise floor. What this means is that a weak signal - just at the noise floor - is more than 3 S-units below the receiver sensitivity. This also means that a receiver that may have been considered to be "Okay" in a noisy, urban environment will be quite "deaf" if it is relocated to a quiet one.
In this case we might think that we would simply increase our gain from 0 dB to +33dB - but you'll notice that even at that setting, the sensitivity will be only -131dBm in 500 Hz - still a few dB short of being able to hear the noise in our "antenna versus no antenna" test.
Too much gain is worse than too little!
At this point I refer to the far-right column in Figure 1 that shows the clipping level: With a gain setting of +33dBm, we see that the RX-888 (Mk2) will overload at a signal level of around -31dBm - which translates to a signal with a strength a bit higher than "S9 + 40dB". While this sound like a strong signal, remember that this signal level is the cumulative TOTAL of ALL signals that enter the antenna port. Thinking of it another way, this is the same as ten "S9+30dB" signals or one hundred "S9+20dB" signals - and when the bands are "open," there will be many times when this "-31dBm" signal level is exceeded from strong shortwave broadcast signals and lightning static.
In the case of too-little gain, only the weakest signals, below the receiver's noise floor will be affected - but if the A/D converter in the receiver is overloaded, ALL signals - weak or strong - are potentially disrupted as the converter no longer provides a faithful representation of the applied signal. When the overload source is one or more strong transmissions, a melange of all signals present is smeared throughout the receive spectrum consisting of many mixing products, but if the overload is a static crash, the entire receive spectrum can be blanked out in a burst of noise - even at frequencies well removed from the original source of static.
Most of the adjustment range is useless!
Looking carefully at Figure 1 at the "noise floor" columns, you may notice something else: Going from a gain of 0 dB to 10 dB, the noise floor "improves" (is lower) by about the same amount - but if you go from 25 dB gain to 33 dB gain we see that our noise floor improves by only 1 dB - but our overload threshold changes by the same eight dB as our gain increase.
What we can determine from this is that for practical purposes, any gain setting above 20 dB will result in a very little receiver sensitivity improvement while causing a dramatic reducing in the ability of the receiver to handle strong signals.
Based on our earlier analysis in a noise "Urban" environment, we can also determine that a gain setting lower than 0 dB will also make our receiver too-insensitive to hear the weakest signals: The gain setting of -25dB shown in Figure 1 with a receive noise floor of -79dBm (500 Hz) - which is about S8 - is an extreme example of this.
Up to this point we have not paid any attention to the PE4312 attenuator as all measurements were taken with this set to minimum. The reason for this is quite simple: The noise figure (which translates to the absolute sensitivity of a receiver system) is determined by the noise generation of all of the components. As reason dictates, if you have some gain in the signal path, the noise contribution of the devices after the gain have lesser effects - but any loss or noise contribution prior to the gain will directly increase the noise figure.
For examples of typical HF noise figure values, see the following articles:
Based on the articles referenced above, having a receiver system with a noise figure of around 15dB is the maximum that will likely permit reception at the noise floor of a quiet 10 meter location. If you aren't familiar with the effects of noise figure - and loss - in a receive signal path, it's worth playing with a tool like the Pasternack Enterprises Cascaded Noise Figure Calculator (link) to get a "feel" of the effects.
I do not have the ability to measure the precise noise figure of the RX-888 (Mk2) - and if I did do so, I would have to make such a measurement using the same variety of configurations depicted in Figure 1 - but we can know some parameters about the worst-case:
- Bias-Tee: Estimated insertion loss of 1dB
- PE4312: Insertion loss of 1.5dB at minimum attenuation
- RF Switch (HF/VHF): 1dB loss
- 50-200 Ohm transformer: 1dB loss
- AD8370 Noise figure: 8dB (at gain of 20dB)
The above sets the minimum HF floor noise figure of the RX-888 (Mk2) at about 12.5dB with an AD8370 gain setting of 20dB - but this does not include the noise figure of the A/D converter itself - which would be difficult to measure using conventional means.
On important aspect about system noise figure is that once you have loss in a system, you cannot recover sensitivity - no matter how much gain or how quiet your amplifier may be! For example, if you have a "perfect" 20 dB gain amplifier with zero noise, if you place a 10 dB attenuator in front of it, you have just turned it into an amplifier with 10 dB noise figure with 10dB gain and there is nothing that can be done to improve it - other than get rid of the loss in front of the amplifier.
Similarly, if we take the same "perfect" amplifier - with 20dB of gain - and then cascade it with a receiver with a 20dB noise figure, the calculator linked above tells us that we now have a system noise figure of 3 dB since even with 20dB preceeding it, our receiver still contributes noise!
If we presume that the LTC2208 A/D converter in the RX-888 has a noise figure of 40dB and no gain (a "ballpark" value assuming an LSB of 10 microvolts - a value that probably doesn't reflect reality) our receive system will therefore have a noise figure of about 22dB.
What this means is that in most of the ways that matter, the PE4312 attenuator is not really very useful when the RX-888 (Mk2) is being used for reception of signal across the HF spectrum, in a relatively quiet location on an antenna system with no additional gain.
Where is the attenuator useful?
From the above, you might be asking under what conditions would the built-in PE4312 attenuator actually be useful? There are two instances where this may be the case - and this would be applied ONLY if you have been unable to resolve overload situations by setting the gain of the AD8370 lower.
- In a receive signal path with a LOT of amplification. If your receive signal path has - say - 30dB of amplification (and if it does, you might ask yourself "why?") a moderate amount of attenuation might be helpful.
- In a situation where there are some extremely strong signals present. If you are near a shortwave or mediumwave (AM broadcast) transmitter that induces extremely strong signals in the receiver that cause intractable overload, the temporary use of attenuation may prevent the receiver from becoming overloaded to the point of being useless - but such attenuation will likely cause the complete loss of weaker signals. In such a situation, the use of directional antennas and/or frequency-specific filtering should be strongly considered!
Returning to an earlier example - our "Quiet Rural" receive site - we observed that even with the gain setting of the RX-888 (Mk2) at maximum, we would still not be able to hear our local noise floor at 30 MHz - so what can be done about this?
Let us build on what we have already determined:
- While sensitivities is slightly improved with higher gain values, setting the gain above 20dB offers little benefit while increasing the likelihood of overload.
- In a "Quiet Rural" situation, our 30 MHz noise floor is about -133dBm (500 Hz BW) which means that our receiver needs to attain a lower noise floor than this: Let's presume that -136dBm (a value that is likely marginal) is a reasonable compromise.
With a "gain" setting of 20dB we know that our noise floor will be around -128dBm (500 Hz) and we need to improve this by about 8 dB. For straw-man purposes, let's presume that the RX-888 (Mk2) at a gain setting of 20dB has a noise figure of 25dB, so let's see what it takes for an amplifier that precedes the RX-888 (Mk2) to lower than to 17dB or so using the Pasternak calculator above:
- 10dB LNA with 7 dB noise figure: This would result in a system noise figure of about 16 dB - which should do the trick.
Again, the above presumes that there is NO loss (cable, splitters, filtering) preceding the preamplifier. Again, the presumed noise figure of 25dB for the RX-888 (Mk2) at a gain setting of 20 is a bit of a "SWAG" - but it illustrates the issue.
Adding a low-noise external amplifier also has another side-effect: By itself, with a gain setting of +33, the RX-888 (Mk2)'s overload point is -31dBm, but if we reduce the gain of the RX-888 to 20dB the overload drops to -18dBm - but adding the external 10dB gain amplifier will effectively reduce the overload to -28dBm, but this is still 5 dB better than if we had turned the RX-888's gain all of the way up!
Taking this a bit further, let's presume that we use, instead, an amplifier with 3dB noise figure and 8 dB gain: Our system noise figure is now about 17dB, but our overload point is now -26dBm - even better!
The RX-888 is connected to a (noisy) computer!
Adding appropriate amounts of external gain has an additional effect: The RX-888 (and all other SDRs) are computer/network connected devices with the potential of ingress of stray signals from connected devices (computers, network switches, power supplies, etc.). The use of external amplifiers can help override (and submerge) such signals and if proper care is taken to choose the amount of gain of the external amplification and properly choose gain/attenuation settings within the receiver, superior performance in terms of sensitivity and signal-handling capability can be the result.
Only mentioned in passing, running a wideband, direct-sampling receiver of ANY type (be it RX-888, KiwiSDR, Red Pitaya, etc.) connected to an antenna is asking a lot of even 16 bits of conversion! If you happen to be in a rather noisy, urban location, the situation is a bit better in the sense that you can reduce receiver gain and still hear "everything there is to hear" - but if you have a very quiet location that requires extra gain, the same, strong signals that you were hearing in the noisy environment are just as strong in the quiet environment.
Here are a few suggestions for maximizing performance under the widest variety of situations:
- Add filtering for ranges that you do not plan to cover. In most cases, AM band (mediumwave) coverage is not needed and may be filtered out. Similarly, it is prudent to remove signals above that in which you are interested. For the RX-888 (Mk2), if you run its sampling rate at just 65 MHz or so, you should install a 30 MHz low-pass filter to keep VHF and FM broadcast signals out.
- Add "window" filtering for bands of interest. If you are interested only in amateur radio bands, there are a lot of very strong signals outside the bands of interest that will contribute to overload of the A/D converter. It is possible to construct a set of filters that will pass only the bands of interest - but this does not (yet?) seem to be a commercial product. (Such a product may be available in the near future - keep a lookout here for updates.)
- Add a "shelving" filter. If you examine the graph in Figure 2 you will notice that as you go lower in frequency, the noise floor goes UP. What this means is that at lower frequencies, you need less receiver sensitivity to hear the signals that are present - and it also means that if you increasingly attenuate those lower frequencies, you can remove a significant amount of RF energy from your receiver without actually reducing the absolute sensitivity. A device that does just this is described in a previous blog article "Revisiting the limited-attenuation high-pass filter - again (link)". While I do not offer such a filter personally, such a device - along with an integrated 30 MHz low-pass filter - may be found at Turn Island Systems - HERE.
- The best HF weak-signal performance for the RX-888 (Mk2) will occur with the receiver configured for "High" gain mode, 0 dB attenuation and a gain setting of about 20dB. Having said this, you should always to the "antenna versus no antenna" test: If you see more than 6-10dB increase in the noise level at the quietest frequency, you probably have too much gain. Conversely, if you don't see/hear a difference, you probably need more gain - taking care in doing so.
- For best HF performance of this - or any other wideband, direct-sampling HF SDR (RX-888, KiwiSDR, Red Pitaya, etc.) additional filtering is suggested - particularly the "shelving" filter described above.
- In situations where the noise floor is very low (e.g. a nice, receive quiet location) many direct-sampling SDRs (RX-888, KiwiSDR, Red Pitaya) will likely need additional gain to "hear" the weaker signals - particularly on the higher HF bands. While some of these receivers offer onboard gain adjustment, the use of external high-performance (low-noise) amplification (along with filtering and careful adjustment of the devices' gain adjustments) will give improved absolute sensitivity while helping to preserve large-signal handling capability.
- Because the RX-888 is a computer-connected device, there will be ingress of undesired signals from the computer and the '888's built-in circuitry. The use of external amplification - along with appropriate decoupling (e.g. common-mode chokes on the USB cable and connecting coaxial cables) can minimize the appearance of these signals.
This page was stolen from ka7oei.blogspot.com.